Genes, Evolution, and Behavior PDF

CHAPTER Genes, Evolution, and Behaviour GENETIC INFLUENCES EVOLUTION AND BEHAVIOUR Chromosomes and Genes Behaviour Genetics Techniques Evolution of Adaptive Mechanisms Evolution and Human Nature Evolutionary Psychology Applications: Gene Therapy and Genetic Counselling CHAPTER OUTLINE Researc...

CHAPTER Genes, Evolution, and Behaviour GENETIC INFLUENCES EVOLUTION AND BEHAVIOUR Chromosomes and Genes Behaviour Genetics Techniques Evolution of Adaptive Mechanisms Evolution and Human Nature Evolutionary Psychology Applications: Gene Therapy and Genetic Counselling CHAPTER OUTLINE Research Foundations: Gender Differences in the Ideal Mate GENETIC INFLUENCES ON BEHAVIOUR Heredity, Environment, and Intelligence Biological Reaction Range, the Environment, Personality, and Intelligence Focus on Neuroscience: The Neuroscience and Genetics of Dyslexia 4 HOW NOT TO THINK ABOUT BEHAVIOUR GENETICS AND EVOLUTIONARY PSYCHOLOGY Frontiers: Heritability, Evolution, and Politics Psychology will be based on a new foundation. —Charles Darwin, 1859 What are the issues here? Where can we find the information to answer these questions? Identical twins Jim Springer and Jim Lewis met for the first time when they were 39 years old. They discovered each other through a landmark University of Minnesota study of twins who had been separated shortly after birth and raised by different adoptive parents. Although they had been raised in different families, the two Jims found that they had many What do we need to know? things in common. Both had married twice and each had a son named James. Both men smoked—and even smoked the same brand of cigarette—and both preferred Miller Lite beer. Both worked as volunteers for their local police departments as part-time sheriffs, favoured poodles as pets, suffered from the same kind of headache symptoms when under stress, and bit their fingernails. Both Jims did woodworking as a hobby, and they were the only people in their respective neighbourhoods to have built a circular bench around a tree in their yard. When given a series of psychological tests, they were strikingly similar in their pattern of personality traits. 1. Differentiate between genotype and phenotype. GENETIC INFLUENCES Our physical development, including the development of the nervous system, is in large part directed by an elaborate genetic blueprint passed on to us by our parents. These biological characteristics set limits on our behavioural capabilities. However, our genetic endowment combines with environmental forces to determine our behaviour. Nature or nurture is not an appropriate dichotomy; it should be nature and nurture. Modern scientists realize that asking whether a particular behaviour is caused by genetic or environmental factors makes no more sense than asking if a triangle is formed by its sides or its corners. Instead, psychologists working in the field of behaviour genetics study the ways in which favourable or unfavourable environmental conditions can affect the genetically inherited potential of an organism. Chromosomes and Genes 2. How does genetic transmission pass on from parents to offspring? How are physical characteristics passed on from parents to their offspring? This question originated in antiquity, and the ancient Greek physician Hippocrates was one of the first to provide a semi-correct answer. Hippocrates suggested that semen contains not body parts, but rather some sort of design for the formation of the offspring. It was not until 22 centuries later that the wisdom of Hippocrates’s answer was confirmed by Gregor Mendel, a monk whose research with garden peas in the 1860s marked the beginning of modern genetic theory. Mendel showed that heredity involves the passing on of specific organic factors, not a simple blending of the parents’ characteristics. These specific factors might produce visible characteristics in the offspring, or they might simply be carried for possible transmission to another generation. In any case, the offspring of one set of parents do not all inherit the same traits, as is evident in the differences we see among brothers and sisters. Early in the 20th century, geneticists made the important distinction between genotype, the specific genetic makeup of an individual, and phenotype, the observable characteristics produced by that genetic endowment. A person’s genotype is like the commands in a computer software program. Some of the directives are used on one occasion, some on another. Some directives are never used at all, either because they are contradicted by other genetic directives or because the environment never calls them forth. Thus, genotypes are present from conception and never change, but phenotypes can be affected by other genes and by the environment. For example, geneticists have discovered that chickens have retained the genetic code for teeth (Kollar & Fischer, 1980). Yet, because the code is prevented from being expressed, hens’ teeth remains an expression for scarcity. The union of two cells, the egg from the mother and the sperm from the father, is the beginning of a new individual. Like all other cells in the body, the egg and sperm carry within them the material of heredity in the form of rodlike units called chromosomes. A chromosome is a tightly coiled molecule of deoxyribonucleic acid (DNA) that is partly covered by protein. Indeed, the DNA is so tightly coiled that if the DNA in a single human cell were stretched out, it would be almost 2 metres long (Masterpasqua, 2009). The DNA portion of the chromosome carries the hereditary blueprint in units called genes (Figure 4.1). The many genes carried on each chromosome are like a giant computer file of information about your characteristics, potentials, and limitations. Every moment of every day, the strands of DNA silently transmit their detailed instructions for cellular functioning. In humans, every cell in the body except one type has 46 chromosomes. The exception is the sex cell (the egg or sperm), which has only 23. At conception, the 23 chromosomes from the egg combine with the 23 from the sperm to form a new cell, Genes, Evolution, and Behaviour Each chromosome contains numerous genes, segments of DNA that contain instructions to make proteins— the building blocks of life. A G T C G Each nucleus contains 46 chromosomes, arranged in 23 pairs. Each human cell (except red blood cells) contains a nucleus. The human body contains 100 trillion cells. T C A G A One chromosome of every pair is from each parent. 