Evolutionary Psychology: Individual Differences (PDF)

13 Evolution and Individual Differences Key Concepts personality niche fitting heritability frequency-dependent selection genome-wide associa- tion studies (GWAS) intelligence general intelligence (g) multiple intelligences Savanna-IQ Interaction Hypothesis single nucleotide polymorphisms A great deal of research in psychology has focused on differences among individuals. Psychologists have been particularly interested in individual differences in personality and intelligence, investigat- ing the underlying causes of these differences and how they might affect other aspects of life such as career development, success in relationships and susceptibility to mental illness. Evolutionary psychology with its focus on ultimate questions asks a different question. What is the function, if any, of individual differences? Why, for example, are some people sensation-seeking extraverts while others are timid stay-at-home introverts? Why are some people smart and others less so? One answer could be that these characteristics reflect differences in upbringing; that they are the result of environmental rather than genetic differences. Intelligent individuals were given more educa- tional opportunities than less intelligent individuals; extraverts were encouraged to be bold and so on. But this cannot be the whole story. Research has shown that many of these traits are heritable, suggesting that at least some of the variation in the aforementioned characteristics is down to the effects of genes. But this poses another problem. We know that natural selection promotes certain genes over others by virtue of their superior phenotypic effects, thereby reducing genetic variability in a species, so why haven’t all of these genetic differences been removed from the gene pool? Is there some hidden benefit in having variability in personality and intelligence within our species? This chapter focuses on these (and other) questions, and in doing so it draws on material covered in previous chapters and points the way to the future of evolutionary psychology; an evolutionary psychology that views genes as dynamic, shaping the phenotype using decision rules that act upon environmental information. Individual Differences and Evolution It is obvious to anyone that people differ from one another. For example, people growing up in France will usually speak French and people growing up in China will tend to speak one of the Chinese languages such as Cantonese or Mandarin, and they will do this regardless of their genetic makeup (see Chapter 10). The same applies to cultural practices, which vary widely across the world (see Chapter 14). But when psychologists speak of individual differences they usually mean some- thing more specific than these differences, they are usually referring to differences in intelligence 354 Evolution and Individual Differences and personality. There is a great deal of evidence that people exhibit differences in personality and intelligence that are stable over time and across different contexts (Cooper, 2012; Larsen and Buss, 2020; Haslam et al., 2017). One might ask whether an evolutionary theory of individual differences is really necessary – might differences in personality and intelligence be just due to environmental differences like the cultural and linguistic variability pointed out above? One reason why evolution might be important is that, as we saw in Chapter 6, there is a considerable body of research indicating that many personality factors are to some extent herit- able (usually between about 0.3 and 0.5 on a scale from 0 to 1) and intelligence has heritability coefficients of 0.4–0.7 (Loehlin et al., 1998; Plomin, 2018). The existence of moderate heritability coefficients suggests individual differences are at least partly due to genetic differences. This being the case it suggests that there is some benefit – in terms of inclusive fitness – to having variability in our offspring because, as we saw in Chapter 2, disadvantageous phenotypes tend to be selected out of the population. There is a potential paradox here. Much of this book has made the claim that selection pressures in the environment of evolutionary adaptation (EEA) led to the evolution of a human nature that was designed to solve the problems encountered by our hunter-gatherer ancestors (see Cosmides and Tooby, 1992; Pinker, 1997; Myers and Bjorklund, 2020). If this is the case then should we not by now all have the same personality and the same intellectual ability (barring brain damage and other unforeseen circumstances)? Does not the very existence of individual differences (particularly those that are apparently genetic in origin) completely undermine the notion of an evolved human nature that evolutionists have worked so hard to promote? No. Such a contention represents a misunderstanding of the very idea of a human nature. There are two possibilities that reconcile the apparent contradiction between the claim of an evolved human nature and the data on individual differences, and these are detailed below. Individual differences make little difference as far as inclusive fitness is concerned and are therefore invisible to natural selection. Here it is suggested that we as humans tend to overemphasise the magnitude of the differences among us: a Martian studying our species might see us all as pretty much identical (barring gross differences such as sex). Thus there are no inclusive fitness benefits of tending towards one personality rather than another, or being more or less intelligent (within reason). Naturally, having a profoundly different personality (for example, having a personality dis- order, such as psychopathy; see later and Chapter 12) or having an abnormally low IQ might make a difference, but for the majority there will be little effect on fitness. In a sense this variability may be thought of as ‘noise’ in the system. There is no single, globally optimal ‘human nature’. Doubtless many of the problems faced by our ancestors had just one best solution. For example, having colour vision is better than not having colour vision; being wary of strangers is – for a child – better than treating them with equa- nimity; being on the look-out for potential cheats is more useful than checking for altruists. In such cases it is easy to see how certain aspects of our nature might have become ‘fixed’. There are many other problems, however, where the best solution (in terms of inclusive fitness) is less clear. Is it better to be bold and venturesome or cautious and considered; to be unremittingly selfish or to be generous with non-kin; to be good at understanding people or good at making tools? Each of the above might be a thoroughly adaptive way of behaving depending on the nature of the environment and what other members of your group are doing. This last point is important, as we shall see later; it might be in your own interests to behave somewhat differently from other members of your group, to Individual Differences in Personality 355 exploit opportunities missed by others. Two such approaches that we discuss later are niche fitting and frequency-dependent selection. Before we begin to discuss in more detail potential ways of resolving the above paradox, we first need to describe in a little more detail the nature of individual differences. We begin with personality. Individual Differences in Personality What Is Personality? To understand personality, we need to make a distinction between psychological states and psy- chological traits. Suppose that a person behaves in a particular way, for example on a single obser- vation that she becomes angry rather quickly. If she behaves in a similar way at different times and over different situations we might conclude that being quick to anger is part of her psychological makeup, known technically as a psychological trait. On the other hand, if this behaviour shows no temporal and situational consistency, then we might conclude it is merely a transient mood, tech- nically known as a psychological state. Evidence for state dependency might be increased if there were some precipitating circumstances, for example the person was under stress. Differences in psy- chological traits (personality) can be thought of as differences in what people are motivated to do or how they respond to a particular state of affairs. Sociable people, for example, are motivated to seek out socially challenging situations, shy people to avoid them; neurotic people might become anxious in stressful situations and hence tend to avoid them, more laid back people might cope better in such situations and so have no reason to avoid them. From an evolutionary point of view we might think of personality as a behavioural strategy; a tendency to respond in specific ways to particular circumstances. Personality tests usually measure ‘traits’ (or sometimes ‘factors’; see later) – that is, enduring facets of a personality such as how ‘outgoing’ or ‘liberal’ a person is rather than states (Schultz and Schultz, 2005; Haslam et al., 2017). Box 13.1 How Is Personality Measured? Personality is measured by personality tests, which are self-report tests that usually require the participant to respond to a set of questions relating to the specific aspect of personality under question. For example, if we are interested in measuring extraversion we might ask participants, questions such as: ‘Do you usually stay in the background at social occasions?’ (YES/NO), or they might be asked, ‘Are you a worrier?’ in order for their level of neuroticism to be determined. After completing a number of such questions the participant is given a score that records, for example, how extravert or neurotic he or she is. These questions are the result of extensive testing to determine whether they predict behaviour – whether they are a valid measure of personality (for example, do people who are categorised as extraverts using these measures really behave in an extravert manner?) (Cooper, 2012; Haslam et al., 2017) and whether they are reliable (for ex- ample, if people complete the tests more than once is their personality measure the same in each case?). Here are some examples from a self-report personality questionnaire (in reality there are usually over 100 questions) that are designed to uncover how anxious/relaxed, extravert/introvert or friendly/unfriendly a person you are: 356 Evolution and Individual Differences Box 13.1 (cont.) 1. Are you outgoing? YES NO 2. Does meeting new people make you happy? YES NO 3. Do you have regular concerns about your health? YES NO 4. Would it make you happy to have others fear you? YES NO 5. Do you lose sleep at times due to work-related worries? YES NO 6. Do you feel good when you hurt someone you love? YES NO Clearly such tests rely on honest answers. Many of these tests have ‘lie detector’ questions – if more than one or two of such questions are answered the ‘wrong way’ then the tester discounts the scores as unreliable. Such lie detector questions are ones that only ‘a saint’ would give a particular answer to such as: 7. Have you ever laughed at a dirty joke? YES NO How Many Personality Traits Are There? In the 1930s psychologist Gordon Allport (Allport and Odbert, 1936) determined that the English dictionary contained no fewer than 17,953 different words for personality traits, but this does not