Developmental Origins of Physical Aggression PDF

# CHAPTER 5 ## Developmental origins of physical aggression ### Richard E. Tremblay ## An Introduction to Developmental Origins of Physical Aggression This chapter describes the state of knowledge on the development of physical aggression from early childhood to adulthood. Unravelling the developme...

# CHAPTER 5 ## Developmental origins of physical aggression ### Richard E. Tremblay ## An Introduction to Developmental Origins of Physical Aggression This chapter describes the state of knowledge on the development of physical aggression from early childhood to adulthood. Unravelling the developmental origins of physical aggression has been important to understand: * When and why humans start using physical aggression * Why some humans suffer from chronic physical aggression * How to prevent the development of this disorder which causes much distress to the aggressors and their victims * Why situational prevention of aggression is important at all ages and in all cultures Longitudinal studies of large population samples were needed to understand the developmental origins of physical aggression. These studies had to start as close as possible to birth and they generally tracked the putative causal factors, correlates, and outcomes. Considering the distress and financial cost of physical violence among humans, there are surprisingly few longitudinal studies that have tried to chart the development of physical aggression from birth onwards. This lack of attention to physical aggression from the early years onwards appears to be the result of a long-held belief that humans start to use physical violence during late childhood and early adolescence as a result of social learning (e.g. Bandura 1977; Zimbardo 2007). The conclusion of the United States Academy of Science Panel on Understanding Violent Behaviour created in the late 1980s is a good example of the social learning perspective for aggressive behaviour: Modern psychological perspectives emphasise that aggressive and violent behaviours are learned responses to frustration, that they can also be learned as instruments for achieving goals, and that the learning occurs by observing models of such behaviour. Such models may be observed in the family, among peers, elsewhere in the neighbourhood, through the mass media... (Reiss and Roth 1993, p. 7). Similarly, the 2002 World Health Organization report on violence concluded: The majority of young people who become violent are adolescent-limited offenders who, in fact, show little or no evidence of high levels of aggression or other problem behaviours during their childhood (Krug et al. 2002, p. 31). This social learning view of the development of antisocial behaviour was very clearly described more than two centuries ago by Jean-Jacques Rousseau (1762/1979). The first phrase of his book on child development and education, Émile, makes the point very clearly: 'Everything is good as it leaves the hands of the Author of things; everything degenerates in the hands of man' (p. 37). A few pages later he is still more explicit and appears to be writing the agenda for twentieth-century research on the development of violent behaviour: "There is no original sin in the human heart, the how and why of the entrance of every vice can be traced' (p. 56). Rousseau's strong stance was in clear opposition to Hobbes (1647/1998), who, a century earlier, described infants as selfish machines striving for pleasure and power, and declared: It is evident therefore that all men (since all men are born as infants) are born unfit for society; and very many (perhaps the majority) remain so throughout their lives, because of mental illness or lack of discipline... Therefore man is made fit for Society not by nature, but by training (p. 25). This Hobbes-Rousseau debate concerning the developmental origin of antisocial behaviour has far-reaching consequences, not only for child development investigators and educators, but also for political scientists, philosophers, and policymakers. Because the underlying debate is clearly grounded in our views of human nature, it is not surprising that investigators are likely to prefer the 'origin of aggression' that best fit their view of human nature, and their political commitment. However, since most political philosophers appear to agree that society must be built on the natural tendencies of man, it is surprising that research on the early developmental origins of violence has not been a priority for the social sciences. There is no doubt that physical aggression is a crucial component of human's behavioural heritage. Our ancestors needed to be skilled in the art of physical aggression to eat, to defend themselves against predators, to compete for mating, to protect their brood, and to acquire resources. However, like all other social animals, humans need to learn to use aggression sparingly because physically aggressive encounters can be fatal, and lack of self-control among social animals leads to social exclusion. Because the life of citizens in developed countries today is very rarely dependent on physical