SPECIFICATION: Hormonal mechanisms in aggression, including serotonin and testosterone

THE ROLE OF HORMONAL MECHANISMS IN AGGRESSION

The information below will help you with other topics like XYY, Testosterone applied to aggression.

You will not be directly asked about the science behind it.

Various neurotransmitters and hormones have been shown to correlate with aggressive behaviour. The most often mentioned of these is the hormone Testosterone. Scientists have for a long time been interested in the relationship between Testosterone and aggressive behaviour. The Y chromosome is important.

ALL ABOUT TESTOSTERONE

• Babies always inherit an X chromosome from their Mothers. The X chromosome is female and comes in the form of an egg.

•  Babies can inherit an X chromosome or a Y from their fathers. The Y chromosome is male in the form of a sperm. The X chromosome is female in the form of a sperm. The Y chromosome is passed exclusively through sperm.

•  An X from the Mother and an X from the Father = a baby girl XX.

• An X from the Mother and a Y from the Father = a baby boy XY.

Testosterone is an androgen. It is made by Leydig cells in the male testis, as well as the adrenal cortex and ovary of both sexes. Testosterone in men is secreted into the bloodstream in spurts so that levels can change dramatically within minutes. The hormone is released in a circadian rhythm in both sexes, highest and most variable in the morning, lower and more stable during the afternoon. Synthetic Testosterone, such as Testosterone propionate, is a synthetic drug absorbed more easily and has longer-lasting effects than the naturally occurring hormone.

Testosterone has

• androgenic (masculinizing) effects

• and anabolic (protein tissue building) exploited by athletes to build muscle mass, reduce fat, and improve performance.

THE EFFECTS OF TESTOSTERONE

DOMINANCE

Testosterone is closely linked to dominance behaviours, enhancing the desire for social status and authority within groups. This hormonal influence is thought to have contributed evolutionarily to increased reproductive success, as individuals displaying dominance could secure better access to resources and mates.

RISK-TAKING

Testosterone is associated with a heightened inclination towards risk-taking. This trait likely offered evolutionary benefits by rewarding behaviours such as hunting, exploration, and engaging in conflicts, which could lead to significant gains.

AGGRESSION

The hormone has a well-established connection with aggression. Evolutionary theory suggests that aggression facilitated competition among males for mates and resources, where assertive strategies could yield reproductive advantages.

PHYSICAL TRAITS

Higher testosterone levels contribute to distinct physical traits like a pronounced square chin and greater muscle mass. These characteristics are thought to enhance mate attraction by signalling health and genetic fitness and improve outcomes in physical confrontations.

LACK OF EMPATHY

Evidence suggests that increased testosterone levels may correlate with reduced empathic responses. Evolutionarily, this could enable more competitive and aggressive behaviours, enhancing survival and reproductive success by prioritising self-interest.

COMPETITIVENESS

Testosterone fosters competitiveness, urging individuals to succeed across various domains, including sports, social standings, and mate acquisition. Historically, this drive for success could increase access to resources and mates, boosting reproductive success.

EVOLUTIONARY REASONS

The various effects of testosterone on behaviour and physical development are seen as evolutionary adaptations. Historically, These adaptations have enabled individuals to compete for mates, resources, and social status, optimising reproductive success in ancestral environments.

There is evidence that androgens affect aggression:

• aggression and androgen levels covary on a seasonal basis -e.g. red deer

• aggression increases with increased levels of androgen at puberty

• generally, males are more aggressive than females

APFC RESEARCH

Mazur & Booth (review studies showing that men with higher levels of Testosterone are more likely to:

• divorce, or remain single

• be arrested for offences other than traffic violations,

• to buy and sell stolen property,

• incur bad debts

• use a weapon in fights

APFC RESEARCH

Reducing testosterone in alpha male mice by castrating them eliminates their dominant social status, and restoring testosterone through injection causes them to regain their social status.  

ANALYSIS

However, administering testosterone to males with less social status does not usually allow them to take over the alpha male position, indicating that there is not a direct relationship between testosterone and position in the dominance hierarchy.

There is some evidence in humans that high testosterone males are more likely to be socially aggressive, but no evidence that they are necessarily more violent. Often, they are successful in professions that thrive on competition, such as successfully leading a company, running for president, or pursuing a sports career. Also, a few psychologists have suggested that females are not necessarily less aggressive than males; they display different aggression. Females are more likely to show non-violent types of aggression, such as ostracizing their peers or spreading false rumours with the intent to cause pain. Thus, while there does seem to be a connection between testosterone and physical aggression, a person's testosterone level will not necessarily be a good predictor of aggressive behaviour.