necessarily mean that there are this many different personality traits. Many of the words are close synonyms (e.g. outgoing, gregarious, sociable, extravert) whereas some are antonyms (e.g. outgoing vs. shy, assertive vs. passive, anxious vs. calm). So it is certainly possible to reduce the thousands of words to substantially fewer (perhaps a hundred or so) personality traits. But this is still too many to be useful. One way of reducing them still further is to look for latent patterns in the still-large set of words. For example ‘gregariousness’ and ‘assertiveness’ might not be synonymous but they might be somewhat related to each other in the sense that someone who tended to be gregarious might also tend towards being assertive, i.e. the two traits were positively correlated. The technical meth- od of reducing a vast number of potential traits to a manageable few is known as factor analysis. A detailed explanation of how this works is beyond the scope of this book, but here is an overview of the process. We begin by presenting a group of participants with a large number of questions relating to personality (such as those above). We then look for intercorrelations between the re- sponses to these questions. Any answers to questions that are highly correlated with each other can be collapsed together to form a more inclusive personality trait (e.g. shyness, aggressiveness, gre- gariousness, impulsivity). We could simply stop there, but we can also carry on by factor analysing these inclusive traits to find still higher-order personality clusters. To do this we look for correlations between the traits; again if the correlation is sufficiently high we can collapse the correlated traits together to form a personality factor. We might find, for example, that gregariousness and shyness are (negatively) correlated, so these would be collapsed into a factor that we might call extraversion. We could presumably keep going until there was only one factor but this is never done, presumably because a single factor isn’t a very meaningful way of explaining human personality. ‘Lumpers’ and ‘Splitters’ Personality theorists tend to fall into two camps: lumpers and splitters. Lumpers tend to produce a Individual Differences in Personality 357 Figure 13.1 Professor Hans Eysenck of the Institute of Psychiatry, King’s College Hospital, London, using a Swed- ish machine for measuring eye blinks in 1968. lumper was Hans Eysenck (see Eysenck, 1990) who reduced all of the variation in personality to just three factors, which are the famous extraversion vs. introversion, neuroticism vs. stability and psychoticism vs. socialised. Extraversion is most often thought of as being concerned with how so- cially outgoing a person is. A person high on extraversion will frequently be outgoing and sociable, whereas a person low on extraversion (or high on introversion) will generally be more reserved. Neuroticism is generally concerned with how psychologically stable a person is. People scoring high on neuroticism are quite highly strung and tend towards anxiousness, people lower on this di- mension being quite laid back and unruffled. Finally, psychoticism is associated with ‘niceness’ (or its polar opposite – ‘nastiness’), people scoring high on this factor tend towards cruelty and can be quite manipulative, lower scorers being kinder and more considerate. 358 Evolution and Individual Differences Table 13.1 The Big Five personality factors with typical characteristics of high and low scorers on these factors Factor High scorer Low scorer Openness to experience tendency to be adventurous, curious tendency to be conservative and (also known as intellect) closed minded Conscientiousness tendency to be self-disciplined and goal tendency to be impulsive, driven directed more by the situation Extraversion (also known as tendency to seek stimulation, including tendency to avoid stimulating surgency) social stimulation situations, reserved Agreeableness tendency to be compassionate, cooperative tendency to be antagonistic and and sympathetic towards others distrustful of others Neuroticism prone to negative emotions such as calmer, less prone to these anxiety, depression, anger emotions Each factor is a continuum rather than a type. A person is not simply assigned to the cat- egory of, for example, extravert or introvert, but rather is given a score on this dimension. Each di- mension is also orthogonal to the others, which means that the score on one factor is not correlated with the score on any other factor when sampled across a large number of individuals. Therefore, a person’s extraversion score cannot be used to predict his or her score on any of the other factors. Other lumpers are those researchers who produced the so-called Big Five personality fac- tors (McCrae and Costa, 1996). The Big Five personality factors are openness to experience, con- scientiousness, extraversion, agreeableness and neuroticism (as a mnemonic they spell the word OCEAN). Table 13.1 lists the key features of these personality factors. In Table 13.1 conscientiousness and agreeableness are essentially sub-factors of Eysenck’s psychoticism factor, kept separate here as it is believed they can explain behavioural characteristics better individually than they can when lumped together as one generic factor. Openness to experi- ence is, in Eysenck’s scheme, incorporated into extraversion. The Big Five separates out the sociable aspects of behaviour – which it calls extraversion – from openness to experience, again because it is believed that these factors account for behaviour better when kept separate. Today most psychologists interested in individual differences subscribe to the Big Five view of personality factors (Haslam et al., 2017). One evolutionary psychologist who has a special interest in personality, David Buss (Buss, 2019; Larsen and Buss, 2020), has argued that the Big Five factors represent a more evolutionarily plausible way of carving up human personality than the other methods such as that of Eysenck and another well-known typology developed by Raymond Cattell (1965). Cattell was a splitter and his typology uses 16 non-orthogonal factors – the non- orthogonality means that some of the factors are correlated with some others. What Buss means by evolutionarily plausible is that each personality type represents a motivational force that is likely to benefit an organism living in the kind of social and physical environment that humans do (or at least did during the EEA). For example, Eysenck’s extraversion pushes together traits concerned with sociability with those concerned with curiosity and imaginativeness. This according to Buss is unsatisfactory, because curiosity is likely to have fitness implications that are entirely different from sociability, and they are therefore better separated as in the Big Five. Two other points are worth mentioning about the personality factors discussed above if personality theory is to be properly understood. First, it is rather too easy to think of scoring highly on some factor as being essentially positive and low as being negative. For example, being agreeable Personality and Evolutionary Theory 359 – where a person is trusting, and cooperative – seems a good thing to be, whereas being disagree- able, with its associated traits of cynicism and uncooperativeness, seems inherently negative. This, however, is a prejudice that we must shake off. If we adopt an evolutionary point of view calling a trait or factor ‘positive’ or ‘negative’ only makes sense to the extent that they have beneficial or detrimental effects on inclusive fitness. Traits or factors that seem negative in some moral sense might well have positive effects on fitness. As an example, one can imagine agreeable people, due to their trusting nature, being more easily exploited by others, whereas cynical people might be more likely to withdraw cooperation in the face of freeriders (see Chapters 2 and 8) and hence avoid the potentially disastrous consequences of exploitation. In this way the environment may act as a trigger to help mould or ‘release’ certain personality traits. The second point is that an individual’s personality (including how they might behave in a given situation) is only captured adequately by knowing his or her score on all of the relevant factors. Exactly how a person high in extraversion will behave will also depend, for example, on whether they are open to experience or narrow-minded, neurotic or stable. Personality and Evolutionary Theory Nature, Nurture and Personality Research from the behavioural genetics tradition (see Chapter 6), which conducts studies on the personalities of twins (monozygotic and dizygotic, reared together and apart) and on adopted indi- viduals, has estimated that the Big Five personality factors show heritability coefficients of between 0.3 and 0.5 (Tellegan et al., 1998; Plomin, 2018). This means that between 30 and 50 per cent of the variation among people in personality is accounted for by genetic differences between them. This therefore means that between 50 and 70 per cent of the variation is down to the non-genetic factors, which is usually thought of as being due to the environment. But remember that the environment is a very broad category that encompasses everything that is not genetic (or correlated with the effects of the genes; see later). Box 13.2 The Consistency of Behaviour across Situations Trait approaches to personality, such as those discussed above, hang on the assumption that our behaviour is relatively consistent over time. For example, people scoring high on extraver- sion should behave in an extravert manner over a variety of different situations. One of the first psychologists to question this assumption was Walter Mischel. After reviewing the evidence in 1968 he found that when behaviour relevant to personality was investigated (e.g. outgoingness, honesty) people behaved somewhat differently across different situations. In other words, a per- son who acts gregariously in one situation might act less so in a different situation. The resultant debate, often referred to as the person–situation debate, is concerned with whether the primary determiner of behaviour is a person’s personality or the specifics of the situation. The debate has continued over the years (see Mischel, 1992; Ross and Nisbett, 1991) but it is important that it is properly understood: no one doubts that there is some level of consistency across situations – there are always some effects of personality – the key question is to what extent personality mat- ters in guiding behaviour. This debate is unlikely to be resolved as there is no a priori generally 360 Evolution and Individual Differences Box 13.2 (cont.) agreed upon number above which, say, one could decide that personality is more important, below which it becomes the situation. Finding a correlation of 0.4 across situations might be sufficient for both sides to claim victory. The evolutionary perspective sees behaviour as part of an evolutionary strategy that pro- moted inclusive fitness in the ancient past. Stable differences in personality represent different life strategies: some people, for example, gain access to resources by dominating others, where- as others do it by being deferential. At the same time behaviour should also be situationally contingent; it would be a poor behavioural strategy, in