ANDROGENS AND VIOLENCE
Since the early history of the vertebrates, the hormone testosterone has mediated aggressive behavior. In humans and nonprimate humans, increased testosterone levels are associated with aggression and dominance behavior. In human males it is associated with violent crime, problems in the workplace, drug use, and other antisocial behaviors. Studies have shown higher than normal levels of adrenal androgens in aggressive children (Giammanco, 2005). Testosterone levels are higher in adolescent boys perceived to be socially dominant. In other species, aggressive acts are often initiated by males of lower social status in an attempt to be more socially dominant. Therefore, the lack of strong evidence for a direct correlation between testosterone levels and aggression may be influenced by the aggression committed by adolescents of a lesser perceived social status (with lower levels of testosterone) who are trying to become more dominant (Ramirez, 2003). One study found that perpetrators of domestic violence who also had alcohol dependence had higher testosterone levels than normal but that this was not true of perpetrators who did not abuse alcohol (George, 2001).
There are a number of hormones which can affect aggression and violence in addition to testosterone. Increased aggressive behavior may also be linked to lower levels of DHEAS, a lower ratio of testosterone to estrogen, lower levels of FSH, lower levels of prolactin, and higher levels of LH, androstenedione, and DHEA. The adrenal gland secretes several hormones which are called androgens because of the similarities of their actions to those of testosterone, androstenedione, DHEA (dehyroepiandrostenedione), and DHEAS (dehyroepiandrostenedione sulfate). Their effects are more easily observed in females since in males their effects are overshadowed by the testosterone secreted by the testes. There is some evidence suggesting that higher levels of adrenal androgens are factors in increased aggression and antisocial behavior (Ramirez, 2003).
OTHER BIOLOGICAL MOLECULES ASSOCIATED WITH AGGRESSION
Nitric oxide serves three main functions in the body. Outside of the nervous system, macrophages can synthesize it to destroy tumor cells and endothelial cells can secrete it to relax blood vessels. In the brain, nitric oxide can affect the activity of neurons and the enzyme which synthesizes it, nitric oxide synthase (NOS), is concentrated in regions of the brain which determine emotions. In animal studies, mutations in NOS greatly increase male aggression, but not female aggression. Testosterone is required for this increase; no such increase is observed in castrated males. In fact, normal female mice do display maternal aggressiveness toward intruders which is reduced in NOS mutant females. Thus it appears that the activity of NOS affects aggression but has opposite effects in males and females. Androgens tend to inhibit NOS and estrogens tend to increase its activity (Chiavegatto, 2003). In mice, absence of the gene nitric oxide synthase affects violence and sexual behavior, in part through effects on the serotonergic pathways and serotonin receptors (Reif, 2003).
There are two genes for monoamine oxidase, MAO-A and MAO-B. Deletions
of MAO-A in humans result in mental retardation, autistic behavior, and
other abnormalities (Reif, 2003). Mutations in MAO-A and 5TTT affect the
organization of the cerebral cortex in mice (Reif, 2003).The absence of
MAO-A expression in mice results in increased levels of some neurotransmitters
(dopamine, serotonin, and NE), higher aggression, and inappropriate sexual
activity in males. In humans, a mutation in MAO-A causes Brunner syndrome
in which males suffer from mild retardation and display a variety of aggressive
and hypersexual behaviors (in addition to other behaviors ranging from
arson to suicidal behavior). This is the only example known which fulfills
the OGOD (one gene, one disease) model for behavioral disorders. Variations
in the promoter region are known to affect panic and depression in females
and aggression in males (Reif, 2003).
The general model is that dopamine facilitates aggression while serotonin
inhibits it. While drugs which are serotonin reuptake inhibitors decrease
aggression in most individuals, there are some in which treatment actually
increases aggression. The same is true of benzodiazepines which facilitate
GABA receptors (Nelson, 2006).
Given rapid changes in levels of violence in human cultures, there must be societal and environmental cues for violence in addition to biological signals. During the 1990s, there was a correlation with homicide rate and unemployment rate while in the 1980s, homicide rates remained constant despite drops in unemployment (Caulkins, 2007). Between 1960 and 1980, homicide frequency in the United States doubled (from 5.1 to 10.2 per 100,000 population) but by the year 2000 had returned to a value near the 1960 value (5.5 per 100,000) (Caulkins, 2007). Some have hypothesized that homicide rates are influenced by the rate of children who experience violence or the availability of abortion (Caulkins, 2007).