105 T C C FIGURE 4.1 The ladder of life. Chromosomes consist of two long, twisted strands of DNA, the chemical that carries genetic information in the form of specific sequences of the substances adenine, thymine, guanine, and cytosine (A, T, G, and C). Every cell in the body (with the exception of red blood cells) carries within its nucleus 23 pairs of chromosomes, each containing numerous genes that regulate every aspect of cellular functioning. Human DNA has about 3 billion chemical base pairs, arranged as A-T or C-G units (Human Genome Project, 2007). the zygote, containing 46 chromosomes. The genes within each chromosome also occur in pairs, so that the offspring receives one of each gene pair from each parent. Every cell nucleus in your body contains the genetic code for your entire body. In all these cells (except for egg and sperm), there are two copies of each gene, one from your mother and one from your father. Alternative forms of a gene that produce different characteristics are called alleles. Genes affect our body’s development and functioning through one general mechanism: genes code for the production of proteins. The estimated 70 000 different types of proteins found in a human (Wahlsten, 1999) control the structure of individual cells and all the chemical reactions that go on within those cells, whether they are reactions necessary to sustain the life of the cell or are changes induced only periodically by experience or maturation. It is estimated that about half of all genes target brain structure and function (Kolb & Whishaw, 2003). Each individual gene carries the code for a specific protein, and when that gene is activated, the cell produces the specified protein. At different points in development, in response to different metabolic demands, or in response to different environmental factors, a gene may be activated and a protein produced or an already active gene may be “turned off,” and the levels of a specific protein will then decrease. As the protein levels within a neuron change, there is a corresponding change in the function of that neuron and the neural circuits in which it participates. Dominant, Recessive, and Polygenic Effects Genotype and phenotype are not identical, because some genes are dominant and some are recessive. If a gene in the pair received from the mother and father is dominant, the particular characteristic that it controls will be displayed; if the gene is recessive, the characteristic will not show up unless the partner gene inherited from the other parent is also recessive. In humans, for example, brown eyes and dark hair are dominant over blue eyes and light hair. Thus, a child will have blue eyes only if both parents have contributed genes for blue eyes. Even if their traits remain hidden, however, recessive genes can be passed on to offspring. In a great many instances, a number of gene pairs combine their influences to create a single phenotypic trait. This action is known as polygenic transmission, and it complicates the straightforward picture that would occur if all characteristics were determined by one pair of genes. It also magnifies the number of possible variations in a trait that can occur. Despite the fact that about 99.9 percent of human genes are identical among people, it is estimated that the union of sperm and egg can result in about 70 trillion potential genotypes, accounting for the great diversity in characteristics that occur, even among siblings. The Human Genome In 1990, geneticists began the Human Genome Project, and in 2001, the genetic map was published, two years ahead of schedule (International Human Genome Sequencing Consortium, 2001; Venter et al., 2001). Canadian geneticists were involved in the Human Genome Project throughout, and a computer called Deep Maple (really!) at Toronto’s Hospital for Sick Children provided the main computer database for the international effort. The genetic structure in every one of the 23 chromosome pairs has been mapped by using methods 3. Compare dominant, recessive, and polygenic influences on phenotypic characteristics. 106 CHAPTER FOUR that allowed the researchers literally to disassemble the genes on each chromosome and study the specific sequence of substances that occur in each gene (A, T, G, and C; Figure 4.1). The 3.1 billion letters in the entire human genome would fill 152 000 newspapers if printed consecutively. The Human Genome Project, along with Celera Genomics, reported a number of surprises when their projects were complete. They discovered humans have fewer genes than expected; a human has approximately 25 000 genes and not the 100 000 originally estimated (Human Genome Project, 2007). Indeed, we have about the same number as a fruit fly. The groups found that approximately 200 human genes may have arisen from genes that bacteria inserted into our early ancestors. As research continues to explore the functions of our genes, a new understanding of our genetic makeup may lead to the development of effective new medical treatments, to a revolution in how therapeutic drugs are developed, and to a whole new understanding of what makes a human and where we came from. Genetic Engineering: The Edge of Creation 4. Describe the methods used in recombinant DNA research. 5. What is the knockout procedure, and how is it used by psychologists to study behaviour? Advances in molecular biology enable scientists to duplicate and modify the structures of genes themselves (Peacock, 2010). In recombinant DNA procedures, researchers use specific enzymes to cut the long threadlike molecules of genetic DNA into pieces, combine them with DNA from another organism, and insert the new strands into a host organism, such as a bacterium. Inside the host, the new DNA combination continues to divide and produce many copies of itself. Scientists have used this procedure to produce human growth hormone, which is very difficult to obtain naturally in large enough quantities to use for therapeutic purposes. In one study, the availability of growth hormone produced through recombinant procedures made it possible to treat 121 children of abnormally short stature who were deficient in the hormone. As a result of the treatment, the children achieved adolescent heights that were only slightly below average, and far beyond what would have been possible without the treatment (Blethe et al., 1997). The positive social and psychological consequences that could occur for the children who received such treatments have interested many psychologists in the application of recombinant technology. Molecular biologists have developed methods for inserting new genetic material into viruses that can infiltrate neurons and modify their genetic structure. These methods are now becoming part of the tool kit of physiological psychologists who wish to study genetic influences on behaviour. Recent gene-modification research by psychologists has focused on processes such as learning, memory, emotion, and motivation (Wahlsten, 1999). One procedure done with animals (typically mice) is to alter a specific gene in a way that prevents it from carrying out its normal function. This is called a knockout procedure because that particular function of the gene is eliminated, or knocked out. The effects on behaviour are then observed. For example, Holmes, Murphy, and Crawley (2003) used mice with a knockout for the mechanism involved in the reuptake of the neurotransmitter serotonin (see Chapter 3 for a discussion of serotonin). Loss of the serotonin reuptake mechanism results in serotonin remaining in the synapse after its release and a consequent alteration in the activity of serotoninreleasing neurons and serotonin receptors. These mice showed increased anxiety-like behaviour and an exaggerated stress response, offering a parallel with human anxiety and depression. Although gene knockout studies are a powerful tool, researchers need to take great care when interpreting their outcomes. Very little behaviour is controlled by a single gene. Thus, the disruption of a behaviour after a gene knockout may help to identify one of the genes involved in the behaviour, but this identification does not mean that one gene is wholly responsible for the behaviour. It is also important to note that knocking out a single gene may disrupt a wide range of functions. Many of the substances found in the body do many