an inclusive fitness sense, for us to ride roughshod over the demands of the immediate situation or our own long- and short-term goals. For example, a person who generally seeks to dominate others might show deference in the pres- ence of someone of high prestige who might be able to offer valuable information and material resources (see Henrich and Gil-White, 2001). A person who is generally deferential might, on the other hand, become more dominant when in the presence of individuals of lower status. The evolutionary viewpoint would ask a different question of the person–situation debate. Rather than asking what the most important determiner of behaviour is, it would ask how stability and variability in behaviour can be seen as part of a strategy that maximises inclusive fitness (or would have done so in the EEA). The environment might include the following: how your parents, peers, siblings or teachers treated you as a child; toxins, drugs, radiation or diseases that make their way into the uterus during pregnancy and disrupt foetal development – technically such substances are known as teratogens and include alcohol, thalidomide, the rubella (German measles) virus and cosmic rays; aspects of the mother’s diet or lifestyle during pregnancy or perhaps even before, for example, there is evidence that a woman’s diet during pregnancy can affect the development of her children and perhaps also her grandchildren (Barker, 1998); birth trauma such as oxygen starvation and other obstetric complications; the intracellular environment; diseases and toxins encountered during childhood; any other experiences, good or bad, physical or psychological, that might have occurred at any point during the lifespan (including the time spent in utero). Some points need clarification. In the case of identical twins, in order for any of the above to appear as effects of the environment in behavioural genetic studies they need to affect one twin and not the other. One of the quirks of behavioural genetic studies is that if, for example, a teratogen affected each identical twin in the same way it would appear as an effect of the genes. In order for an effect to appear as environmental (or not genetic) it has to drive identical twins in different directions. Later we shall see that even in this case labelling such a finding an environmental effect might be misleading. In some cases the environment might merely serve as a trigger that leads to phenotypic change under the control of evolved mechanisms (see epigenetics in Chapter 2). Personality and Evolutionary Theory 361 The Search for an Evolutionary Theory for Variation in Personality – What Needs to Be Explained? Let us remind ourselves of what an evolutionary theory of personality needs to explain, and what a theory might look like. First of all we need to explain the observation that at least 30 per cent of vari- ation among individuals in most personality factors (e.g. the Big Five) is accounted for by genes. We therefore need to explain why particular personality traits are, to some extent, passed down the genera- tions. Evolutionary theory, and the rest of this book, teaches us that heritable traits (physical or psycho- logical) are usually passed on for a reason; if they have negative fitness consequences they usually die out over the generations. Second, we know that something like 50–70 per cent of the variation among people is not down to genes (or, at least, not directly due to genes) and this variation is usually thought of as being down to the environment. If there are real benefits in passing on personality traits (as we might assume from the first of these points) then why is so much seemingly left to chance? We there- fore have yet another paradox. If we try to explain heritable variation by assuming that the traits are fitness enhancing (in some, as yet unspecified, way) we are left with the question as to why personality is not completely inherited. An evolutionary theory of personality differences therefore should account for both of these features of personality: heritable and non-heritable (or environmental) variation. Table 13.2 Summary of the different accounts of individual variation depending on its source (heritable vs environmental) and its effect (adaptive, non-adaptive, maladaptive) Source of individual variation Heritable variation Non-heritable variation Adaptive frequency-dependent adaptive early environmental calibration ‘weather strategies (e.g. Mealey’s primary forecasting’ (e.g. the Belsky/Chisholm psychopathy) view of attachment) genetically based niche filling. strategic niche specialisation (e.g. frequency-dependent strategies such as Mealey’s secondary psychopathy) variation because of changing environmentally based niche filling, e.g. environments peer socialisation (Harris), birth order effects (Sulloway) Proposed effect differential susceptibility to rearing none yet proposed of the individual influence (Belsky, 2005) variation variation due to fitness trade-offs (e.g. chance variation due to incomplete survival, reproduction, childrearing) specification of phenotype (see Pinker, 2002) Non-adaptive phenotypic variation because of sexual social learning (e.g. Social Learning recombination Theory) by-product of an adaptation (e.g. homosexuality in men might be a side effect of genes which when in females increase their fertility) Maladaptive genetic abnormalities (e.g. environmental trauma (e.g. foetal alcohol Huntington’s chorea) syndrome), physical or psychological trauma 362 Evolution and Individual Differences There are three general categories of evolutionary explanation for these two types of var- iation. Adaptive theories suggest that there is some benefit in terms of inclusive fitness in having these types of variation; non-adaptive theories suggest that in some ways the differences might be due to noise, or knock-on effects of other adaptations; and maladaptive accounts would suggest that variation might have deleterious effects on inclusive fitness, such as might be found in some psy- chopathologies (see Chapter 12). These accounts are not mutually exclusive: some accounts might work for some phenomena and other accounts might work for different phenomena. Table 13.2 is adapted from the work of David Buss (Buss and Greiling, 1999) and tries to explain the heritable and non-heritable variation in personality by virtue of whether the variation may be adaptive (is ben- eficial for fitness), non-adaptive (has no effect on fitness) or maladaptive (has negative fitness con- sequences). This table summarises the various positions that we discuss over the next few sections. Explaining the Heritable Component of Individual Difference Non-adaptive Variation due to Sexual Recombination and Mutation As we discussed in Chapters 2 and 3, the shuffling of genes that occurs during sexual recombination means that each offspring has a unique combination of their parents’ genes. Genes have various effects on each other; some genes switch other genes off or on at different points in development for example. Moreover, each of us has on average around 100 mutated genes. The net result could be that a large degree of variation that is genetic occurs as a result of an epiphenomenon – that is, as a side effect of the process of recombination, sex and mutations, rather than as a direct result of natural selection. Hence some individual differences might merely be due to the side effect of sexual reproduction and the interactions among genes. Adaptive Variation as a Result of Changing Environments As all of us are doubtless aware, environments change over time. During the time of the dinosaurs, the earth’s climate was known as ‘greenhouse earth’ as there was no permanent ice; the North and South Poles enjoyed tropical conditions. Sometime later the earth was plunged into several ice ages with ice-sheets over a large proportion of the planet; a situation that, for the time being at least, exists today as the poles are still frozen. Environments change on a smaller scale too; there might be periods in which food is scarce, followed by periods of comparative plenty. One possible account for the genetic variation among individuals leading to individual differences in personality is that some phenotypes might profit in certain conditions and others in quite different conditions (this may even be the reason that sex exists; see the Red Queen hypothesis in Chapter 3). For example, spec- ulatively, hard times might favour individuals who are likely to cooperate – there is a powerful need to work together to eke out a living from a hostile environment – whereas good times might favour individuals who are somewhat more selfish – since there is plenty for all, individuals can more easily lead a productive life on their own. If we imagine that the environment favours cooperators, then this phenotype will profit at the expense of more selfish individuals. If such conditions were to obtain for a long period of time then selfish individuals would probably die out, but if the environment fluctu- ates back and forth comparatively rapidly (over a cycle of, say, 100 years or so) then under different conditions the selfish individuals might find themselves at a fitness advantage and their numbers would increase at the expense of cooperators. Explaining the Heritable Component of Individual Difference 363 When we discuss ‘personality’ in non-human animals later in this chapter we point out some instances in which the environment changes somewhat more rapidly than discussed above (e.g. every year or few years) and how this seems to lead to the existence of multiple personality types in a population. Variation in Personality as a Non-adaptive Side Effect Above we discussed that variation in offspring might be beneficial in an uncertain world but it is possible that heritable variation in personality is merely a side effect of some other factors that are beneficial. Many genes, as we have discussed, are pleiotropic, meaning that they have a variety of phenotypic effects (see Chapters 2 and 12). For example, there is some evidence that it pays to have offspring whose immune systems vary from one another to prevent the parents putting all their ge- netic eggs in one basket. It is possible that other sources of variation give rise, as an epiphenomenon, to differences in personality. If the environment becomes more predictable there will be a strong selection pressure on the most beneficial of these characteristics leading to them becoming the popu- lation norm. Although this argument might sound somewhat tortuous, there is some evidence to sup- port the general point, if not the specifics. For example, we know that on average women live longer than men and, due to this sex difference, we might construct an argument that this is an adaptation that has affected only females. However, we know that a shorter life in males is partly due to the effect of testosterone that leads to men developing broader chests and greater upper-body strength in early adulthood. Thus, there may be no benefit for females living longer – this is simply a side effect of the fact that they have less testosterone than men. Putting it another way, the genes that control testosterone in men also shorten their lives. Hence these genes are pleiotropic; in this case, because the same genes have beneficial effects early in life and negative effects later on in life, a situation which is known as antagonistic pleiotropy. We can clearly see that the genes have been sexually selected in the ancestral past in order both to impress women and to compete with other men. Thus such genes are kept in the population despite their negative non-adaptive (antagonistic) side effect of causing men to die younger (Nesse and Dawkins, 2010; Nesse, 2019; see also Chapter 12). Adaptive Variation due to the Existence of Different Ecological Niches and Frequency Dependency As we discussed above, one reason for individual differences might be that there is no one universally optimal human nature, just as there is no one universally optimal design of car. In evolutionary personality theory this might mean that having a different personality from other members of the group may even be beneficial because there is the possibility that you can occupy a behavioural niche with less competition from others of similar personality. Across species such niche fitting is commonplace, but it might also occur within species, in particular species with a high degree of sociality such as humans. Put simply, there is more than one way to win at the game of life where winning means, in evolutionary terms, leaving behind surviving offspring (and other kin). Therefore people high in psychopathy might profit from their ability to manipu- late others, whereas people low in psychopathy might profit from their ability to gain the trust of others and hence engage with them in reciprocal exchange (Furnham and Kanazawa, 2020). In extreme cases this may help to explain why psychopaths remain in the population (see Chapter 12 and below). 