Social competition and testosterone levels

The reverse direction of causality – from social aggression to testosterone levels – seems stronger. Men’s fluctuating short-term testosterone levels respond to competitive situations, such as tennis or a wrestling match, a chess game, or a competitive task in a psychology laboratory. Levels rise in preparation for the competition and then go up afterwards in winners and down in losers. This effect does not depend on direct aggression. It applies to changes in an individual’s perceived status in a social hierarchy. Winning or losing a physical fight often has that effect, but so do other competitions. The testosterone high of competitive victory has been measured in males participating in a ceremony to receive their MD degrees and even in sports fans when their team wins. (One study of US males in various professions found the highest testosterone levels among trial lawyers and lowest levels among ministers – i.e., among the most and least competitive professions.)

The effect on testosterone levels depends on subjective judgments about triumph or defeat and is strongest when a victory is decisive and results from an individual’s efforts. For example, testosterone changes after professional basketball games correlated not with the game’s outcome but with the player’s assessment of his contribution to a win or loss and his attribution of the outcome to internal or external causes. Similarly, post-match testosterone significantly correlated not with the outcome but with the individual’s satisfaction in judo competitors.

Outcomes of aggressive interactions affect testosterone levels among animals. When male rodents fight over status and territory, the fight's winner produces more testosterone, and the loser produces less. In rhesus monkeys, researchers studied whether testosterone levels, before forming a group from unfamiliar males, would predict the eventual status hierarchy that emerged in that group. They did not. But once that hierarchy was established, the testosterone levels in the top monkey rose dramatically, as much as tenfold. After fighting, defeated males’ testosterone levels dropped to 10–15 per cent of the prior level. In one study, the top quartile in the dominance hierarchy had significantly higher testosterone levels than the other three-quarters. In long-established and stable hierarchies, however, high-ranking and low-ranking males did not differ in testosterone levels. Thus, testosterone levels appear to reflect status changes – i.e., winning and losing. Similarly, their testosterone levels were unaffected in experiments where male monkeys displayed aggression but did not win or lose an encounter.

In one pleasant experiment, five men were confined on a sailboat for 14 days and had their testosterone levels monitored. They had similar testosterone levels before and after the trip. Still, towards the end of the trip, the higher-ranking men (in the social hierarchy that emerged during the trip) had more testosterone than the others. These results parallel those in rhesus monkeys.

Another experiment found that men’s testosterone levels are higher than usual during and immediately after having intercourse but only slightly higher, if at all, after masturbation. This suggests that testosterone levels respond not just to the physiology of sex, but to contextual aspects such as cultural meanings, feelings, or pheromones. Perhaps even sex (intercourse) is counted under competition, but not masturbation.

Since winning social conflicts increases testosterone levels, winners are presumably more sexually motivated than losers. In some species, high-status males who win conflicts (and, sometimes, control territory) do most of the breeding. This may be the original evolutionary reason for testosterone to rise in winners – a higher status in the social hierarchy implying more sexual opportunities. The lingering effects on our physiology could help explain both Henry Kissinger’s claim that “power is the great aphrodisiac” and the expansive sexual proclivities of many male political leaders.

Does the testosterone response to competition occur in women and men? Evidence is scant but suggests it does not. Testosterone levels rose before a male–male competition in a video game but not before a female competition. (Neither gender showed a post-outcome response in this experiment, however.)

But Testosterone levels are higher in women who commit aggressive crimes or are deemed aggressive by their peers than in non-aggressive females. However, no specific response of Testosterone levels to competition was observed in female athletes, although a mood difference was noted.

One problem in this area is the time that elapses between the aggressive act(s) and the analysis of hormone levels. Nevertheless, it does appear that there is a positive correlation between violence in male and female prisoners and circulating levels of androgen. However, it is unclear whether androgens:

• facilitate aggression or,

• encourage social dominance, competitiveness, and impulsiveness

Mazur & Booth argue that "high levels of endogenous Testosterone (T) seem to encourage behaviour intended to dominate -- to enhance one's status over -- other people. Sometimes dominant behaviour is aggressive, its apparent intent being to inflict harm on another person, but often dominance is expressed non-aggressively. Sometimes dominant behaviour takes the form of antisocial behaviour, including rebellion against authority and law-breaking... T not only affects behaviour but also responds to it. The act of competing for dominant status affects male T levels in two ways. First, T rises in the face of a challenge as if it were an anticipatory response to impending competition. Second, T rises in winners and declines in losers after the competition. Thus, there is a reciprocity between T and dominance behaviour, each affecting the other. "

There may be important individual differences in our response to Testosterone. In a recent report, Harrison et al. (2000) administered Testosterone to 56 men aged 20 to 50. Testosterone treatment significantly increased aggressive responses to a frustration-inducing computer game involving a fictitious subject. This effect, however, was not uniform across individuals; most showed little psychological change, whereas a few developed prominent effects. The Handbook of Crime Correlates, a review of crime studies, states most studies support a link between adult criminality and Testosterone. However, the relationship is modest, and nearly all studies of juvenile delinquency and Testosterone are not very significant and only correlational.

