Brain Development and Physical Aggression: Regardless of gender, children’s physical aggressiveness peaks at 2-4 years of age but then starts diverging, as girls learn more quickly than boys to suppress such behaviors

Brain Development and Physical Aggression: How a Small Gender Difference Grows into a Violence Problem. Lise Eliot. Current Anthropology, Volume 62, Number S23, February 2021. https://www.journals.uchicago.edu/doi/full/10.1086/711705

Abstract: Of the various behavioral differences between males and females, physical aggression is one of the largest. Regardless of gender, children’s physical aggressiveness peaks between two and four years of age but then starts diverging, as girls learn more quickly than boys to suppress such overt behaviors. By puberty there is a sizable gender difference in physical aggression and violence. Studies in rodents suggest that sex differences in aggression and rough-and-tumble play are mediated by the amygdala, whose volume in males is enhanced by prenatal testosterone. However, efforts to extend this model to humans have shown limited success. This paper takes a critical look at current assumptions about gender differences in aggression and their neural basis. Aggression and empathy are competing impulses that engage much of the same cortico-limbic circuitry and are highly sensitive to social factors and early adversity. Learning, or neuroplasticity, is both a primary cause and key intervention for minimizing male aggression and violence.

Rerouting the Neurodevelopment of Male Violence

This proposed trajectory for the incubation of violence accords with our understanding of brain development and neuroplasticity. Given the potency of early experience, it is likely that an initial biologic tilt, compounded by social factors and environmental stressors, leads to a divergence in neural circuitry and increased risk for violence among the minority of highly aggressive boys. Research on the neurobiology of aggression and violence has uncovered various neural correlates but no clear-cut sex difference in the limbic circuitry that underlies such behaviors. Again, the hub of this circuit is the amygdala, which integrates a wide range of sensory and contextual signals to detect emotion and drive motivated behavior through both autonomic and cortical outputs. The amygdala is about 10% larger in adult men compared to women, but this difference is proportional to overall brain volume and disappears when analyses correct for individual differences in brain or body size (Marwha, Halari, and Eliot 2017). Similarly, boys’ amygdala volume is about 7% larger than girls’ in infancy, but this is proportional to their larger head and brain size at birth. Another developmental difference is that the amygdala reaches its peak volume about 1.5 years later in boys compared to girls at the end of the prepubertal period (Uematsu et al. 2012), which also parallels the sex difference in bodily growth that peaks one to two years earlier in girls.

In principle, males’ larger absolute amygdala volume could contribute to the gender difference in physical aggression and violence. However, studies of adult humans have actually detected an inverse correlation between aggression and amygdala volume (Rosell and Siever 2015). That is, the amygdala is reliably reduced in volume in association with aggression in both sexes—contrary to the findings in rats. The amygdala, in turn, is controlled by the orbital prefrontal cortex (PFC), a largely inhibitory connection that appears weaker in individuals who tend toward high levels of anger expression. Like most of the frontal lobe, the orbital PFC matures quite slowly, so only gradually gains potency at suppressing anger and aggression as children grow.

Early research suggested that women have a proportionately larger orbitofrontal cortex than men (Gur et al. 2002), which was believed to underlie their stronger emotional regulation. However, more recent research using very large samples has failed to confirm this; if anything, the medial orbitofrontal cortex is slightly larger in men, and there is no sex difference in volume of the lateral orbitofrontal cortex (Potvin, Dieumegarde, and Duchesne 2017, 2018; Ritchie et al. 2018). Furthermore, studies of infants from zero to two years of age have failed to detect sex differences at the level of brain structure or connectivity in the amygdala, orbitofrontal cortex, and their associated circuits, suggesting that any such male-female brain differences emerge later in development, coincident with the period of gender socialization and segregation across childhood and adolescence (Geng et al. 2017; Gilmore, Knickmeyer, and Gao 2018).