aggression, we easily forget that the life of our ancestors was, in the words of Hobbes, 'solitary, poor, nasty, brutish and short' (Hobbes 1651/1999, p. 110). Historical analyses of homicide rates indicate that physical violence has systematically and substantially decreased among European citizens over the past 500 years (Eisner 2003). Homicides in European ### SECTION 2 THE DESCRIPTIVE EPIDEMIOLOGY OF VIOLENCE cities decreased from 40 to 1 per 100,000 citizens per year. We can conclude with Elias (1939) that the civilizing process has brought numerous advantages to humans, although, surprisingly, we often look back nostalgically to our primitive nature. Interestingly, the estimated rate of 'homicide' among our closest non-human relatives, chimpanzees, is 261 per 100,000 (Wrangham et al. 2006). ### What is 'Physical Aggression'? The simplest way to define 'physical aggression' is to follow the ethological approach and list the physical aggressions in agonistic encounters (Restoin et al. 1985). However, it is difficult to make a complete list of the multitude of means humans invented to physically hurt other humans. Examples of the most basic physical aggressions should be sufficient to make the concept clear: hitting, slapping, kicking, biting, pushing, grabbing, pulling, shoving, beating, twisting, choking. Some scales use terms such as fighting and bullying to summarize these behaviours. Threatening to physically aggress, use of objects and weapons to aggress is also included in the definition used by ethologists. In a playful context these behaviours are defined as playful aggression. For example, the Olympic Games reward humans who are best at the basic war-like behaviours of a 2-year-old: run, throw, hit, push, pull, etc. Thus an agonistic interaction context is needed to conclude that the behaviour is a physical aggression. Chronic physical aggression can be defined as a tendency to use physical aggressions significantly more frequently than the large majority of a birth cohort over many years. Thus repeated assessments over many years (longitudinal studies) of random samples of a population birth cohort are needed to estimate the prevalence of chronic physical aggression during specific developmental periods. Such studies provide an opportunity to assess the different developmental trajectories of physical aggression in a population and estimate the proportion of individuals on a chronic trajectory (Nagin and Tremblay 1999). In contrast, occasional physical aggression can be defined as the use of physical aggression at a rate that corresponds to the rate of a relatively large number of members in a population cohort. As will be described, the rate of physical aggression for the majority of members in a birth cohort varies substantially with age and subgroups of occasional physical aggression can be identified. However, risk factors that discriminate between chronic and occasional physical aggression are easier to identify than risk factors that discriminate subgroups of occasional physical aggression. ### Development of Physical Aggression #### During Early Childhood Studies of physical aggression during infancy have clearly shown that humans start to use physical aggression towards the end of the first year after birth when they have acquired the motor coordination to push, pull, hit, kick, etc. (Hay et al. 2011; Tremblay et al. 1999). The first population birth cohort to study the developmental trajectories of physical aggression from infancy onwards was the Québec Longitudinal Study of Child Development (Tremblay et al. 1999, 2004). Figure 5.1 illustrates the results of the physical aggression developmental trajectory analyses from 17 to 60 months with this birth cohort (Côté et al. 2007). We can see that half of the children are in the middle trajectory of physical aggression frequency, a third are on a low trajectory, while 17 per cent are on a high trajectory. Based on the earlier definition, we can consider the high trajectory as a chronic physical aggression trajectory and the two others as occasional physical aggression trajectories, while taking into account that the frequencies of aggressions varies depending on age. These analyses are based on prospective repeated assessments of physical aggressions reported by mothers over many years. From this perspective developmental trajectories should be a better estimate of a chronic behaviour problem than an assessment at a given point in time, even if that assessment attempts to reconstruct past behaviour. Longitudinal data have shown that within a year mothers do not recall the age of onset of their children's physical aggressions (Tremblay 2000). In a clinical study of boys between 7 and 12 years of age, the mean age of physical aggression onset reported by parents was 6.75 years (Frick et al. 1993). Retrospective information collected in the Pittsburgh Youth Study (Loeber and Stouthamer-Loeber 1998) compared to prospective data is a good example of the problem with retrospective dating of physical aggression onset. The subjects (N = 503) represented the Pittsburgh public schools' male eighth graders and were close