Biological Risk Factors for Challenging Behavior
Scientists are leaning more and more toward the view that a gene specifically "for" a disorder or condition such as antisocial behavior is very unlikely (Rutter, Moffitt, and Caspi, 2006). Dean Hamer, director of the Gene Structure and Regulation Unit at the National Cancer Institute, writes, "Human behaviors, and the brain circuits that produce them, are undoubtedly the product of intricate networks involving hundreds to thousands of genes working in concert with multiple developmental and environmental events" (2002, p. 72).
Because there are so many elements involved and they interact with one another in such complex ways, it is extremely difficult to disentangle the influence of genes from the influence of the environment. To tease out these different strands and estimate their relative power, behavioral geneticists use twin studies, comparing identical twins (who share all their genes) and fraternal twins (who share about half their genes). The genetic influence on a characteristic such as intelligence, temperament, personality, cognitive style, or psychophysiology is greater when the trait is more similar in identical twins than in fraternal twins. Researchers also study adopted children to see if they are more like their biological parents (with whom they share genes) or their adoptive parents (with whom they share the environment). These studies, which are becoming increasingly analytical and sophisticated, show that antisocial behavior is moderately heritable (Moffitt, 2005), especially antisocial behavior that begins early in life (Arseneault et al., 2003; Rhee and Waldman, 2002).
In addition, scientists have discovered that some genes interact with a particular environment to actually produce a disorder (Rutter et al., 2006); some genes are expressed or turned on (or not) because of physical, social, and cultural factors in the environment; and some genes—for example, those that influence difficult temperament, impulsivity, novelty seeking, and lack of empathy—predispose people to be exposed to environmental risks. Genes even help shape the environment. Genes influence how parents bring up their children; genes affect the responses that children evoke from their families and the others around them; and, as children grow older, genes sway their choice of companions and surroundings (Caspi and Silva, 1995; Plomin, Owen, and McGuffin, 1994).
It's important to remember that heredity is not destiny. With the right environmental interventions at the right time, even a trait with a strong genetic foundation (such as antisocial behavior) can be altered.
Almost all experts agree that boys are at greater risk for physical aggression than girls (Underwood, 2003), and several large longitudinal studies bear this out (Broidy et al., 2003). Boys seem more susceptible to many of the risk factors for aggressive behavior—difficult temperament, ADHD, learning disabilities, and nervous system dysfunction, for example (Moffitt and Caspi, 2001; Rutter, Giller, and Hagell, 1998) —and the prevalence rate for aggression in boys is three to four times as high as the rate for girls (American Psychiatric Association, 1994). The social context has a strong influence on how and whether aggressive behavior appears (Maccoby, 2004). Parents treat anger and aggression differently in their sons and daughters and use more physical punishment and power-assertive discipline with their boys (Zahn-Waxler and Polanichka, 2004). Boys hit, push, kick, tease, and insult each other more; spend more time in rough-and-tumble play; and accept aggressive behavior more readily than girls. Physical aggression starts to taper off as boys get older and develop more impulse control (Maccoby, 2004).
Like boys who don't renounce physical aggression before they enter school, girls who continue to act aggressively face the prospect of school failure and rejection by their peers and are also more likely to be depressed (Underwood, 2003). They often join groups of boys, fight with boys, and eventually date—and marry—boys who act aggressively. Without the social and problem-solving skills they need to sustain an intimate relationship, they may find themselves in increasing danger as the boys grow bigger and stronger (Pepler and Craig, 1999). They are also more likely to become teenage mothers (Pepler and Craig, 1999). Although arrest statistics indicate that the rate of physical violence among girls is rising, other studies show that this is a myth: It is not the level of violence that has changed but the labeling of offenses (Chesney-Lind and Belknap, 2004).
As researchers turn their attention to aggressive behavior in girls, they are looking hard at indirect and relational aggression, where the goal is to damage another's self-esteem, social status, or both (Underwood, 2003). Covert tactics—exclusion, back-stabbing, gossiping, belittling, and the like—become more sophisticated and prevalent in middle childhood and are fairly widespread among girls during adolescence. Relational aggression provides a way for them to act on their angry feelings, seek revenge, and assure themselves that they're accepted by the group (Underwood, 2003).
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