different things in different areas of the brain and the body. Nonetheless, gene-modification techniques may one day enable us to alter genes that contribute to psychological disorders, such as schizophrenia. Genetic engineering gives humans potential control over the processes of heredity and evolution. But these revolutionary techniques also give birth to a host of ethical and moral issues (Lucassen, 2012; Reiss & Straughan, 1998). How and when, if ever, should these techniques be used? To prevent genetic disorders? To propagate desirable human characteristics? To duplicate or clone exceptional people? What are the social and environmental consequences of using genetic engineering to greatly extend the healthy lifespan of people? Questions such as these are already topics of intense discussion, as scientific and technological advances carry us toward uncharted genetic frontiers. Behaviour Genetics Techniques Knowledge of the principles of genetic transmission tells us how genetically similar people are, depending on their degree of relatedness to one another. Recall that children get half of their Genes, Evolution, and Behaviour 107 In Review • Heredity potential is carried within the DNA portion of the 23 pairs of chromosomes in units called genes. Genotype and phenotype are not identical because some genes are dominant while others are recessive. Many characteristics are polygenic in origin; that is, influenced by interactions of multiple genes. genetic material from each parent. Thus, the probability of sharing any particular gene with one of your parents is 50 percent, or 0.50. Brothers and sisters also have a probability of 0.50 of sharing the same gene with one another, since they get their genetic material from the same parents. And what about grandparents? Here, the probability of a shared gene is 0.25 because, for example, your maternal grandmother passed on half of her genes to your mother, who passed on half of hers to you. Thus, the likelihood that you inherited one of your grandmother’s genes is 0.50 3 0.50, or 0.25. The probability of sharing a gene is also 0.25 for halfsiblings, who share half their genes with their biological parent but none with the other parent. An adopted child has no genes in common with his or her adoptive parents, nor do unrelated people share genes in common. Behaviour geneticists are interested in studying how hereditary and environmental factors combine to influence psychological characteristics. One important question is the potential role of genetic factors in accounting for differences between people. The extent to which variation in a particular characteristic within a group can be attributed to genetic factors is estimated statistically by a heritability coefficient. It is easy to confuse two terms in this discussion. Heredity means the passage of characteristics from parents to offspring by way of genes; heritability means how much of the variation in a characteristic within a population can be attributed to genetic differences. It is important to note that heritability refers to differences, or variance, in the trait across individuals and not to the trait itself. If a characteristic, such as weight, has a heritability coefficient of 0.60, this number does not mean that 60 percent of my body weight is due to my genes and 40 percent is due to my environment. If you look around at the other students in your psychology class, you will see a range of body weights. The heritability coefficient is a way of estimating how much of that variation is attributable to genetic factors. Furthermore, heritability applies only to differences within a group, not • Genes influence the development, structure, and function of our body, including our brain, by controlling the production of proteins. • Genetic engineering allows scientists to duplicate and alter genetic material or, potentially, to repair dysfunctional genes. to differences between groups. Consider the range of weights apparent within your psychology class, and now think of a different group, such as a group of individuals from a traditional hunter–gatherer society. Differences in body weight between your class and the hunter–gatherer group are most likely attributable to differences in the environment, such as differences in the availability of high-sugar and high-fat foods and the amount of physical exercise. You could calculate a heritability coefficient for each group and obtain estimates for the importance of genetic factors in explaining individual differences within each group, but your results could not be used to explain differences between groups. This point is widely misunderstood and misreported in the popular media. In considering heritability estimates, such as those shown in Table 4.1, it is important to know TABLE 4.1 Heritability Estimates for Various Human Characteristics Trait Heritability Estimate Height 0.80 Weight 0.60 Intelligence (IQ) 0.50–0.70 School achievement 0.40 Extraversion 0.36 Conscientiousness 0.28 Agreeableness 0.28 Emotional stability 0.31 Activity level 0.25 Impulsivity 0.45 Antisocial behaviour 0.41 Major depression 0.37 Anxiety disorder 0.35 Smoking 0.52 Problem drinking 0.26 Sources: Bouchard et al., 1990; Dunn & Plomin, 1990; Malouf et al., 2008. 6. What is the percentage of genetic resemblance between parents and children, identical and fraternal twins, brothers and sisters, and grandparents and grandchildren? 108 CHAPTER FOUR 7. How are adoption and twin studies used to achieve heritability estimates? What have such studies shown? 8. Why are studies of twins raised together and apart especially informative? What findings have occurred in such studies? what group was studied (the heritability estimates shown in Table 4.1 were obtained from studies of mostly middle-class North Americans). Why does knowing the group matter? If, for example, you were to obtain a heritability coefficient for intelligence from a group of highly advantaged children, those with plentiful resources, enrichment, and educational support, then you would find a heritability coefficient with a high value. On the contrary, if you studied children with diverse backgrounds, those whose backgrounds range from impoverished to privileged, then you would find a heritability coefficient with a much lower value. How can the same characteristic, intelligence, have two very different heritability coefficients? Remember that the heritability coefficient is a statistical estimate of how much of the variability within a group is due to genetic factors. For the group of children from advantaged backgrounds, environmental factors that influence intelligence would be very similar from one individual to the next and so would be unable to explain individual differences. If the environment does not account for the variation in intelligence within this group, then the difference could be due to genetic factors, and the heritability coefficient would estimate a high value. Within the second group, which included children from a wide range of backgrounds, more of the differences can be attributed to differences in the environment, and hence the heritability estimate would be low. Knowing the level of genetic similarity in family members and relatives provides a basis for estimating the relative contributions of heredity and environment to a physical or psychological characteristic (Plomin, 1997). If a characteristic has higher concordance, or co-occurrence, in people who are