364 Evolution and Individual Differences Furthermore, the successfulness of a ‘personality type’ will, up to a point, be dependent on the strategies adopted by other members of the population. Driving to work early in order to miss the rush is only an effective strategy so long as few people do it. If large numbers of people adopt the same strategy then it becomes increasingly less effective. When the effectiveness of an inherited strategy depends upon the number of individuals adopting the same strategy it can lead to frequency-dependent selection. For example, Linda Mea- ley’s claims that psychopathy might be a frequency-dependent behavioural strategy (Mealey, 1995). Psychopathy is characterised by manipulativeness, sensation seeking, and a lack of consideration for the feelings of other people (see Chapter 12). These characteristics can often lead the psychopath into a life of crime and thus it is usually considered to be a pathological condition. Mealey argues that there are two sub-groups of psychopathy: primary psychopathy and secondary psychopathy. Primary psychopaths, she suggests are born not made, whereas secondary psychopathy is produced by an interaction between a genetic predisposition and specific environmental factors (we shall discuss secondary psychopathy later, when we discuss the non-heritable component of personality). Focusing, for the moment, on primary psychopaths, Mealey argues the psychopathic per- sonality is, in fact, an adaptive strategy: psychopaths might use their manipulative tendencies in order to exploit others, gaining resources including sex (recall from Chapter 12 that the majority of psychopaths are male). If all of this seems rather far-fetched, the pre-eminent researcher in psychop- athy Robert Hare (see Hare, 1980; 1993; 2006) has argued that many of the character traits exhibited by psychopaths (manipulativeness, charm, tough-mindedness) are just the sort of traits that lead to success in high-level business executives. The crucial point, however, is that the effectiveness of the psychopathic strategy depends on how many psychopaths there are. So long as there are compara- tively few psychopaths their victims will tend to be trusting and hence easily exploited (you might recall from Chapter 12 that the frequency of psychopaths in the population is estimated to be be- tween 1 and 3 per cent). If, however, the number of psychopaths increases due to it being an effective strategy, potential victims will become more wary and hence less easily exploited (imagine if 20 per cent of the population were psychopathic, then we would meet them almost every day and be in a constant state of vigilance). As a result of this psychopathy becomes a victim of its own success and psychopaths leave behind fewer offspring leading to a reduction in numbers over the generations. At some point psychopaths will become sufficiently rare for the strategy to become effective again and numbers will rise. We can see, therefore, that frequency-dependent selection keeps the number of psychopaths relatively small. Leading on from this explanation for psychopathy, it is entirely possible that other extremes of personality are maintained in the population through this frequen- cy-dependent selection including people who are overly conscientious and agreeable. This means that frequency-dependent selection might help to explain extremes of variation, but only where numbers of those with such personality profiles are relatively small (Wolf and McNamara, 2012). Before discussing explanations for the non-heritable component of personality, we first look at research that attempts to identify the genetic basis for individual differences in some of the personality factors described above. Differential Susceptibility to Rearing Influence We discussed Belsky’s recent theory of child development in Chapter 6 (Belsky, 2005). To recap, Belsky’s argument is that children might be genetically ‘programmed’ to respond differently to parental influence. Some children are strongly influenced by parental input (including, perhaps, their parent’s personality), whereas others are less responsive. Belsky’s argument is that this state of Explaining the Heritable Component of Individual Difference 365 Figure 13.2 Some researchers have argued that psychopathic tendencies might be adaptive in the business world. affairs might have been adaptive if the environment were unpredictable (periods of comparatively little change punctuated by periods of rapid change). Those children that learn from their parents might be at an advantage during periods of stasis, whereas children who are less influenced by parents might be at an advantage during periods of rapid change when creativity is at a premium (it’s worth mentioning at this point that Judith Harris has also argued that children may differ in the extent to which they respond to parental influence; see Harris, 2006). Variation as a Result of Cost–Benefit Trade-offs A final explanation for individual differences in personality derives from the fact that for any one environment there are multiple strategies for ensuring that genes survive into future generations. One strategic choice is discussed later in this chapter and in previous chapters (Chapters 3 and 6): the C–F (or related r–K) continuum. Recall that, according to life history theory, the C–F continuum describes whether an individual opts to maximise its current fitness (having a lot of children but in- vesting comparatively little in them) or future fitness (having few children but investing a great deal in them). As we saw in Chapter 6, this is also known as a ‘slow’ to ‘fast’ continuum (breed later or focus on this now). Recall, also, that we discussed how the effectiveness of choosing current fitness (C or fast) over future fitness (F or slow) is dependent on how much environmental risk is present (Chisholm, 1996; Belsky, 1997, see later). However there may also be alternative strategies that are equally workable within a single environment. Nettle (2005) discusses the case of extraversion. As we saw above, extraverts tend to be more socially outgoing and more prone to risk taking compared to introverts. Nettle argues that such a difference could have consequences for fitness, with extra- verts tending to maximise current reproductive success and introverts tending to maximise future 366 Evolution and Individual Differences reproductive success. Consistent with these predictions it was found that in a study of 545 adults extraversion was positively correlated with the number of sexual partners throughout the lifetime, and the number of children borne by other partners and the number of extramarital relationships. On the negative side, extraversion was also positively correlated with the number of hospitalisa- tions, and extraverts were more likely to have visited the doctor four or more times in the past two years. Nettle therefore argues that extraverts are acting to maximise the number of children they have at the expense of their ability to care for their children as a result of premature death or serious disability. Thus, it is claimed that although extraversion has its benefits in terms of fitness – more sex with more people so presumably more copies of genes in the next generation, this is offset by an increased likelihood that parents may die or become disabled due to their risky behaviour, and therefore less likely to be able to care for their children and foster these genes into the next genera- tion. Extraverts are also more prone to infidelity and relationship breakup. Swings and roundabouts. These findings (and their interpretation) are not without their potential pitfalls. First, al- though some of the predictions were supported by the data, there was no evidence that extraverts had more children, or that they were more likely to die as a result of their own actions. There was also no evidence that extraverts spent less time caring for their children than introverts (although, given they do have more extra-marital affairs, this is of course possible). On a different line, even if future data more unambiguously support this hypothesis, it does not entirely explain why such individual differences arose. Two plausible explanations are either that this is due to some version of the ‘noise’ explanation given above, or that such personality profiles are conditional adaptations. You might recall from Chapter 6 that conditional adaptations consist of various potential outcomes which are dependent on the particular circumstances occurring during development (Myers and Bjorklund, 2020). It might, for example, be advantageous to grow up to be more introverted in a family where extravert behaviour leads to poor outcomes (and vice versa for extraverts). This expla- nation is currently speculative, but if supported, would provide a good example of how genes and environment interact. The Search for the Genetic Underpinnings of Personality If individual differences are, in part, the product of evolution then we must as a species have a genet- ic foundation for these differences. As we saw in Chapters 2 and 6, behavioural geneticists generally attempt to understand the genetic contribution to differences in personality. Up until around 30 years ago their methods were indirect, such as comparing twins and other relatives on personality scales. Since the mid 1990s, however, behavioural geneticists have begun to identify specific genes implicated in scores on personality traits. Then, well into the current century, a further step forward was made as geneticists began to develop tools to explore the entire genome. We will examine each of these developments in turn. Where a single gene (allele) is implicated in having a causal role in differences