The Biology of Aggression
Aggression varies too widely from culture to culture, era to era, and person to person to be considered an unlearned instinct. But biology does influence aggression. We can look for biological influences at three levels—genetic, neural, and biochemical.
Do guns in the home save or take more lives? “Personal safety/protection” is the #1 reason U.S. gun owners give for firearm ownership (Swift, 2013). Yet in the last 40 years, well over one million Americans have suffered nonwar firearm deaths—more than all American war deaths. Compared with people of the same sex, race, age, and neighborhood, those who keep a gun in the home have been twice as likely to be murdered and three times as likely to commit suicide (Anglemyer et al., 2014; Stroebe, 2013). States and countries with high gun ownership rates also tend to have high gun death rates (VPC, 2013).
Genetic Influences
Genes influence aggression. Animals have been bred for aggressiveness—sometimes for sport, sometimes for research. The effect of genes also appears in human twin studies (Miles & Carey, 1997; Rowe et al., 1999). If one identical twin admits to “having a violent temper,” the other twin will often independently admit the same. Fraternal twins are much less likely to respond similarly.
Researchers continue to search for genetic markers in those who commit violent acts. One is already well known and is carried by half the human race: the Y chromosome. Another such marker is the monoamine oxidase A (MAOA) gene, which helps break down neurotransmitters such as dopamine and serotonin. Sometimes called the “warrior gene,” people who have low MAOA gene expression tend to behave aggressively when provoked. In one experiment, low (compared with high) MAOA gene carriers gave more unpleasant hot sauce to someone who provoked them (McDermott et al., 2009; Tiihonen et al., 2015).
“It’s a guy thing.”
Neural Influences
There is no one spot in the brain that controls aggression. Aggression is a complex behavior, and it occurs in particular contexts. But animal and human brains have neural systems that, given provocation, will either inhibit or facilitate aggression (Falkner et al., 2016; Moyer, 1983; Wilkowski et al., 2011). Consider:
- Researchers implanted a radio-controlled electrode in the brain of the domineering leader of a caged monkey colony. The electrode was in an area that, when stimulated, inhibits aggression. When researchers placed the control button for the electrode in the colony’s cage, one small monkey learned to push it every time the boss became threatening.
- A neurosurgeon, seeking to diagnose a disorder, implanted an electrode in the amygdala of a mild-mannered woman. Because the brain has no sensory receptors, she was unable to feel the stimulation. But at the flick of a switch she snarled, “Take my blood pressure. Take it now,” then stood up and began to strike the doctor.
- Studies of violent criminals have revealed diminished activity in the frontal lobes, which play an important role in controlling impulses. If the frontal lobes are damaged, inactive, disconnected, or not yet fully mature, aggression may be more likely (Amen et al., 1996; Davidson, 2000; Raine, 2013).
Biochemical Influences
Our genes engineer our individual nervous systems, which operate electrochemically. The hormone testosterone, for example, circulates in the bloodstream and influences the neural systems that control aggression. A raging bull becomes a gentle giant when castration reduces its testosterone level. Conversely, when injected with testosterone, gentle, castrated mice once again become aggressive.
Humans are less sensitive to hormonal changes. But as men’s testosterone levels diminish with age, hormonally charged, aggressive 17-year-olds mature into quieter and gentler 70-year-olds.
Facial width is testosterone-linked. A high facial width-to-height ratio is a predictor of men’s aggressiveness and prejudicial attitudes (Carré et al., 2009; Hehman et al., 2013; Stirrat & Perrett, 2010). Women apparently pick up on this by perceiving men with higher facial width-to-height ratios as more dominant (Valentine et al., 2014). Other high-testosterone-linked traits among males include irritability, assertiveness, impulsiveness, hard drug use, and low tolerance for frustration (Dabbs et al., 2001; McAndrew, 2009; Montoya et al., 2012; Olweus et al., 1988). Drugs that sharply reduce testosterone levels subdue men’s aggressive tendencies.
“ We could avoid two-thirds of all crime simply by putting all able-bodied young men in cryogenic sleep from the age of 12 through 28.”
David T. Lykken, The Antisocial Personalities, 1995
Another drug that sometimes circulates in the bloodstream—alcohol—unleashes aggressive responses to frustration. Across police data, prison surveys, and experiments, aggression-prone adults are more likely to drink, and to become violent when intoxicated (White et al., 1993). Alcohol is a disinhibitor—it slows brain activity that controls judgment and inhibitions. Under its influence, people may interpret ambiguous acts (such as being bumped in a crowd) as provocations and react accordingly (Bègue et al., 2010; Giancola & Corman, 2007). Alcohol has been a factor in 73 percent of homicides in Russia and 57 percent in the United States (Landberg & Norström, 2011).
A lean, mean fighting machine—the testosterone-laden female hyena The hyena’s unusual embryology pumps testosterone into female fetuses. The result is revved-up young female hyenas who seem born to fight.
Those who just think they’ve imbibed alcohol can become more aggressive (Bègue et al., 2009). But so, too, will those who unknowingly ingest alcohol slipped into a drink. Thus, alcohol affects aggression both biologically and psychologically (Bushman, 1993; Ito et al., 1996; Taylor & Chermack, 1993).