more highly related to one another, then this points to a possible genetic contribution, particularly if the people have lived in different environments. One research method based on this principle is the adoption study, in which a person who was adopted early in life is compared on some characteristic both with the biological parents, with whom the person shares genetic endowment, and with the adoptive parents, with whom no genes are shared. If the adopted person is more similar to the biological parents than to the adoptive parents, then a genetic influence is suggested. If greater similarity is shown with the adoptive parents, then environmental factors are probably more important. In one study of genetic factors in schizophrenia, Seymour Kety and colleagues (1978) identified formerly adopted children who were diagnosed with the disorder later in life. They then examined the backgrounds of the biological and adoptive parents and relatives to determine the rate of schizophrenia in the two sets of families. The researchers found that 12 percent of biological family members also had been diagnosed with schizophrenia, compared with a concordance rate of only 3 percent of adoptive family members, suggesting a hereditary link. Twin studies are one of the more powerful techniques used in behaviour genetics. Monozygotic (identical) twins develop from the same fertilized egg, so they are genetically identical (Figure 4.2). Approximately 1 in 250 births produces identical twins. Dizygotic (fraternal) twins develop from two fertilized eggs, so they share 50 percent of their genetic endowment, like any other set of brothers and sisters. They occur once in 125 births. Twins are usually raised in the same familial environment. Thus, we can compare concordance rates or behavioural similarity in samples of identical and fraternal twins, assuming that, if the identical twins are far more similar to each other than are the fraternal twins, then a genetic factor is likely to be involved. Of course, it is always possible that, because identical twins are more similar to each other in appearance than are fraternal twins, they might be treated more alike and therefore share a more similar environment. This environmental factor could partially account for greater behavioural similarity in identical twins. To rule out this environmental explanation for greater psychological similarity, behaviour geneticists have adopted an even more elegant research method. Sometimes they are able to find and compare sets of identical and fraternal twins who were separated very early in life and raised in different environments (Lykken, Bouchard, McGue, & Tellegen, 1993). This design permits a better basis for evaluating the respective contributions of genes and environment. Both adoption and twin studies have led behavioural geneticists to conclude that many psychological characteristics, including intelligence, personality traits, and certain psychological disorders, have a notable genetic contribution. Adoptive children frequently are found to be more similar to their biological parents than to their adoptive parents, and identical twins tend to be more similar to each other on many traits than are fraternal twins, even when they have been reared in different environments (Bazzett, 2008; Loehlin, 1992; Genes, Evolution, and Behaviour 109 Identical twins (1 in 250 births) Sperm Egg One sperm and one egg Zygote divides Two zygotes with identical chromosomes (a) (b) Fraternal twins (1 in 125 births) Two eggs and two sperm Two zygotes with different chromosomes (d) (c) FIGURE 4.2 Identical (monozygotic) twins come from a single egg and sperm as a result of a division of the zygote. They have all of their genes in common. Fraternal (dizygotic) twins result from two eggs fertilized by two sperm. As a result, they share only half of their genes. Applications GENE THERAPY AND GENETIC COUNSELLING Until recently, biological psychologists had to be content with studying genetic phenomena that occurred in nature. Aside from selective breeding of plants and animals for certain characteristics or studying the effects of genetic mutations, scientists had limited ways to study the effects of specific genes on behaviour. Technological advances now enable them not only to map the human genome and measure the genotypes of individuals but to modify genes themselves (Peacock, 2010). In one gene-manipulation approach, the recombinant DNA procedure discussed earlier, scientists can join together segments of DNA from different sources, creating sections of DNA that are not found in nature. This new genetic material can then be inserted into a bacterium to produce many copies of the new DNA. The DNA can then be inserted into a virus that can enter the CNS and alter the genetic makeup of neurons within the brain. Modified genes have been used to study processes such as learning and memory, and to study disorders such depression and Alzhiemer’s disease. For example, gene knockout procedures have been used to prevent neurons from producing a chemcial thought to be involved in the realease of the neurotransitter glutamate, and the effects on brain function and behaviour have been tested (Ohira et al., 2013). Researchers can also use a knock-in procedure to insert a new gene into an animal, rather than to remove the actions of an existing gene, as is done with a kockout procedure. For example, researchers have inserted a gene associated with Alzhiemer’s disease into the brain of mice and later tested the impact on neurotransmission, brain structure, and behaviour (Dumanis et al., 2013). continued 110 CHAPTER FOUR As we learn more about the human genome, the assessment and modification of genes heralds advances in the form of genetic screening and therapy. Currently, more than 1000 DNAbased genetic tests for specific diseases have been developed (National Institutes of Health, 2010). These include tests for susceptibility to Alzheimer’s disease, cancers, and arthritis. Some tests are used to assist in diagnosis, other tests allow couples to assess the likelihood of conceiving children with gene-related health problems, and others help to identify a person’s risk for cancers, heart disease, or some psychiatric disorders. There are now private companies that, for a fee, will process your DNA sample (usually obtained from a saliva sample) and report to you your risk for alcholism, cancers, Alzhiemer’s disease, Parkinson’s disease, coronary heart disease, and other disorders. This capability, however, brings with it serious practical and ethical issues (Lucassen, 2012). For example, the tests are not infallible and many tell you only about susceptibility or risk. Erroneous results or misinterpretation of results could cause great psychological suffering. Medical ethics experts also fear what would happen if insurance companies and employers had access to genetic testing results and the danger of having those kinds of decisions based on genetic screening. Canada is currently the only G8 nation that does not have laws against genetic discrimination. That is, there are no laws in Canada to prevent businesses, such as insurance companies, from using the results of genetic testing in a discriminatory way, such as denying insurance coverage for someone who carries the genetic risk for a specific disease. Embryonic screening gives parents increased knowledge of what their offspring might be like. Are parents entitled to make abortion