Biology of Aggression

Biology can help us to distinguish the roles of nature, nurture, and their interaction in understanding human aggression (Nelson 2000). Aggression has a biological component in both the structure of the brain and the operation of the hormone system.

Brain structure relates to aggression, anxiety, dominance, and fear in ways not fully understood. Panksepp (1998) has identified neurochemical pathways in the brain associated with fear, panic, and rage. Two of the four parts of the limbic system, which developed early in evolutionary history and have changed little since, seem most involved.

The hypothalamus probably is the place where the sensation of anger originates in humans. In the 1940s, Bard and Mountcastle concluded from studies of cats with brain lesions that the hypothalamus organizes attack behavior and the cerebral cortex normally inhibits the release of aggression In the 1960s, Flynn found that electrical stimulation of the lateral hypothalamus elicits predatory aggression in cats, whereas stimulation of the medial hypothalamus elicited vicious attack behavior. Tumors in the human hypothalamus are associated with irritability. People may not be fully aware of or in control of their behavior under stress to the extent that the limbic system controls emotions (Feldman 1997, Schaffhausen n.d.).

The amygdala alerts the individual to threats and has a role in governing emotions including anger and fear hardwired into our brains (Arendal 2000). Eggar and Flynn found that attack behavior produced by stimulating the hypothalamus can be inhibited by stimulating the basomedial nucleus of the amygdala but can be facilitated by stimulating the lateral nucleus of the amygdala. An amygdalectomy reduces emotional reactions including those associated with violence.

Apparently, this attack prompted researchers to begin keeping records of mass shootings. The Independence Institute reported twenty mass shootings between 1966 and 1979, eighteen in the 1980s, 54 in the 1990s, and 87 in the 2000s. There were thirteen in 2012 alone. One possible explanation for the increase is ready availability of guns (Chapter 16). Another is increasingly graphic and violent entertainment, especially movies and video games that have become much more accessible due to cable TV and the Internet. Still another is the breakdown of the family and the stability it provides. A fourth is the culture fostered by many psychologists who emphasize self-esteem and catharsis and professors who justify and encourage protests. Together, they create a culture in which self-restraint and personal responsibility are devalued and social and personal dysfunction is tolerated (Chapter 4).

The fifth possibility, relevant to this chapter, is deinstitutionalization of the violent mentally ill. ACLU lawyers with extreme views on civil liberties and radical psychologists and psychiatrists who denied the existence of mental illness successfully worked to effectively end involuntary commitment and care for those who resist treatment, although a third of the interstate variation in homicide rates is attributable to the strength or weakness of involuntary civil commitment laws.

An estimated 70,000 violent, mentally ill individuals are free in the US at any given time. Most perpetrators of mass slayings are males, 17-24, introverted individuals who dwell significantly in the videogame world. Beyond that, we know too little of the underlying causes of murder-suicides and mass shootings. Answers will remain elusive without systematic research and national monitoring as suggested by Peter Marzuk of Cornell over twenty years ago.

Hormones and aggression are related in ways that are not fully understood. It is unclear whether hormones facilitate aggression or encourage social dominance, competitiveness, and impulsiveness that result in aggression. The effort to unravel the complexities has involved medical and chemical studies on animals and humans as well as interviews, questionnaires, and study of criminal records (Nelson, 1995).

Adrenaline, also known as epinephrine, prepares the body for fight or flight. When released, heart rate and systolic blood pressure go up, blood flow can double, and blood sugar rises while metabolism is inhibited. In humans, there also is a feeling of anxiety. Intriguingly, those who argue that fighting is instinctive tend not to attend much to the related, often preferred, instinct for flight, nor do any of these hormones explain which choice is made and why.

Estrogen seems to reduce aggressiveness unless the female is defending its young, but the effects vary among species. Of greater interest, estrogen has proven effective in reducing hostility and aggression in male sex offenders and in demented patients (Kyomen 2002).

Testosterone is a logical hormone to suspect of links to aggression because men generally are more aggressive than and have testosterone levels eight to ten times higher than women have. In humans, violence occurs largely in males from adolescence into their forties. Then, testosterone concentrations tend to decrease, while serotonin, which inhibits aggression, increases.

Mazur and Booth (1998) reviewed a large body of research that links high testosterone levels with rebellion against authority, criminal behavior, poor work history, spousal abuse, extramarital affairs, and divorce. In a study of marital status among 2,100 male Air Force veterans over a ten-year period, testosterone levels fell and remained low with marriage, and rose with divorce. A study of 692 men in prison found those with high testosterone levels to have been more likely to have committed crimes of violence, have more convictions and denials of parole, and have more prison rule violations. High testosterone levels may contribute to risk seeking behavior (Chapter 2).