to 14 years old (mean age = 13.8; SD = 0.80) at the first data collection. The cumulative age of onset of physical aggressions reported by the mothers and the boys at that first data collection indicated that by age 5 years less than 5 per cent of the boys had initiated use of physical aggressions and almost no one had initiated fighting. In sharp contrast, the prospective data represented in Figure 5.1 on physical aggression from 17 months after birth indicate that children who do not initiate physical aggression before 3 years of age are extremely rare. These prospective studies suggest that the peak frequency in use of physical aggression for most humans is somewhere between 2 and 4 years of age (see Figure 5.2 and NICHD Early Child Care Research Network 2004). The recall problem suggests that retrospective assessments of children or adolescents cannot identify the age of onset and developmental trajectories of physical aggression use or of chronic physical aggression (chronic physical aggression). Hence the conclusion reached by the World Health Organization ### Developmental Trajectories of Physical Aggression after Early Childhood The developmental trajectories of physical aggression after early childhood have now been studied in many different cultures. From these studies we can expect between 7 per cent and 11 per cent of elementary school children to be on a chronic physical aggression trajectory (Broidy et al. 2003; Campbell et al. 2010; Nagin and Tremblay 1999). That percentage tends to be higher for preschool children (Côté et al. 2007; Tremblay et al. 2004) and lower for adolescents (Brame et al. 2001). This decrease in chronic physical aggression cases with age corresponds to the general decrease in frequency of physical aggression with age after the peak in early childhood (see Figure 5.2 from Tremblay and Côté 2009). Most children use physical aggression during the preschool years, but most children also learn to use alternatives to physical aggression with age, and this applies to a number of chronic cases during early childhood and preadolescence (Nagin and Tremblay 1999). In fact there is good evidence that the learning process to gain control over physical aggression continues throughout adulthood (Sampson and Laub 2003). Crime records from the middle ages to modern times suggest that this phenomenon is not new. The likelihood of committing a homicide and most other crimes has always decreased from late adolescence and early adulthood to old age (Eisner 2003). Trajectories of physical aggression covering different age periods (early childhood to childhood, childhood to adolescence, adolescence to adulthood) also indicate that chronic physical aggression very rarely onsets after early childhood (NICHD Early Child Care Research Network 2004; van Lier et al. 2009). ### Outcomes of Chronic Physical Aggression Trajectories During Childhood Longitudinal studies of physical aggression trajectories during childhood have been used to study how well they predict future outcomes such as school performance, social adjustment, mental health, and violent behaviour. The first longitudinal study to describe developmental trajectories of physical aggression from school entry to adolescence (Nagin and Tremblay 1999) reported that boys on a teacher-rated high trajectory of physical aggression from 6 to 15 years of age were at highest risk of self-reported violence as well as other forms of delinquency at 17 years of age, even after having controlled for hyperactivity and oppositional behaviour. The chronically aggressive boys were also at highest risk of school drop-out. A study which used six longitudinal studies from Canada, New Zealand, and the United States (Broidy et al. 2003) reached the same conclusion for male adolescent violent delinquency, but not for female adolescent violent delinquency. The authors attributed the sex difference in prediction to the fact that the prevalence of female adolescent violent delinquency was too low. However, a later analysis of one of the female samples (Fontaine et al. 2008) reported that elementary school girls who were on a chronic physical aggression trajectory combined with a chronic hyperactivity trajectory were more likely than others to report physical and psychological aggression towards intimate partners by age 21 years. They were also more likely to report early pregnancy, welfare assistance, nicotine use problems, and low educational attainment. A more recent analysis of a population sample of males and females (Pingault et al. 2013) reported that the 9.5 per cent of children on a high physical aggression trajectory between 6 and 12 years, according to mother and teacher rating, represented 28.2 per cent of all those who had a criminal record by age 24 years. In addition, they represented 45.9 per cent of all recorded criminal charges and 57.4 per cent of the violence charges. Therefore, children on a high trajectory of physical aggression during elementary school are not only more likely to have a criminal record but also to have more criminal charges. There is evidence that the criminal outcomes of childhood physical