decisions based on results that tell them whether a child is likely to be emotionally reactive, possibly obese, or lacking some characteristic valued by the parents (Valverde, 2010)? Genetic testing combined with the ability to modify the genetic makeup of cells presents enormous potential for treating some of our most serious illnesses. Current gene therapy, however, is experimental and has not proven very successful in clinical trials (National Institutes of Health, 2010). Scientific work continues on the development of effective therapies. Gene-modification techniques may one day enable us to alter genes that contribute to psychological disorders, such as bipolar disorder and schizophrenia (McGuffin et al., 2005). Genetic counsellors help people deal with issues that can arise from genetic testing (Wilkin, 2011). A genetic counsellor provides information on the inheritance of illnesses; addresses the concerns of patients, their families, and their healthcare providers; and supports patients and their families dealing with illness. Genetic counsellors usually work as part of a team, typically with a geneticist, physicians, and healthcare professionals from other specialties such as oncologists, obstetricians, dietitians, social workers, and nurses. The goal of genetic counselling is to assist individuals in making decisions about healthcare. Clients may seek advice because they have a disorder or because of a family member’s illness. Couples with a child affected by a genetic disorder may seek advice as they plan another pregnancy, and couples who are planning their first pregnancy may want to understand their future child’s disease susceptibility, especially if they are planning a pregnancy late in life. Currently healthy clients may seek advice about lifestyle changes if they are at risk for developing a disease. Genetic counselling in Canada is provided by individuals trained specifically as genetic counsellors or by nurses with additional training in genetic counselling. The Canadian Association of Genetic Counsellors was formed to support the development of genetic counselling in Canada and to increase public awareness of the issues involved. It also serves as the national accrediting body for genetic counsellors. Currently, four Canadian universities offer accredited Master’s degree programs in genetic counselling: McGill University, the University of British Columbia, Université de Montréal, and the University of Toronto. Students entering these programs have a variety of undergraduate backgrounds, most commonly an undergraduate degree in biology, psychology, or social work. As genetic screening becomes more commonly available and more genetic tests are developed, the demand for genetic counselling is sure to grow. FIGURE 4.3 A genetic counsellor works with other healthcare professionals, such as obstetricians, to provide advice and support to a couple for whom pregnancy presents special risks because the unborn child may be affected by a genetic disorder. Plomin & Spinach, 2004). Figure 4.4 shows the results of one such comparison. Three groups of twins—identical twins reared together and apart, and fraternal twins reared together—completed personality tests of extraversion (sociability, liveliness, impulsiveness) and neuroticism (moodiness, anxiousness, and irritability). The higher correlation coefficients reveal that the identical Genes, Evolution, and Behaviour Correlations within twin pairs 0.60 Extraversion 0.50 Neuroticism 0.40 0.30 0.20 0.10 0.00 Fraternal twins Identical twins Identical twins reared together reared apart reared together FIGURE 4.4 Degree of similarity on personality measures of extraversion and neuroticism of 24 000 pairs of twins who were reared together and apart. Data from Loehlin, 1992. twins are more similar to each other than are the fraternal twins, and that the degree of similarity in identical twins on the trait of neuroticism is almost as great when they are reared in different environments as when they are reared together (Loehlin, 1992). On the other hand, behaviour genetics studies also have demonstrated that environmental factors interact with genetic endowment in important ways. For example, one adoption study compared the criminal records of men who were adopted at an early age with the criminal records of their biological fathers and their adoptive fathers. A low incidence of criminal behaviour was found in the sons whose biological fathers had no criminal record, even when the adoptive fathers who reared them had criminal records. In contrast, the criminal behaviour of sons whose biological fathers had criminal records was very high, even when their adoptive fathers had no criminal records. This pattern clearly points to a genetic component in criminality. But one additional finding deserves our attention: The level of criminality was highest of all for those sons whose biological and adoptive fathers both had criminal records, suggesting a combined impact of genetic and environmental factors (Cloninger & Gottesman, 1987). In this case, heredity and environment combined to create a double whammy for society. This finding underscores the conclusion that genetic and environmental factors almost always interact with each other to influence behaviour. In Review • The field of behaviour genetics studies contributions of genetic and environmental factors in psychological traits and behaviours. The major research methods used in attempts to disentangle heredity and environmental factors are adoption and twin studies. The most useful research strategy in this area is the study of identical and fraternal twins who were separated in early life and raised in different environments. GENETIC INFLUENCES ON BEHAVIOUR Our unique characteristics as individuals arise from the combination of our learning experiences and the environment in which we behave acting on a substrate provided by our genetic makeup. All our behaviours reflect the interaction between genes and the environment. The best known and most fully explored (although still incomplete) studies of the genes–environment interaction involve intelligence and personality. Indeed, our growing understanding of intelligence has helped to elucidate how favourable and unfavourable • Behaviour genetics techniques allow a heritability coefficient to be determined for different characteristics. The heritability coefficient indicates the extent to which variation in a particular characteristic can be attributed to genetic factors. environmental conditions act on genetically determined potential. Studies of personality have helped to illustrate how opportunities provided by the environment influence the expression of genetically based differences. Heredity, Environment, and Intelligence One of the most controversial questions in the history of psychology is the question: To what extent are differences in intelligence due to genetic factors, and to what extent does environment determine differences in intelligence? Since the 19th century, this question has been at the centre of controversy and, at times, bitter debate. Proponents 111 112 CHAPTER FOUR 9. What evidence supports a genetic contribution to intelligence, and how much IQ group variation is accounted for? 10. How does the concept of reaction range illustrate the interaction between heredity and environment? on each side of this debate have marshalled strong arguments and sound supporting data. But can both sides