in person- ality (or intelligence) between people it is known as a candidate gene (note – some experts restrict the term ‘candidate’ to cases where the gene is implicated in disease or mental health issues). Hunting for Genes There are two ways in which a candidate gene can be identified – through linkage or association. Linkage analysis involves studying large portions of the genome by making use of common varia- tions in the DNA and combining this with knowledge of family genealogies. Linkage refers to the The Search for the Genetic Underpinnings of Personality 367 Figure 13.3 All humans share 99 per cent of their genes. This variability in 1 per cent sounds tiny, but, given that there are 3 billion ‘rungs’ (base pairs) on the human DNA ‘ladder’, this means that we can differ by as many as 30 million base pairs. Clearly that 1 per cent plays a massive role in the individual differences we perceive. tendency for genetic markers that are close to each other on a chromosome to be inherited together. A genetic marker is a DNA segment (genes and other genetic material) that is in a known physical location. It is broader than a single gene and may or may not have a known function. The point is that specific genes that may be involved in facets of personality (such as sensation seeking) or disorders (such as schizophrenia) are passed on to offspring along with the known genetic markers due to their proximity to the markers. In linkage studies geneticists examine large families, some of which will have the trait and some won’t. They then determine the genotype of each person and in particular the genetic markers (maybe 300). Next they establish which marker alleles are shared by members of the family with the trait but not by those without the trait. In this way they can then determine whether an individual has a specific candidate gene. Association studies, rather than focusing on members of the same family, make use of two samples from the population – one sample that has the trait and one that does not. Individuals from the two groups are then genotyped to establish differences in allele frequencies between the groups. In this way association studies are able to localise a genomic region (and possibly a specific gene) involved in differences between people for a particular trait. So far linkage and association studies have largely been confined to genetic disorders since they are expensive and time consuming. But they have also been used for facets of personality such as novelty-seeking and proneness to anxiety. Linkage and association studies have led to the identification of a number of candidate genes some of which are implicated in differences between people in personality traits. During the early years of these new technological developments a number of candidate genes were identified that were considered to be involved in differences in personality between people by producing var- iations in the receptor sites for neurotransmitters or by affecting the release or re-uptake of neuro- transmitters. People appear to be polymorphic for the genes that code for these structures (i.e. there are a number of different genes that can potentially occur in the same position on the chromosome; see Chapter 2). Two genes in particular were implicated in personality differences among people. 368 Evolution and Individual Differences D4DR AND THE DOPAMINERGIC SYSTEM. On the long arm of chromosome 11 there is a gene that has been labelled D4DR. This gene codes for the production of a protein that protrudes from the membrane of various neurones in the brain and receives the neurotransmitter molecule dopamine. This protein is called the D4 receptor (i.e. it was the fourth type of dopamine receptor to be discov- ered). This dopamine receptor picks up dopamine molecules and, should sufficient numbers of these be trapped by these receptors, then the neurone will discharge an electrical signal (i.e. it will ‘fire’). Of course, individual neurones work in combination with others to form bundles or pathways. Do- pamine pathways are very much involved in motivation and arousal. It is now known that the D4DR gene varies between individuals in its length and that this length affects how well the dopamine receptor it codes for works. The longer the D4DR portion of the chromosome the less responsive to dopamine an individual’s neurones are. People who inherit a longer D4DR gene do not appear to feel the effects of dopamine as much as those with a shorter D4DR. When assessed on a sensation-seeking scale, the longer the gene the greater the craving for new and exciting experiences (Ebstein et al., 1996; Eichhammer et al., 2005). Putting it crudely, it is suggested that those individuals with a long D4DR gene, being unable to feel the full effects of their dopamine pathways, are the sensation seekers – constantly trying to turn up their levels of this arousal-enhancing chemical. This means that a single gene that naturally occurs in different forms appears to have a profound effect on personality, making a person more likely to act in an extravert manner. 5-HTT AND SEROTONIN ACTIVITY. A second gene that has been implicated in differences between people in personality factors codes for a protein that affects serotonin activity. The 5-HTT gene ex- ists in two forms – known as the ‘long’ and ‘short’ forms (L and S alleles). Which form you inherit determines how well serotonin is transported (i.e. how well serotonin that has been released into the synaptic gap is then transported back into the nerve cell to be reused). This, in turn, is believed to affect how shy a person is. The gene is on chromosome 17 and a portion of it is known as the ‘serotonin transporter promoter regulatory region’ or 5-HTTLPR (5-HT is the abbreviation that is traditionally used for serotonin). Some researchers have claimed that alleles of this gene may be involved in a number of disorders such as obsessive–compulsive disorder, bulimia, alcoholism and even autism. A number of studies suggested that the long form (or the heterozygous form SL) is re- lated to shyness, anxiety and even eating disorders (Benjamin et al., 2002; Monteleone et al., 2006). Such studies might suggest that there is a relationship between the combination of the alleles you inherit and your level of shyness. Limitations of Single Gene Studies Such findings sound like a breakthrough for areas such as evolutionary psychology that rely in some sense on a genetic foundation for behaviour and internal states. In recent years, however, the picture of single gene effects has become less positive. First, each time a study is published that reports a significant relationship between a candidate gene and a personality score, another study appears to follow that disputes this finding (Flint, Greenspan and Kendler, 2020). Second, a number of me- ta-analyses (reviews that combine the findings from a large number of previous studies to reanalyse a large body of data) suggest that these single gene effects either do not stand up to scrutiny or that the amount of variation they account for between people is really very small (Munafò et al., 2008; Mathieson et al., 2012; Webb et al., 2012). Why might this be the case? One reason is that each of The Search for the Genetic Underpinnings of Personality 369 these single gene effects is likely to interact with other single gene effects. It is known for exam- ple that dopamine and serotonin interact in complex ways, with the former being regulated by the latter (Toates, 2011). Hence variations of the 5-HTT and D4DR genes are likely to interact in their phenotypic effects on personality. These two genes are also likely to interact with other genes – so disentangling the effects of the various genes involved in personality is going to be a very com- plex business. We also have to factor in the fact that many aspects of individual differences such as personality and intelligence are likely to be highly polygenic (i.e. they depend on many genes, potentially thousands; Heck et al., 2009; Plomin, 2018). Finally, having these genes may predispose people to a type of personality profile but that the predisposition may not be that strong. Returning to the sources of variation outlined in Table 13.1, in a sense the activity of these genes may be context dependent – that is, dependent on the early environment (see earlier). A particular early environment might lessen the effects of these alleles or even reverse them. It is also worth mentioning at this point that the twin studies suggest that even for anxiety-related personality traits heritability is between 40 and 60 per cent. And this is one of the highest heritability findings in the field of behaviour genetics. In contrast, the degree of heritability of other personality dispositions, such as ‘agreeableness’ and ‘extraversion’, is a little lower – around 30–50 per cent (see Plomin, 2018). Finally, when considering the effect of single genes on personality, it is important to recall, as discussed earlier, that many genes are pleiotropic – that is, they code for more than one pheno- typic effect. Hence single genes that affect personality might have originally been selected for other phenotypic effects (see also Chapter 2). Examining the Whole Genome – Genome-Wide Association Studies The various limitations and equivocal findings from single gene studies have led, understandably, to questions concerning their reliability. This is particularly true of linkage studies of candidate genes. Although there are some rare cases where a single gene can have a large effect (Huntingdon’s dis- ease is a case in point), over the last 20 years it has become increasingly apparent that personality traits and intelligence are affected by many genes. By 2006, rather than a field of dreams, behaviour- al genetics appeared to have developed into a field of disappointment. Beginning in 2007, however, the tide began to turn as, due to technological advances, behavioural geneticists were for the first time able to examine the entire genome (Flint, Greenspan and Kendler, 2020). By this point it was well established that association studies were statistically more reliable than linkage studies (i.e. they were more likely to be replicated). New techniques began to emerge which, in effect, allowed expanded association studies to consider the entire human genome (Plomin, 2018; Flint, Greenspan and Kendler, 2020). These genome-wide association studies (GWAS) made use of huge samples and proved to have a high level of reliability. GWAS are based on identifying variations between people in their individual nucleotides (A, T, G or C, which as pairs, form the ‘rungs’ of the DNA ‘ladder’; see Chapter 2) which are known as single nucleotide polymorphisms (SNPs) and deter- mining whether each of these is associated with a trait. Note that polymorphism, when used here, means that one of a number of nucleotides can occupy a given locus (position) on a chromosome. (Confusingly the term is also used to describe varieties of form within a species such as brown or yellow domestic chicks). By searching the entire genomes of a large sample, the identification of these SNPs allows geneticists to determine which alleles lead to differences in traits between people and hence which genes are associated with variation in specific traits. In a groundbreaking study a team from the Wellcome Trust used a GWAS to demonstrate