aggression during adolescence and adulthood are preceded by a large range of negative social and academic outcomes by the end of elementary school for boys and girls (Campbell et al. 2010). ### Early Risk Factors and Prevention of Chronic Physical Aggression Trajectories Sex of the child is one of the most important risk factors for chronic physical aggression. When children start using physical aggression at the end of the first year after birth there are no significant differences in frequency of physical aggressions between boys and girls (Hay et al. 2011); however, the differences appear soon after and increase until adolescence (e.g. Côté 2007). Between 10 and 15 years of age boys are close to 20 times (OR = 18.84) more at risk than girls of being on a chronic physical aggression trajectory (van Lier et al. 2009). Twin studies have become important tools to understand the contributions of environmental and genetic factors in the development of human characteristics, including aggression. However, to date there appears to be only one longitudinal study that used a large sample of twins from infancy onwards to study the contributions of genetic and environmental factors in the development of aggression. The study reported that 19 months after birth 58 per cent of the variance in frequency of physical aggression rated by mothers could be attributed to genetic contributions and 42 per cent to common environmental contributions (Dionne et al. 2003). Physical aggression at 72 months after birth was rated by kindergarten teachers and genetic contribution to variance in frequency was estimated to be 66 per cent with 34 per cent attributed to common environmental factors (van Lier et al. 2007). Although these results suggest a substantial contribution of genetic factors in the use of physical aggression from infancy to school entry, environmental factors are also very important. The developmental trajectories of physical aggression previously described indicate that the environmental conditions are essential to learn alternatives to physical aggression during early childhood. Studies of physical aggression trajectories during early childhood with singletons have identified the following types of environmental risk factors: (i) Maternal characteristics, including life style and mental health; (ii) family characteristics; (iii) maternal parenting; (iv) child characteristics (Campbell et al. 2010; Côté et al. 2006; Hay et al. 2011; NICHD Early Child Care Research Network 2004; Tremblay et al. 2004). Maternal and family characteristics are key for planning preventive interventions because they can be used to identify pregnant women at risk of having children on a chronic physical aggression trajectory (e.g. Olds et al. 1998). The maternal characteristics identified include mothers' young age at birth of their child, mothers' smoking during pregnancy, mothers' antisocial behaviour during adolescence, mothers' depression, and mothers' low level of education. Family characteristics include low income, family dysfunction, and the presence of siblings. High-risk maternal parenting behaviours include mothers' hostile-coercive-harsh parenting and lack of sensitivity. Finally, as expected, boys were found to be at higher risk than girls of being on the chronic physical aggression trajectory. It is important to note that these studies of environmental risk factors were not done in the context of a genetically informative design (e.g. twin studies or sibling studies), hence we do not know to what extent the significant environmental risk factors are correlated or interact with genetic factors (Szyf et al. 2009). Nonetheless, the environmental risk factors identified by these studies can be used to identify at-risk groups for preventive experiments. Such experiments are useful to test the effectiveness of the interventions as well as test causal hypotheses (Tremblay 2003). Maternal and family characteristics are especially key for early preventive interventions because they can be used to identify at-risk pregnant women (e.g. Olds et al. 1998). The challenge for the future is to integrate preventive experiments within genetically informative longitudinal studies of risk factors. The discovery of environmental effects on gene expression (epigenetics) is providing the tools to meet this challenge (Tremblay 2010). The term 'epigenetic' refers to the mechanisms which programme genes and can change gene function without modifying gene sequence, mainly through changes in DNA methylation and chromatin structure. This programming is responsive to environmental effects, especially during foetal and early post-natal development. Thus, environments can impact phenotypes through their chemical impact on programming of gene function (Szyf et al. 2009). Epigenetic effects are well known in cancer research and have recently been shown to possibly explain the obesity epidemic and behaviour regulation (Meaney and Szyf 2005; Szyf et al. 2009). Epigenetic studies focus specifically on the physical effects of the environment on gene expression at a given moment in time. The classic example for effects of neglectful