be right? Let’s first examine the genetic argument. Suppose that intelligence is totally determined by genes. (No psychologist today would maintain that it is, but examining the extreme view can be instructive.) In that case, any two individuals with exactly the same genes would have identical test scores, so the correlation between the test scores of identical (monozygotic) twins would be 11.00. Nonidentical brothers and sisters (including fraternal twins, who result from two fertilized eggs) share only half their genes. Therefore, the correlation between the test scores of fraternal twins and other siblings should be substantially lower. Extending the argument, the correlation between a parent’s test scores and his or her children’s scores should be about the same as that between siblings, because a child inherits only half of his or her genes from each parent. What do the actual data look like? Table 4.2 summarizes the results from many studies. As you can see, the correlations between the test scores of identical twins are substantially higher than any other correlations. Identical twins separated early in life and reared apart are of special interest because they have identical genes but experienced different environments. The correlation for identical twins raised apart is nearly as high as that for identical twins reared together, and higher than that for nonidentical twins raised together (Bouchard et al., 1990; Plomin et al., 2007). Moreover, as Table 4.2 shows, IQs of adopted children correlate as highly with their biological parents’ IQs as with the IQs of the adoptive parents who reared them. The pattern is quite clear: The more genes people have in common, the TABLE 4.2 more similar they are in IQ. This strong evidence suggests that genes play a significant role in intelligence (Petrill, 2003). Notice, however, that the figure for identical twins raised together is higher than the figure for identical twins raised apart. The same is true for other types of siblings raised together and raised apart. These findings rule out an entirely genetic explanation. Although one’s genotype seems to be an important factor in determining intelligence test scores, it probably accounts for only 50 to 70 percent of the IQ variation among people in the United States (Bouchard et al., 1990; Plomin & Spinath, 2004). Thus, environment, too, contributes significantly to intelligence. Obviously, then, the question with which this section began is too simplistic. The real question should be as follows: How do heredity and environment interact to affect intelligence? Biological Reaction Range, the Environment, Personality, and Intelligence The concept of reaction range contributes to our understanding of genetic–environmental interactions. The reaction range for a genetically influenced trait is the range of possibilities—the upper and lower limits—that the genetic code allows. Thus, to say that intelligence is genetically influenced does not mean that intelligence is fixed at birth. Instead, it means that an individual inherits a range for potential intelligence that has upper and lower limits. Environmental effects will then determine where the person falls within these genetically determined boundaries. Each of us has a range of Correlations in Intelligence among People Who Differ in Genetic Similarity and Who Live Together or Apart Relationship Percentage of Shared Genes Correlation of IQ Scores Identical twins reared together 100 0.86 Identical twins reared apart 100 0.75 Nonidentical twins reared together 50 0.57 Siblings reared together 50 0.45 Siblings reared apart 50 0.21 Biological parent—offspring reared by parent 50 0.36 Biological parent—offspring not reared by parent 50 0.20 Cousins 12.5 0.15 Adopted child–adoptive parent 0 0.19 Adopted children reared together 0 0.02 Sources: Based on Bouchard & McGue, 1981; Bouchard et al., 1990; Plomin et al., 2007; Scarr, 1992. Genes, Evolution, and Behaviour 113 Focus on Neuroscience THE NEUROSCIENCE AND GENETICS OF DYSLEXIA Dyslexia is defined as difficulty learning to read that cannot be explained by general intellectual impairment, educational opportunity, or sensory deficits. Dyslexia represents a meeting of an evolved behaviour, language, and a cultural invention: literacy (Pennington & Olson, 2005). Although first identified and studied more than a century ago, our understanding of dyslexia has increased dramatically within the past decade as both functional brain-imaging and behavioural and molecular genetics studies have investigated this disorder. Dyslexia was first described in the mid-1890s, and reports that dyslexia tended to run in families began to appear as early as 1905 (Pennington & Olson, 2005). The early studies of the genetics of dyslexia, however, suffered from a number of methodological flaws and it is difficult to draw any firm conclusions from this early research. More recently, large, methodologically sound studies of reading ability among family members of those with dyslexia have contributed to our understanding of the genetics of dyslexia. In one large study of children identified as having difficulty reading, DeFries and colleagues found strong evidence for a familial transmission of dyslexia (DeFries, Singer, Foch, & Lewitter, 1978). Compared with the relatives of children whose reading ability was within the normal range, the relatives of children identified as having reading difficulties were significantly more likely to also have problems reading. These results have been confirmed by numerous studies, and current evidence indicates that genetic factors are very important in the development of dyslexia, especially among individuals with both a high IQ and dyslexia (Davis, Knopik, Olson, Wadsworth, & DeFries, 2001; Pennington & Olson, 2005). Twin studies also indicate a strong genetic component, with heritability estimates ranging from 0.5 to as high as 0.93 (DeFries, Fulker, & LaBuda, 1987; Petryshen et al., 2001). Behaviour genetics research on dyslexia has found that there is a genetic contribution to the entire range of reading ability, not only to dyslexia. That is, variations in reading skill, including dyslexia, are, in part, heritable. We must next determine what gene or genes contribute to the heritability of dyslexia. Drs. Tracey Petryshen and Bonnie Kaplan of the University of Calgary and their colleagues at the University of Calgary and the University of British Columbia have been among the leaders in the search for the genetic components of dyslexia (e.g., Petryshen et al., 2001; Tzenova, Kaplan, Petryshen, & Field, 2004). In one study of 100 Canadian families tested for reading ability, they evaluated potential genetic linkages and found that two measures of dyslexia in particular showed strong evidence of genetic linkage (Tzenova et al., 2004). These two measures were spelling and phonological coding (i.e., the ability to translate letters and syllables into sounds). Their research found a specific dyslexia susceptibility gene on chromosome 6; at least four independent research groups have confirmed this finding (see Pennington & Olson, 2005). Research in behavioural and molecular genetics has found that there are at least six loci, one on each of chromosomes 1, 2, 3, 6, 15, and 18, that influence reading ability. It is interesting to note that none of these gene locations are on the X chromosome. It is popularly reported that dyslexia is a sex-linked disorder, appearing