how a large number of identified genes were involved in the development of seven different major 370 Evolution and Individual Differences disorders (including Type 2 diabetes and bipolar disorder). While most studies since then have, understandably, been concerned with disease-associated alleles (affecting both physical and mental health), in recent years behavioural geneticists such as Robert Plomin have made use of GWAS to study aspects of personality and intelligence. Plomin and von Stumm (2018), when considering a sample size of more than 1 million have, for example, reported a large number of alleles which account for 10 per cent of the variance in intelligence. Although this is a big step forward, given that twin studies have demonstrated around 50 per cent of the variance in intelligence is attributed to differences in genes between individuals, it does, however, mean that 40 per cent of the genetic variance is yet to be discovered. But it’s a start and behavioural geneticists are now growing in confidence that the, as yet missing, genetic inheritance will be uncovered (Plomin, 2018). Other GWAS have recently found a vast number of genes are involved in aspects of personality such as extraversion and neuroticism. In contrast to the aims of the single gene studies of the late 1990s and early 2000s, it is becoming increasingly clear that differences in the big five between people are due, not to a small number of genes having a large effect, but to a very large number, each of which has a tiny effect (Sanchez-Roige et al., 2018; Plomin, 2018; Flint, Greenspan and Kendler, 2020). By ‘a very large number’ we are probably talking about tens of thousands. The take home message from GWAS is that, when it comes to personality, all traits are highly polygenic; that is, many genes contribute to their development. Explaining the Non-heritable Component of Individual Difference Non-adaptive Differences due to Social Learning B. F. Skinner’s behaviourism has already been examined in some detail when we discussed language in Chapter 10. His claims for language apply equally to personality. Personality, in Skinner’s view, is just a set of behaviours that are learned by the various processes of reward and punishment: children who are rewarded for their gregariousness and punished for being withdrawn become extraverts, those for whom the opposite applies become introverts. This idea was modified somewhat by Albert Bandura who proposed that children could learn their personalities by observing other people, via a process of social learning. The heritability estimates shown above contradict the strongest version of social learning theory, but the substantial non-heritable component to personality suggests that the environment is likely to play an important role. Non-adaptive Variation due to Chance The brain is a hugely complex organ containing approximately 100,000 million neurons, each of which interfaces with many other neurones. Such is the interconnectivity of the brain, that it has been estimated that the mature adult brain has 150 million million synaptic connections (Pakken- berg and Gundersen, 1997). It is therefore almost impossible that such a complex wiring diagram could be specified by a genome that contains, at current estimates, around 20,300 genes (see Chap- ter 2), approximately one-third of which are expressed primarily in the brain (the highest proportion of genes expressed in any part of the body; National Institutes of Health, 2010). It is therefore not surprising that although monozygotic twins are genetically identical, their brains appear to be different from birth, even at the fairly gross level of the size of particular brain regions. Given these facts it seems reasonable to assume that the behavioural traits of monozygotic Explaining the Non-heritable Component of Individual Difference 371 twins should be different, up to a point, which is indeed what we find. The correlations between pairs of identical twins on personality measures are usually between 0.6 and 0.7 (which leads to the variances of 0.3–0.5 presented above, variance being the square of the correlation coefficient); the correlations for intelligence usually being slightly higher (around 0.75–0.76; Plomin, 2018; see Box 13.4). Steven Pinker (2002) speculates that these differences might be due to blind chance: One twin lies one way in the womb and stakes out her share of the placenta, the other has to squeeze around her. A cosmic ray mutates a stretch of DNA, a neurotransmitter zigs in- stead of zags, the growth cone of an axon goes left instead of right, and one identical twin’s brain might gel into a slightly different configuration than the other’s. (396) Because the genes cannot specify the wiring of the brain in its entirety (nor the rest of the body for that matter), some of the brain’s structure is left to the whims of the environment. In support of this, Pinker cites evidence that a range of organisms such as mice, fruit flies and roundworms raised in carefully controlled laboratory conditions still show variations in a number of areas between genetically identical individuals. For example, differences in the number of bristles in the case of fruit flies, or life expectancy in the case of roundworms (Austad, 2000). This account for the non- heritable component is therefore non-adaptive – variation arises simply as a result of the impossi- bility in specifying an entire organism (including, most crucially for the discussion here, the brain) with only a limited amount of information provided by the genes. Adaptive Variation due to Early Environmental Calibration or ‘Weather Forecasting’ In addition to explaining the heritable component of traits, changing environments have also been used to explain variations in personality that are not, on the face of it, the result of genes. According to the theory of early environmental calibration, a capricious environment means that natural selec- tion cannot specify entirely a successful phenotype. One reason, therefore, why personality might only be partly genetically specified is that in an uncertain world there is a risk of producing subop- timal phenotypes that offer a poor fit to the environment. Therefore, rather than specifying a com- plete personality, they specify psychological mechanisms that are capable of tuning or calibrating a personality that better fits the environment in which an individual develops as one might calibrate a scientific instrument, or set a watch, by attending to local conditions (some others also refer to this phenomenon as demonstrating that traits are ‘facultative’). An example will help to illustrate this principle. Certain species of grasshopper have the handy knack of blending in with their surroundings (Rowell and Cannis, 1972; see also Bateson and Martin, 1999). A juvenile grasshopper that is placed into a dark coloured environment will, when it next sheds its skin, itself become dark. (Grasshoppers, like all organisms with an exoskeleton, have to shed it in order to grow, with growth occurring in the period immediately following the moult while the new skin is still soft.) If one then places the now-dark grasshopper into a light-coloured environment then following the next moult (technically: ecdysis), it develops a light colouration (see Figure 13.4). This variation is apparently completely due to the environment and has seemingly nothing to do with the genes, as two genetically identical grasshoppers reared in different coloured environments would turn out to be different colours as above. In behavioural genetics terms, genes therefore account for none of the variation, and the environment 100 per cent of it. Does this there- fore mean that the environment ‘causes’ the colour change? It depends on what is meant by ‘cause’. 372 Evolution and Individual Differences Figure 13.4 Bright green grasshopper matching back- ground. Forest fires are, it turns out, a commonplace in the environment inhabited by these particular insects, blackening the previously light-coloured landscape. Against a backdrop of charred plants a pale coloured grasshopper would be easy pickings for any predators. Therefore any mutation that enabled the simple colour switch would be favoured by natural selection, which is what seems to have happened. So the environment is really only a trigger for a set of genetically specified processes that lead to the switch. The key environmental variable being the amount of light reflected from the ground, its intensity being positively related to the lightness of colour of the grasshopper. This ability of organisms to change their characteristics according to environmental signals is technically known as phenotype switching, because the or- ganism can literally change its physical characteristics on the basis of environmental information. (Note this can also be termed a ‘conditional adaptation’ as discussed in Chapter 6; Myers and Bjorklund, 2020). To return to humans, it has been claimed that some of the variation among individuals is due to a similar mechanism. As we saw above, if the environment is predictably unpredictable (meaning that there is a fixed set of environments but it is not clear from the outset which you will be born into) then, as with grasshoppers, humans might be born with mechanisms designed to sam- ple key features of the environment which lead to a set of processes that affect the direction of the developmental trajectory. We have already discussed two versions of this theory at length in Chapter 6. As we saw, Belsky (1997) and Chisholm (1996) both argued that early experiences might lead to different reproductive strategies. Parental cues that signal high-investment childrearing practices might lead to children themselves starting on a pathway of high-investment parenting (for example, higher age of first sexual encounter, fewer sexual partners, stable relationships and fewer offspring). Conversely, cues that signal that the child’s parents are low investors might lead them to develop along a low-investment pathway (such as lower age of first sexual encounter, more sexual partners). There is evidence that parental behaviour can affect children’s development (see, again, Chapter 6), most of which has been conducted under the strictures of ‘attachment theory’. The modifications by Belsky and Chisholm suggest that parental cues in childhood might signal what is likely to happen in the future (Are parents likely to still be around when I’m a teenager? Will they still be caring Explaining the Non-heritable Component of Individual Difference 373 for me?) and evolved mechanisms possessed by the children use these cues as a ‘weather forecast’ (Bateson and Martin, 1999; Myers and Bjorklund, 2020) and act accordingly, changing personality and other factors. Secondary Psychopathy as a Gene–Environment Interaction In addition to primary psychopaths, discussed above, whose cheating behaviour is specified by their genes, Mealey has proposed that so-called secondary psychopaths inherit a predisposition to adopt the cheating strategy – whether they become psychopaths depends upon the environment in which they grow up. Individuals with this predisposition might need certain environmental triggers such as growing up in an environment that has few opportunities for gaining resources by cooperating, a dense population (so that there are many opportunities for cheating) and