Why Are People Aggressive

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Aggression
Department of Justice, 2005). Further, as many as one-third of married and unmarried
couples in the United States have engaged in aggressive acts toward each other that
range from pushing, shoving, and slapping to beatings and the threatened or actual use
of weapons (Archer, 2000; Durose et al., 2005).
Why Are People Aggressive?
Sigmund Freud proposed that aggression is an instinctive biological urge that builds
up in everyone and must be released. Evolutionary psychologists offer a different view,
suggesting that in prehistoric times, aggression helped people compete for mates, thus
ensuring the survival of their genes in the next generation (Malamuth & Addison,
2001). Through natural selection, they say, aggressive tendencies have been passed on
through countless generations.
Evolutionary theories of aggression are popular, but even evolutionary theorists recognize that “nature” alone cannot fully account for aggression. “Nurture,” in the form
of environmental factors, also plays a large role in when and why people are aggressive. We know this partly because there are large differences in aggression from culture
to culture. The murder rate in the Philippines, for example, is forty-six times higher
than in China or Finland; and the U.S. murder rate is almost nine times higher than
in those latter two countries (Barclay & Tavares, 2002). These data suggest that even if
aggressive impulses are universal, the appearance of aggressive behavior reflects the
influence of both nature and nurture (Malamuth & Addison, 2001). No equation can
predict exactly when people will be aggressive, but years of research have revealed a
number of important biological, learning, and environmental factors that combine in
various ways to produce aggression in various situations.
There is strong evidence for hereditary
influences on aggression, especially in animals (Cairns, Gariepy, & Hood, 1990). In one
study, the most aggressive members of a large group of mice were interbred; then the
most aggressive of their offspring were also interbred. After this procedure was followed
for twenty-five generations, the resulting animals would immediately attack any mouse
put in their cage. Continuous inbreeding of the least aggressive members of the original group produced animals that were so nonaggressive that they would refuse to fight
even when attacked (Lagerspetz & Lagerspetz, 1983). Research in which human twins—
reared together or apart—were rated on aggressiveness suggests that there is a genetic
component to aggression in people as well (Hudziak et al., 2003). However, other
research suggests that people do not necessarily inherit the tendency to be aggressive.
Instead, they may inherit certain temperaments, such as impulsiveness, or certain aspects
of brain chemistry that in turn make aggression more likely (Hennig et al., 2005).
Several parts of the brain influence aggression (Anderson & Bushman, 2002a). One
of these is the limbic system, which includes the amygdala, the hypothalamus, and
related areas. Damage to these structures may produce defensive aggression, which
includes aggressiveness to stimuli that are not usually threatening or a decrease in the
responses that normally inhibit aggression (Coccaro, 1989; Eichelman, 1983). The cerebral cortex may also be involved in aggression (Pietro et al., 2000).
Hormones such as testosterone—the masculine hormone that is present in both
sexes—may also play an important role in aggression (Dabbs & Dabbs, 2001). Experiments have shown that aggressive behavior increases or decreases dramatically with the
amount of testosterone in an animal’s body (Pope, Kouri, & Hudson, 2000; Yates, 2000).
Among criminals, those who commit violent crimes have higher levels of testosterone
than those whose crimes are nonviolent. And among murderers, those with higher levels of testosterone are more likely than others to have known their victims and to have
planned their crimes before committing them (Dabbs, Riad, & Chance, 2001).
Testosterone may have its most significant and durable influence through its impact
on early brain development. One natural test of this hypothesis occurred when pregnant women were given testosterone in an attempt to prevent miscarriage. As a result,
their children were exposed to high doses of testosterone during prenatal development.
Genetic and Biological Mechanisms