more commonly among males than females, but the evidence does not support this often reported idea. The widely reported male susceptibility to dyslexia is mostly a difference in reporting and, if corrected for selection biases, there is only a small preponderance of males (1.5:1 male: female), not the three or four times greater rate among males that is sometimes reported (Pennington & Olson, 2005). If there are genes that influence one’s susceptibility to dyslexia, what do these genes do? The functions of the specific genes associated with dyslexia are not yet known, although several intriguing possibilities are currently being explored. The linkage to chromosome 6, the best replicated finding for dyslexia, implicates genes that code for several different substances important in brain development. That is, the alleles associated with dyslexia may act to cause subtle changes in brain development. If the gene variants associated with dyslexia influence brain development and later brain function, then there should be differences in brain activity between those with dyslexia and those who read within the normal range when these individuals are reading. An important challenge with this research, however, is separating the different processes that are involved when reading aloud. When you read aloud you necessarily involve visual sensory processes to see the written words, the cognitive abilities used in reading itself, and then the motor demands of saying the words. Some of the potential confounds could be avoided by asking participants to read silently, but then the researchers would not know whether the word was read correctly, or even whether the participants had read the word at all. In this research, it is also difficult to separate the abnormal processing that underlies dyslexia (i.e., the cause of the reading disorder) from differences that are due to performance deficits (i.e., the consequences of dyslexia). Dyslexia research that uses brain-imaging techniques, such as PET scans and fMRI, have discovered that many different brain areas are involved in reading aloud. Although the results from these studies do not always agree, activity in two brain areas consistently has been found to differ in readers with continued 114 CHAPTER FOUR dyslexia and readers without dyslexia (i.e., controls). Dyslexics have been found to show less activation than controls in two areas of the left temporal lobe: the left inferior temporal cortex and the middle temporal cortex (see Figure 4.5; Price & McCrory, 2005). Studies report reduced temporal lobe activity when participants with dyslexia were reading aloud or silently, making decisions about word or syllable sounds, and making decisions about word meanings. Furthermore, the level of activation within these areas of the left temporal lobe correlates positively with reading ability across a wide range of reading skill, not only with dyslexia (Price & McCrory, 2005). Whether dyslexia means that the activation within these areas falls below some minimum threshold necessary for reading within the normal range, or whether dyslexia affects other, yet unidentified brain areas, is currently under debate (Price & McCrory, 2005). The results of behaviour genetics studies of dyslexia have found evidence for important heritable components. The results of molecular genetics research have identified six different genetic locations that represent risk factors for the development of dyslexia, and the potential gene products of these loci include factors that are involved in brain development. The results of brain-imaging studies suggest that differences in the activity of the left temporal lobe are associated with reading ability. There are no large differences in brain activity associated with dyslexia or evidence of brain abnormalities; there are only subtle differences in the activation of neural systems involved in reading between dyslexics and controls. Although there is still a great deal to learn about dyslexia, progress is being made by using the tools available in genetics and neuroscience. As our understanding of dyslexia increases, it will inform more effective and efficient means of intervention. MT IT Temporal lobe FIGURE 4.5 Brain-imaging research has found two areas in which the levels of activation during reading differ consistently between those with dyslexia and those who read within the normal range. These areas are both within the left temporal lobe: the inferior temporal (shown as IT) and the middle temporal (MT) areas. Activity within the IT and MT areas of the left temporal lobe have been found to be lower among dyslexics, and research indicates that the level of activation within these areas correlates with reading ability across a range of reading skill. intellectual potential that is jointly influenced by two factors: our genetic inheritance and the opportunities our environment provides for acquiring intellectual skills. The diverse abilities measured by intelligence tests are undoubtedly influenced by large numbers of interacting genes, and different combinations seem to underlie specific abilities (Luciano et al., 2001). At present, genetic reaction ranges cannot be measured directly, and we do not know if their sizes differ from one person to another. But studies of IQ gains associated with environmental enrichment and adoption programs suggest that the ranges could be as large as 15 to 20 points on the IQ scale (Dunn & Plomin, 1990). If this is indeed the case, then the influence of environmental factors on intelligence would be highly significant. Some practical implications of the reaction range concept are illustrated in Figure 4.6. First, consider persons B and H. They have identical reaction ranges, but B develops in a very deprived environment and H in an enriched environment with many cultural and educational advantages. Person H is able to realize her innate potential and has an IQ that is 20 points higher than person B’s. Now compare persons C and I. Person C actually has greater intellectual potential than person I, but ends up with a lower IQ as a result of living in an environment that does not allow that potential to develop. Finally, note person G, who was born with high genetic endowment and reared in an enriched environment. His IQ of 110 is lower than we would expect, suggesting that he did not take advantage of either his biological capacity or his environmental advantages. Evidence for these types of environmental effects comes from studies of children who are removed from deprived environments and placed Genes, Evolution, and Behaviour Genetically determined reaction range 58 Measured IQ 150 E C F 130 120 D 125 B 110 110 IQ 100 90 A I 130 G 100 H 106 86 70 58 50 Deprived Enriched Average Quality of environment for intellectual growth FIGURE 4.6 Reaction ranges, environment, and intelligence. Genetic endowment is believed to create a reaction range within which environment exerts its effects. Enriched environments are expected to allow a person’s intelligence to develop to the upper region of his or her reaction range, whereas deprived environments may limit intelligence to the lower portion of the range. The reaction range may cover as much as 15 to 20 points on the IQ scale. in middle- or upper-class adoptive homes. Typically, such children show a gradual increase in IQ on the order of 10 to 12 points (Schiff & Lewontin, 1986). Conversely, when deprived children remain in their