the potential for com- paratively anonymous interactions (so that your cheating is less likely to be found out). In a sense, unlike the primary psychopaths (see above) that are maintained in the population due to frequen- cy-dependent selection, secondary psychopaths are more flexible and their behavioural repertoire is ‘calibrated’ to their early environment. Once again, secondary psychopathy can be thought of as an example of a conditional adaptation. The notion of early environmental calibration is a relatively new area and also an excit- ing one. Exciting because it suggests that some of this 50–70 per cent environmental variation in personality discussed by behavioural geneticists might be at least partly the result of evolved, envi- ronmentally contingent cognitive and emotional mechanisms that sample the world and affect the phenotype based upon a best-guess prediction of the future. Incidentally, this perspective provides further artillery against the view that evolutionary thinking is necessarily determinist (see Chapter 1). In this case the environment makes an important difference but, at least in some cases, it doesn’t pound us into shape – instead development can be seen as a process where a phenotype is selected from a battery of alternatives (Myers and Bjorklund, 2020). Adaptive Niche Filling Two otherwise quite different theories claim that some of the non-heritable variation in personality might be the result of a person occupying a particular niche as it becomes available. We discussed Harris’ group socialisation theory in Chapter 6. Part of this theory suggests that children’s person- alities might be affected by the availability of particular niches in their group. For example, a group might have space for only one leader, so if that position is already occupied a child will either com- pete for leadership with the present incumbent or occupy a different ‘ecological niche’ (e.g. as a jok- er, a pragmatist or some other role). One researcher who has been influenced by evolutionary theory and has used the notion of ‘fulfilling different ecological niches’ is Berkeley academic Frank Sullo- way. Sulloway has developed a radical theory to account for some of the differences in personality that occur between siblings. Sulloway’s theory is rather different from Harris’, as it is squarely based within the family. Frequently firstborn children will act as surrogate parents, helping to bring up the younger siblings and, Sulloway argues, to help them achieve this end they are equipped with an appropriate personality (genes that help other copies of themselves, recall, will tend to propagate). Firstborns therefore usually adopt what Sulloway terms the ‘responsible achiever’ strategy, being dominant, conscientious and somewhat conservative in their approach to life. Laterborns, on the other hand, are born into an environment where there is at least one older child than them, and that older sibling will usually be physically stronger and more experienced in gaining parental attention. 374 Evolution and Individual Differences Rather than attempting to compete directly using the responsible achiever strategy – which is un- likely to succeed given their older sibling’s advantages – they tend towards a rather different strategy. Laterborns, according to Sulloway, win parental attention by being more sociable and more creative and less respectful of the status quo (Sulloway, 1996; 2011; 2020, see Box 13.3). Evaluation of Evolution and Personality Above we have presented an evolutionary explanation for the variation in personality that accounts for both the heritable and non-heritable components. Identical twins are similar in personality whether they are reared together or apart. This alone suggests that genes are involved in the dif- ferences between people in personality. That personality is somewhat heritable suggests that there might be inclusive fitness advantages for having a population that has some degree of diversity. It is easy to get drawn into thinking that this might be of benefit to the group as a whole, but such a group selectionist point of view is unnecessary. A soccer team has a number of different roles (strik- er, defender, goalkeeper and so on) and of course it benefits the team that there is this variety: think of a soccer team made entirely of goalkeepers. But this variation also benefits the individuals in it. Each individual in that team benefits from winning matches in terms of status, financial reward and so forth, so it is of benefit to a striker that there is a goalkeeper who is in turn helped by the presence of defenders. In this way it is not necessary to resort to group selectionism to explain the benefit of variety, it also makes sense at the individual (or gene) level of selection. Box 13.3 Birth Order and Personality Frank Sulloway is a historian of science but is perhaps most widely known for his 1996 book Born to Rebel in which he argued that birth order has a significant effect on personality. As a his- torian it is perhaps natural that he looked to the past to find some of his most interesting data. He noticed that many of the most iconoclastic and creative people in history tended to be laterborns, including Lenin, Thomas Jefferson, Fidel Castro, Charles Darwin, Bill Gates and Rousseau. In fact of 6556 ‘revolutionary’ historical figures, more were laterborns than would be expected by chance. He also conducted a meta-analysis of 196 personality studies of which 72 supported his predictions, 14 contradicted them and 110 showed no effect either way (Sulloway, 1995). PER CENT SUPPORT Figure 13.5 Personality order and 100% personality receptivity to evolutionary Laterborns theory by year and birth order. 80 Darwin’s Origin of Species (1859) 60 SUPPORT OPPOSITION 40 Firstborns 20 0 1700 1725 1750 1775 1800 1825 1850 1875 1900 YEAR Explaining the Non-heritable Component of Individual Difference 375 Box 13.3 (cont.) The reception of evolutionary theory from 1700 to 1875 by birth order (N = 433). During the long period of debate preceding publication of Darwin’s Origin of Species (1859), individual laterborns were 9.7 times more likely than individual firstborns to endorse evolution. These group differences are corrected for the greater frequency of laterborns in the population. The likelihood of these birth order differences arising by chance is less than one in a billion. Note that firstborn conversions to Darwinism peaked soon after publication of the Origin. Those firstborns capable of being converted to Darwin’s theories were converted quickly, leaving fewer open-minded firstborns to be converted later on. Sulloway (1999; 2001) conducted a study in which he asked participants to rate their own personalities and that of another sibling using questions related to the Big Five personality fac- tors. Firstborns rated themselves and a younger sibling; laterborns themselves and an older sib- ling. The correlation of each of these scores with birth order is shown below. All five of the correlations between trait variation and birth order are in the predicted direction and statistically significant. Firstborns are usually more conscientious and neurotic (less stable) but less agreea- ble, less open to experience, less extravert (in the sense of gregarious). See Table 13.3. You might have noticed that although significant, the correlations are rather small. Corre- lations, recall, vary between –1 and +1 – a correlation of 0.8 (or –0.8) is usually considered to be strong, whereas 0.3 is weak. Some of the correlations presented above are very weak indeed. Thus although the correlations are significant (unlikely to be a fluke effect), birth order, at least in this study, accounts for very little of the variation in personality. You can find out what percentage of the variance of one variable is explained by another by squaring the correlation and multiply- ing by 100. A correlation of 0.1, as above, means that 1 per cent of the variance in agreeableness is explained by birth order. Sulloway addresses this point (see Sulloway, 2001) first by arguing that using variance is misguided and might underestimate the importance of a factor such as birth Table 13.3 Partial correlations of the Big Five personality factors with birth order Personality factor (and predicted effect of birth order) Partial correlation Significance with birth order level, p < CONSCIENTIOUSNESS Firstborns are more deliberate, −0.18 0.001 dutiful, effective, energetic, hardworking, organised, self- disciplined and under control AGREEABLENESS Laterborns are more acquiescent, 0.10 0.001 cooperative, easy-going, modest, straightforward, unassertive/submissive, tender-minded and trusting OPENNESS TO EXPERIENCE Laterborns are more 0.08 0.001 aesthetically inclined, prone to fantasy, attentive to inner feelings, untraditional, attracted by novelty and drawn to ideas EXTRAVERSION Laterborns are more affectionate, 0.14 0.001 excitement seeking, fun loving and gregarious NEUROTICISM Firstborns are more anxious as well −0.04 0.01 as more prone to depression. Laterborns are more self- conscious. Source: Adapted from Sulloway (2001). 376 Evolution and Individual Differences Box 13.3 (cont.) order. While he may have a point, most psychologists would still consider the correlations above to be rather small. Second, he suggests that many factors (e.g. sex, age, social class) can affect personality and although small (in conventional statistical terms) birth order is still important. There might be something in his argument, as a fitness advantage of even 1 per cent might be enough for one genotype to win out over its competitors in the long run. Some have criticised Sulloway’s interpretation of the data, perhaps most prominently Judith Harris who, among other things, takes issue with his low effect sizes. Others have argued that his theory is incompatible with evolutionary theory itself. For example, Steven Pinker (2002) suggests that strategies that work within the family are unlikely to work outside the family: your parents will usually give you resources (including love) no matter how poorly behaved you are, but non-kin are unlikely to be so kind. Evolution should have provided us with strategies for dealing with kin, and strategies for dealing with non-kin. In response Sulloway has pointed out that, throughout the vertebrate species, from fish to mammals, we observe sibling competition with older siblings having an advantage. This, he suggests, is likely to have a noticeable effect on later behaviour and is compatible with evolutionary theory (Sulloway, 2020). The observation that personality is not 100 per cent heritable (identical twins do not have the same personalities) has an adaptive explanation too. Perhaps variation in the environment and the availability of different niches leads the individuals to adopt a partial ‘wait and see’ strategy by constructing the best behavioural phenotype it can based on the environment (early environmental calibration and niche fitting). Whether this is an evolved strategy, as some of the life history re- searchers have claimed (including Belsky, Chisholm, Mealey; Myers and Bjorklund, 2020), or is simply the result of the inability of the genome to specify each and every synaptic connection in the central nervous system, is an empirical question. The variability we see in personality between people may have led some to consider