environments comes from an experimental study of maternal behaviour in rats which showed that rat pups insufficiently licked by their mothers in the days following birth (i.e. neglected) have increased methylation of the gene encoding the glucocorticoid receptor in the hippocampus, resulting in reduced expression (Weaver et al. 2004). The study further showed that this gene methylation effect had downstream effects on the hypothalamic-pituitary-adrenal axis, which regulates stress responses in the body. Epigenetic mechanisms are especially important because they provide a powerful explanation for early maternal and family effects on the development of physical and mental health problems, including chronic physical aggression. Furthermore, DNA methylation changes over time can be used as markers of environmental effects during development, including assessment of preventive and corrective intervention effects. This discussion of early risk factors has shown that trajectories of chronic physical aggression are specifically related to maternal characteristics: maternal age at first pregnancy, history of behaviour problems, education, smoking, depression, coercive parenting, etc. This can easily be understood from the traditional environmental perspective: a poor early environment has an impact on the developing foetus and infant. Mother characteristics turn out to be more important risk factors than father characteristics because the former carry the child in their womb during foetal life and are more involved in care giving during early childhood. However, the exact bio-psycho-social mechanisms linking poor quality environment to disorganized behaviour remain unclear, to say the least. The epigenetic story provides a basic mechanism that has the advantage of being parsimonious, testable, and promising for prevention. The most fascinating aspect of this mechanism is that it provides an environmentally based explanation of intergenerational transmission for physical and mental disorders which involves genes but is not genetically transmitted. These mechanisms are still far from being clearly understood, but they provide a challenging alternative perspective to the traditional gene vs. environment and gene-environment interaction hypotheses. The first epigenetic study of children with chronic physical aggression (Wang et al. 2012) used subjects from the longitudinal study of boys living in low socio-economic environments described previously (Nagin and Tremblay 1999; Tremblay et al. 1994). Blood was collected from two groups of the boys when they were 27 years old to assess DNA methylation patterns (in monocytes and T cells) of the serotonin transporter (5-HT). The first group included boys who were on a high trajectory of physical aggression during childhood. The second group included boys who were on a normal trajectory of physical aggression. Brain imaging of serotonin synthesis was also obtained from the same boys around age 27 years. We found that chronic physical aggression during childhood was associated with increased DNA methylation in specific CpG sites in monocytes and T cells of the serotonin transporter gene. Interestingly, we also found associations between measures of serotonin synthesis in the brain and differential DNA methylation in T cells and monocytes in the same CpG sites that revealed association with chronic physical aggression during childhood. These findings are the first evidence of the association between environmentally related differences in DNA methylation in white blood cells and in vivo measures of 5-HT in the living human brain. Experimental studies with rats and monkeys which started at birth (e.g. Weaver et al. 2004) are suggesting that the associations between DNA methylation and aggression observed in humans were caused by perinatal environmental effects. ### Conclusions The available data on the development of physical aggression from early childhood to adulthood lead to the following conclusions: * The vast majority of humans have used physical aggression; * The vast majority of humans also learn with age to use means of solving problems other than the use of physical aggression; * Some need more time than others to learn; * Females learn more quickly than males; * By adolescence not much more than 5 per cent of males can be considered cases of chronic physical aggression, while female cases are exceptional; * Most of the chronic physical aggression cases during adolescence were on a chronic physical aggression trajectory since early childhood; * Since most humans used physical aggression and learned alternatives during childhood, most adults are at risk of using physical aggression if in a given situation it appears to be the 'best' solution to the problem. Thus, to continue to reduce the incidence of physical aggression among humans, societies need to implement effective situational preventive strategies as well as effective early preventive interventions of chronic physical aggression. The latter preventive approach will benefit from our new understanding of environmental effects on gene expression during pregnancy and early childhood.