Social rank (hierarchy in higher primates, usually education, and socioeconomic status in humans) mitigates the effects of testosterone. More important, it also is associated with many positive qualities, such as self-confidence and a sense of well-being. James Dabbs (2000) found in both physical (e.g. soccer) and non-physical competitions (e.g. chess) that testosterone increases in the winners but declines in losers and even in their fans, leading him to conclude that testosterone levels may be a result rather than a cause of aggression. In what anyone who has watched the polar swings in interpreting research on human behavior for a couple of decades will recognize as a fairly common phenomenon, researchers now suggest that testosterone deficiency may cause anxiety, depression, diminished libido, insomnia, irritability, physical weakness, and poor memory.

There is some inconclusive evidence that hormones, and thus any impact they have on aggression, vary seasonally. As the temperature rises, so do tempers. Ferri and Intezeslogou found that the number of attacks, murders, rapes, and robberies rise in the spring and peak in the summer at about 20% above the winter rate, at least in Greece, perhaps due to a seasonal increase in testosterone levels, perhaps in interaction with cultural norms.

If hormones are linked to aggression, then one might expect that diseases of the endocrine system might be linked as well. Hyperthyroidism appears linked to unusual levels of violence. Premenstrual Syndrome [PMS] has been associated with aggression, and has been used in court defenses of women accused of violent crimes, although there is little evidence for it as a direct factor in criminal activity. It has four forms and three effects. The first form is associated with fluid retention [PMS-H] and results in bloated stomachs and swollen ankles, knees, hands, and face. The second form [PMS-C] is associated with physical symptoms such as headaches, pains in the legs, arms and back, nausea, and food cravings. The third [PMS-A] and fourth [PMS-D] forms are associated with anxiety, depression, disturbed sleep, irritability, mood swings, and tension (Reid & Yen 1981).

Some may think the infamous “Twinkie Defense” to be relevant to this discussion—but it never happened. Dan White murdered San Francisco Mayor George Moscone and City Councilman Harvey Milk. The defense team argued that White had been suffering from long-term untreated depression that diminished his capacity to distinguish right from wrong, and called Dr. Martin Blinder as a witness to testify that the conversion of the previously health-conscious White to a junk-food junkie was evidence of, not the cause of, his depression.

Drug abuse also has a strong association with violence, though perhaps less with actual use than with crimes committed to get the cash to buy them or with wars among sellers. The actual situation is more complex. Marijuana appears to reduce aggression. Hallucinogens except PCP are associated with aggression only when they lead to disorganized behavior or panic. Opiates produce euphoria and lethargy although controlled laboratory experiments have found them to increase the likelihood of unprovoked aggression. Some researchers report that amphetamines stimulate delusions, emotional volatility, hyperactivity, impulsivity, paranoid psychosis, and rage, but their studies are retrospective, involve small samples, and often involve users of multiple drugs, so causation is unclear.

Alcohol use has long been associated with violence and aggression. The upper limits in studies of the association between the two report as many as 86% of murderers, 57% of men and 27% of women involved in marital violence, 60% of sexual offenders, and 42% of people who committed other violent crimes had been drinking. A person may become aggressive because he drinks, but a person who intends to commit crime, violent or not, may drink to bolster his courage. Alcohol may increase the risk of violent behavior for some but not all individuals or subpopulations under some but not all situations and social or cultural influences (Alcohol Alert 1997 and 2000, Potter-Effron 1997).

One explanation of these data suggests that alcohol weakens brain mechanisms that restrain impulsive behavior. The drinker is less able than the non-drinker (even if the same person) to judge risks, more likely to misjudge social cues as aggressive, and more likely to overreact. Another explanation focuses on individual differences in brain chemistry to explain why alcohol results in aggression in some persons but not in others. Research to refine and correct models such as these should gradually improve our understanding of the environmental and biological aspects of violence.

Reliable and valid models are essential to developing effective strategies not just for alcohol-related violence, but also for other problems such as domestic violence, sexual assault, and child abuse and neglect. The research suggests that some violent behavior is preventable and some is treatable. One study found decreased levels of marital violence in couples that completed behavioral therapy for alcoholism and remained sober. Another suggests that the reduced consumption following a 10% increase in the beer tax reduces murder, rape, and robbery. Although the reductions were miniscule, and the effects may be temporary given the tendency of people to become accustomed to higher prices and return to their usual behavioral patterns, the findings suggest directions for research. Some anticonvulsants (e.g., carbamazepine), mood stabilizers (e.g., lithium), and antidepressants, (e.g., fluoxetine) have similar potential.

Some blame sex chromosome abnormalities for aggressive personalities. There are two sex chromosomes, labeled X (female) and Y (male). An individual inherits one from each parent. If both are X, the child will be a normal female; if one is X and one is Y the child will be a normal male. In about one case in two thousand, a male child inherits an extra Y-chromosome, resulting in Jacobs (or XYY) syndrome. Such individuals are usually tall and thin; many experience severe acne during adolescence, and their IQ usually is significantly below that of siblings. For a while, there was a lot of interest in sex chromosome abnormalities as a cause of aggression. But, a thorough review of the literature combined with a double blind study of 4591 men found no association between XYY or XXY abnormalities and violence (Schiavi, et al. 1984).