Aggression can also be increased through environmental variables. Mice become more aggressive when isolated and isolation is required for increased aggression in nitric oxide synthase mutants to become manifest (Chiavegatto, 2003).
Many of the social explanations for crime rates cannot explain why crime rates dropped during the 1990s. One model does account for this change: crime as facilitated by childhood exposure to lead. Lead is a neurotoxin and there is association between areas with high lead exposure and crime rate, after controlling for other variables. Since the elimination of lead based paint and leaded gasoline, lead exposure in U.S. children has dropped (Nevin, 2007).
There in interest in whether dietary factors can increase aggression. Although it has been observed that low blood sugar increases irritability and aggressive individuals may have low blood sugar, this is a correlation and may not represent a causal relationship. For example, another variable, such as serotonin levels, might increase aggression and lower blood sugar. The inclusion of omega-3 fatty acids in diet has been shown to decrease aggression and some studies have linked aggression to food intolerance (Benton, 2007).
VIDEO GAMES AND MOVIES
There is conflicting evidence on whether cartoon violence increases aggression
in children. One factor in the contradictory findings of studies may stem
from the variations between the studies (such as whether the cartoon was
comedic or whether aggression was measured as acts against peers or against
a toy). Some have concluded that cartoon violence can disinhibit aggressive
behaviors by making children more likely to display behaviors they have
already learned (Kirsh, 2006).
Studies have concluded that violent video games increase aggression,
anger, aggressive thoughts, and desensitization to violence. About 85%
of games involve some degree of violence and about half involve intense
violence (Carnagey, 2007). Exposure to video game violence decreased empathy
and increased aggressive attitudes (Funk, 2004). Violence in film has
been shown to result in desensititzation to violence (Carnagey, 2007).
Individuals who rate high on aggressiveness respond more to violence in
film, video games, than nonviolent men (Giumetti, 2007).
In some religious and ethnic groups, 'honor killings' take the lives of women who have acted differently than the norms set by the family or community. The UN estimates that 5000 women are killed a year (Meetoo, 2007).
VIOLENCE IN APES
Biologically, humans are most closely related to apes and descriptions of ape violence can suggest parallels to human violence. An estimated 10% of gorilla infants are killed by an adult male gorilla who displaces another male as leader of a harem of females. The majority of males commit infanticide at least once during their lives and most females will have an infant killed by an adult male gorilla (Bagemihl, 1999; Wrangham, 1996).
Chimps can attack each other in lethal gang attacks. High ranking female chimps have been observed to kill low ranking females and to commit infanticide of the offspring of low-ranking females. Aggression is often displayed towards immigrant females. High-ranking females mature faster than low ranking females, perhaps because they are better nourished (De Waal, 2001).Male chimpanzees are known to kill the infants of chimpanzees who belong to other groups. A few cases are known in which males killed infants in their own group if the female was a new immigrant to the group. Infanticide was not committed on the offspring of immigrant females once they had mated with the males of the group (De Waal, 2001).
Chimpanzees and humans are unusual among animals in that they are prone to kill individuals of their own species which belong to different social groups (De Waal, 2001). In 1974, the first case in which non-humans deliberately sought for an individual of their own species in order to attack and kill them was recorded. A group of common chimps silently passed into the territory of a neighboring group, encountered a solitary male, and beat him to death. In 1977, the members of one group of common chimps fatally assaulted the males of a neighboring group one by one over time and abducted the females until the neighboring group no longer existed (Wrangham, 1996).
Compared to common chimpanzees, Bonobo males are less aggressive and form closer bonds with females which are not limited to the period of estrus. Unlike common chimp males, Bonobo males do not form lethal raiding parties or commit infanticide (Savage-Rumbaugh, 1998).
In both orangutans and common chimpanzees, heterosexual intercourse can involve forceful coercion of the female. In orangutans, female coercion may be a factor in as many as 1/3 of matings. Once, an orangutan male raped a human woman. In common chimpanzees, females can be assaulted by males of the group which apparently results in a female being more likely to submit to the male (Wrangham, 1996).