Other research has focused on neurochemical differences among highly aggressive individuals, which include reduced serotonin levels and greater prevalence of the short-allele form of the serotonin uptake transporter (the 5-HT transporter-linked polymorphic region, or 5-HTTLPR), which results in less efficient serotonin reuptake from the synapse (Rosell and Siever 2015). Importantly, research on both the 5-HTTLPR and the serotonin catabolic enzyme, monoamine oxidase A (MAOA), has identified relationships between low-activity alleles and aggressive or antisocial behavior, but only through interaction with adverse childhood experience (maltreatment). In the case of MAOA, this interaction between short allele (lower serotonin degradation) and maltreatment predicted antisocial behavior in males but trended the opposite way in females, suggesting an early-seeded sex difference in the impact of adversity on the development of antisocial behavior (Byrd and Manuck 2014).

These are just some of the neuroanatomical and neurochemical pathways that participate in violent behavior and are being shaped during the developmental period that normally sees a suppression in overt aggression. Given the high rate of adverse childhood experiences (ACEs) among incarcerated juveniles (Baglivio et al. 2014) and the many links between such adversity and neurobiological dysfunction, it is clear that violence can be one result of neuroplasticity gone awry—a failure to develop the inhibition that increasingly holds aggression in check as children grow. The impact of ACEs on risk-taking, mental health, and violence has become a particular focus of public health efforts, with the goal of identifying and evaluating strategies to counteract their threat to healthy limbic system development (Centers for Disease Control and Prevention, US 2016).

It is clear that such interventions must begin very early, if not a full generation in advance. Antisocial violence is transmitted intergenerationally, and by the time an abused child becomes a parent it is often too late for their own children to escape its shadow. Nonetheless, some interventions are beginning to show promise. Evaluating these is complex, especially when the outcome of interest, violent aggression, is relatively rare and not apparent until decades later. But a few strategies have proven beneficial for reducing child abuse and improving long-term behavior outcomes, mostly involving parent support and education about child development and positive discipline (Tremblay, Vitaro, and Côté 2018).

One program out of Dublin that has been heavily researched is called Preparing for Life. It works with parents from pregnancy through the first four to five years of life using a combination of home visiting, infant massage, and positive parenting education. A large, randomized trial of Preparing for Life (Côté et al. 2018) found benefits of treatment to children’s cognitive development and externalizing behaviors, a common precursor to aggression and violence. Another intervention conducted in the United States, the Nurse-Family Partnership, has been found to reduce child abuse (Chen and Chan 2016) and improve cognitive outcomes, especially for boys (Heckman et al. 2017). Additionally, high-quality preschool has been found to lower the incidence of crime among both boys and girls (García, Heckman, and Ziff 2019). Such interventions cannot end at preschool but must continue when children enroll in elementary and high school. In Canada, a two-year project was developed for seven- to nine-year-olds that included home-based parent training and school-based pairings of at-risk boys with a handful of carefully chosen, prosocial peers. Long-term benefits of this program included a reduction in delinquency and criminal records by age 24, although the intervention did not impact violent crime rates (Tremblay, Vitaro, and Côté 2018).

I conclude with another promising program: a low-cost, holistic intervention developed in Canada and known as Roots of Empathy. At the heart of this program is a human infant, recruited with his or her parent to join an elementary school classroom once per month and serve as the emotional “teacher.” Students cluster around a large blanket and watch as the infant’s abilities and personality unfold month by month. The curriculum is centered on attachment and the importance of a caring, trusted nurturer for every human to grow and thrive physically and emotionally. Most importantly, the children in the class—boys and girls alike—develop a genuine bond and protective affection for their infant teacher. Rigorous assessments of Roots of Empathy have found several benefits in reducing bullying and aggression and increasing empathy and prosocial behaviors within classrooms. Importantly, the benefits are equally great for boys as girls, contrary to teachers’ initial expectations (Connolly et al. 2018).