impoverished environments, either they show no improvement in IQ or they may even deteriorate intellectually over time (Serpell, 2000). These results remind us that intellectual growth depends not only on genetic endowment and environmental advantage, but also on personal characteristics that affect how much we take advantage of our gifts and opportunities. Behaviour Genetics and Personality Increasingly, personality theorists are working to trace differences in personality characteristics to specific differences in brain activity. Hans Eysenck was one of the first modern personality theorists to suggest a biological basis for major personality traits. Eysenck argued that personality differences could be traced to differences in brain development or function. The personality dimension extraversion-introversion, for example, was argued to reflect differences in brain arousal (Eysenck, 1967). If such personality differences can be traced to specific aspects of brain development or function, then at least some genetic component would be expected. Since Eysenck’s pioneering work, other research has indeed found evidence for specific genetic components of some personality characteristics. For example, a relationship between neuroticism and a gene allele that increases the action of the neurotransmitter serotonin has been reported (Lesch et al., 1996), as has a relationship between novelty seeking and a single gene allele that decreases the action of the neurotransmitter dopamine (Benjamin et al., 1996). One prominent personality trait theory is called the Five Factor Model (see Chapter 14). Five-factor theorists such as Robert McCrae and Paul Costa (2003) believe that individual differences in personality can be accounted for by variation along five broad personality dimensions or traits known as the Big Five: (1) ExtraversionIntroversion (sociable, outgoing, adventuresome versus quiet, inhibited, solitary), (2) Agreeableness (cooperative, helpful, good-natured versus antagonistic, uncooperative, suspicious); (3) Conscientiousness (responsible, goal-directed, dependable versus undependable, careless, irresponsible); (4) Neuroticism (worrying, anxious, emotionally unstable versus well-adjusted, secure, calm); and (5) Openness to experience (imaginative, artistically sensitive versus unreflective, lacking in intellectual curiosity). Twin studies of the heritability of the Big Five personality traits have found heritability coefficients ranging from 0.42 (Agreeableness) to 0.57 (Openness) (Bouchard, 2004). These results are consistent with studies of other personality variables, indicating that between 40 and 50 percent of the personality variations among people are attributable to genotype differences (Kandler, 2012). Although personality characteristics do not show as high a level of heritability as is found for intelligence, it is clear that genetic factors account for a significant amount of personality difference. As discussed earlier, twin studies are particularly informative for studying the role of genetic factors because they compare the degree of resemblance between two individuals who are genetically identical—monozygotic, or identical, twins—and two who are not—dizygotic, or fraternal, twins (Rowe, 1999). As noted briefly in the section “Behaviour Genetics Techniques,” across many psychological characteristics monozygotic twins are more similar to each other than are dizygotic twins, suggesting a role for genetics. The issue, however, is complicated by the possibility that identical twins 115 11. Apart from genetic makeup, how else are monozygotic twins similar or the same? 116 CHAPTER FOUR 12. According to the results of the Minnesota Twin Study, what factors were the most important in determining personality? 13. How might genes influence the tendency to enjoy reading or participating in organized sports? FIGURE 4.7 Identical twins may be more similar because people treat them similarly, influenced by their identical appearance, size, and even clothing. may also have more similar experiences than fraternal twins. Because identical twins are more similar than fraternal twins in appearance, size, and physical characteristics, others may treat them more similarly. Indeed, some parents even dress identical twins in the same clothes, making it almost impossible for the twins to be treated differently within many contexts (Figure 4.7). Even someone who knows the twins may confuse one for the other. One of us is married to an identical twin. Although she and her sister did not dress alike or even wear their hair the same, from her childhood to her adulthood, her grandparents called her by her own name about half of the time and by her twin’s name about half of the time. The ideal approach would be to compare personality traits in identical and fraternal twins who either were raised together or reared apart. If identical twins who were reared in different environments, by different adoptive families, are as similar as those reared together, a powerful argument could be made for the role of genetic factors. Moreover, this research design would allow us to divide the total variation among individuals on each personality trait into three components: (1) variation attributable to genetic factors; (2) variation due to a shared family environment among those reared together; and (3) variation attributable to other factors, such as unique individual experiences. The relative influence of these sources of variation can be estimated by comparing personality test correlations among four groups of twins: identical twins reared together, identical twins reared apart, fraternal twins reared together, and fraternal twins reared apart (Plomin & Caspi, 1999; Plomin et al., 2007). Several studies have used this powerful research design to assess the genetic contribution to a range of personality traits (Lykken et al., 1993; Pederson et al., 1988; Rhee & Waldman, 2002; Tellegen et al., 1988; Yamagata et al., 2006). These studies have shown that identical twins are far more similar in personality traits than are fraternal twins, and it makes little difference whether they were reared together or in different adoptive families. Contrary to what many personality psychologists had expected, family environment had little influence on personality differences in these studies. One of the best known and largest of these studies was conducted by Lykken, Tellegen, and colleagues at the University of Minnesota. The socalled “Minnesota Twin Study” (in reference to the university and the participants, not the baseball team) assessed more than 400 pairs of twins, including Jim and Jim who we met at the start of this chapter. For those twins who were separated and reared apart, the median age at separation was 2.5 months, demonstrating relatively little shared experience within the same family environment. The results of this study are shown in Table 4.3. The four types of twin pairs completed measures of 14 different personality traits. Genetic factors accounted for 39 to 58 percent of the variation among people in personality trait scores. Surprisingly, the degree of resemblance did not differ much whether the twin pair were reared together or apart, showing that general features of the family environment, such as emotional climate and degree of affluence, accounted for little or no variation in any of the traits. The absence of important effects of family environment, however, does not mean th