that it is free of evolutionary processes. Evolutionists, however, have argued that this malleability or developmental plasticity is an adaptation in itself, which allows us to alter our strategies in order to exploit the circumstances in which we grow up. In addition to personality factors people also vary in how clever they appear to be. If evolutionary psychology is to make a contribution to individual differences, then it will also have to help explain this variation. Do Non-human Animals Differ in Personality? Recent research has demonstrated that it is not only humans that have individual differences. In one sense, this has been known for centuries. Any pet owner will tell you how no two dogs are the same or that their cats are like chalk and cheese. What is different is that the recent evidence shows that some differences are, as for humans, heritable. For example, O’Steen et al. (2002), have shown that guppies that live in fast-flowing water are bolder than those living in calmer environments, the reason seemingly being that fast-flowing water is less likely to contain predators such as pike. In the absence of such predators it pays guppies to be bold as this opens up more opportunities for obtain- ing food and for mating, the presence of pike mean that excess boldness can be a disadvantage as it increases the chances of predation. These are not learned behaviours. If offspring of guppies from Explaining the Non-heritable Component of Individual Difference 377 populations from fast-flowing waters are placed in pike-infested territory, they show the same bold- ness, and catastrophic insouciance when they eventually meet their nemesis. Put simply, timidity is an anti-predator adaptation. Similar findings were obtained in research on the great tit (Parus major) with an interesting sex-related twist. It has been found that great tits vary in their degree of what is called explora- tion; high scorers are bold, inquisitive and show high aggression, low scorers being more reserved (Dingemanse et al., 2002). As with guppies this trait is heritable (heritability estimates range from 0.3 to 0.6). Again as with guppies, it seems that there are fitness consequences of this trait. For females, being high in exploration benefits them when food supplies are scarce as such individuals are more likely to search and find food supplies. When food is plentiful, however, the extra risk of predation of moving farther afield and reduced inter-species conflict tends to favour those low in exploration. For males the opposite is true. When food supplies are poor, mortality through star- vation means that there are comparatively few other males to compete with so males scoring low in exploration tend to do well, plentiful food means plentiful males and increased competition for mates and resources which favours aggressive males high in exploration (Dingemanse et al., 2004). There are many other examples of such heritable individual differences in non-human ani- mals (see Nettle, 2006) but the question is, are these the same kind of thing that we see in humans? Should we even refer to such consistent differences as personality? There are a number of objec- tions, many of which are familiar to researchers on human personality (see Katsnelson, 2010). Are these behaviours consistent over time, or just ‘random noise’ (see Box 13.2 on the person–situation debate); are these traits the actual cause of enhanced or decreased fitness or are they merely correlat- ed with some other behaviour that is causally related? For example, aggressive male great tits might be larger, and it may be their increased size not aggression that leads to success in competition. All of these questions are scientifically answerable by the usual means of removing confounding variables and using controlled experimentation. Others, such as Giraldeau (cited in Katsnelson, 2010) suggest that researchers into non-human animal personality have not produced a convincing explanation for how such differences might have evolved. One way of gathering evidence to support or refute the conception of animals having variation in personalities is to determine whether such differences are related to similar underlying proximate mechanisms that have been uncovered for humans. Interestingly, there is now clear evidence that in great tits there are similar patterns of re- sponse which are related to differences in the D4DR gene (as discussed above) between individuals. In this sense ‘similar patterns of response’ means how bold and sensation seeking an individual is in both species. Timm et al. (2015) found evidence that how bold a particular great tit was depended on which form of the D4DR gene the bird possessed. The fact that similar variation in underlying genes appears to be related to broadly similar variation in patterns of response adds weight to the notion of personality existing in other species. In addition to various scientific objections, however, there are those who are queasy about use of the word ‘personality’ when applied to non-human animals (see, again, Katsnelson, 2010). To be sure, personality has all sorts of non-technical connotations relating to a person’s character such as when we say that someone has a ‘larger-than-life personality’ or ‘no discernible personal- ity’. However, when used technically in the way that personality theorists use it, it is hard to see what – beyond anthropocentric chauvinism – people are objecting to. If by personality we mean relatively stable differences in measurable traits that affect behaviour (which is what we mean when we talk about humans) then, with the important proviso that the above research is correct, animals surely have personality. Alternatively, we could join in with those nay-sayers, but stipulate that if we cannot describe non-human animals as having a personality, then we should refrain from applying 378 Evolution and Individual Differences Figure 13.6 A ‘bold’ great tit (Parus major) taking on a much larger blackbird (Turdus merula). this word, in its technical sense, to humans. One of the important contributions of evolutionary psychology is that it retains barriers between species only when it is theoretically meaningful to do so but removes those that are based merely on prejudice. As Richard Dawkins (1976) points out, we are all survival machines and although different species are likely to have evolved different solutions to survival, it is also likely that similar strategies might be used. Having differences in personality might be one of them. Intelligence What Is Intelligence and How Is It Measured? Intelligence is a difficult concept to define. Even among psychologists there is much debate. In 1986, 24 of the world’s leading experts on intelligence met up to produce a definition of intelligence. They failed. Or, in a sense, they succeeded rather too well – inasmuch as they came up with many (Sternberg and Kaufman, 2002). Despite this failure what did emerge from the debate was that there were clearly ‘overlapping themes’ which suggests that, despite differences of opinion, there is some degree of consensus. Interestingly for our purpose, the most common theme to emerge was ‘adap- tation to the environment’. This might suggest that experts on intelligence have been influenced by evolutionary theory. For the most part this is not strictly true because, while this ‘adapting to the environment’ sounds superficially Darwinian, the adapting is generally ontogenic (development of the individual) rather than phylogenic (development of the species). In fact few psychometricians (people who measure psychological abilities) have considered the role of evolution in the devel- opment of human intelligence. A typical modern-day definition of intelligence that we feel most psychologists would broadly accept is provided by Plomin (2018): Intelligence 379 [I]ntelligence is the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly, and learn from experience. (53) History of Intelligence Testing The measurement of intellectual ability has had a long history dating back to tests that were devised by the Chinese civil service around 2000 BC (Cooper, 2012). Hence it is likely that ever since our ancestors developed the notion of division of labour we have tried to use some form of assessment to rank and select people (Ridley, 1996). In the history of psychology, the first intelligence test was devised by the French psychologists Alfred Binet and Theodore Simon in 1905 and become known as the Binet–Simon intelligence scale. Binet and Simon initially produced this first intelligence test in order to identify schoolchildren who needed special attention, but later versions of it were used to test and rank mainstream school pupils. Binet and Simon realised that problem-solving abilities increase with age during childhood. They therefore created a series of tasks which drew on memory, judgement and comprehension and which ranged from simple to complex. How well a child did on these tasks determined their mental age – that is, where they were in comparison to an average child at each age. In this way a ‘bright’ six-year-old whose performance on the Binet-Simon test is at the typical level of an eight-year-old would have a mental age of eight. Binet and Simon’s early intelli- gence test was later further developed by Lewis Terman working at Stanford University in 1916 and hence this become known as the Stanford–Binet IQ test. IQ stands for ‘intelligence quotient’, ‘quo- tient’ being the mathematical term for the process of dividing one number by another. In the case of IQ mental age is divided by chronological age, thus the average person’s mental age should be the same as their chronological age which would give a quotient of 1, whereas a person whose mental age was lower than their chronological age would have a quotient of less than 1. For reasons of convenience IQ scores are derived by multiplying the quotient by 100, so that average IQ is always 100. This means a child with a mental age of 12 and a chronological age of 10 would have an above average IQ of 120 (12 divided by 10 multiplied by 100), whereas a child with a mental age of 5 but a chronological age of 10 would have a below-average IQ of 50. Since this form of IQ is based on the ratio of mental age to chronological age it became known as ratio IQ. Ratio IQ has subsequently been superseded by deviation IQ which is a measurement of how far an individual deviates from the norm of 100. Norms are derived from a large number of people and a person’s score is derived mathematically by looking at how much they deviate (above or below) from the middle of a normal distribution – or, if you like, a bell-shaped curve (Haslam et al., 2017). One Intelligence or Many? The Search for ‘g’ When we think of intelligence we usually consider it to be a unitary phenomenon, as being some- thing that an individual has more or less ‘of’, and many psychological theories of intelligence have conceptualised intelligence in a similar way. The search for this general intelligence or ‘g’ began with British psychologist Charles Spearman (1923) who observed that children tended to score at a similar level on a number of different sub-tests of Binet and Simon’s original test such as arithmetic and vocabulary. Although Spearman had uncovered a high level of positive correlations bet

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