Most studies have also found Testosterone to be associated with behaviours or personality traits linked with criminality, such as antisocial behaviour and alcoholism. Which of these variables is the most important?

In one source, it was noted that the concentration of Testosterone most clearly correlated with aggressive responses involving provocation. In adulthood, it is clear that Testosterone is not related to any consistent methods of measuring aggression on personality scales, but several studies of the concentration of blood Testosterone of convicted male criminals who committed violent crimes compared to males without a criminal record or who committed non-aggressive crimes revealed in most cases that men who were judged aggressive/dominant had higher blood concentrations of Testosterone than controls.

According to the reciprocal model, Testosterone levels vary as a function of a person's dominance.

In a study of marital status among 2,100 male Air Force veterans who received four medical examinations over ten years, Testosterone levels fell and remained low with marriage and rose with divorce. These results are consistent with the reciprocal model (Mazur & Booth.

In short, for those who find the above a little challenging

 •       Thought to act on areas of the brain which control aggression from young adulthood onwards

•       Thought to be the primary biochemical influence on aggression

•       Dabbs et al. (1987) – salivary Testosterone levels were able to differentiate between violent and non-violent crimes

•       Lindman et al. (1987) – aggressive behaviour in drunk males positively correlated with Testosterone levels

•       Wagner et al. (1979) castrated male mice showed decreased aggression which then increased when given Testosterone again

•       Archer (1991) and Book et al. (2001) – meta-analyses with slightly different results

•       Wingfield et al. (1990) – argue that Testosterone levels rise in monogamous species in response to social challenges

Experiments in rodents suggest that androgens increase aggression in male and female mice

Male aggression & Testosterone in male mice

• castration reduces aggression

• Testosterone restores aggression in castrated mice

• androgens have an organizational effect on aggression in infancy

• exposure to androgens in infancy increases the sensitivity of the adult brain to androgen

There is evidence that high levels of Testosterone are necessary but not sufficient to trigger aggression. There are individual differences in the aggressive behaviour of mice. In one experiment, male mice were rated as aggressive or non-aggressive. They were then castrated, which reduced their aggression. When they were given Testosterone replacement therapy, only those mice who were initially rated as aggressive showed a restoration of aggressive behaviour. In other words, Testosterone is necessary for aggressive mice to exhibit aggression, but injecting Testosterone is not sufficient to turn a previously non-aggressive mouse into an aggressive mouse.

Can you think of an explanation for this finding based on the idea that early exposure to Testosterone organizes the brain?

Female aggression & Testosterone in rats

Redrawn from Carlson (1998)

In female mice

• Testosterone - but not oestrogen - increases aggressiveness in ovariectomised females (van de Poll et al., 1988)

• Exposure to androgens prenatally - due to intrauterine position - increases aggressiveness in adulthood.

Evidence conflicting on the role of Testosterone

Studies showing a positive correlation between small samples and self-report measures

Studies are mainly correlational (e.g. Wagner), so it is impossible to conclude that Testosterone causes aggression.

Also, Testosterone is not always associated with negative characteristics: improved sporting and spatial abilities have also been found.

Need to distinguish between aggression & dominance.

The issue relates to the internal validity of the research as studies may be measuring different things, making a comparison of results difficult.

Gender bias is evident as most research is done on males (animals and humans) whereas it is known that there are differences between the genders in androgens.

Also, cultural bias, as most research conducted in Western countries

Physiologically reductionists consider the role of biochemistry alone

Also, we need to consider genetic factors and brain structure

E.g. Phineas Gage suffered a brain injury and showed heightened levels of aggression.

Also need to consider the contribution of environmental factors such as situational cues, temperature, noise, overcrowding, and the role of learning.

A very important & useful area of bio-psychological research and theory as aggression is associated with many anti-social phenomena in society, e.g. numerous forms of crime, violence

If the role of biochemistry can be understood, it can perhaps be treated or managed, but is this a scary thought?

Although there would be ethical issues associated with giving people drugs to alleviate aggression (i.e. for social control), or male castration, even if it may be in the interests of multiple parties

Could perhaps be treated  more ethically through diet (tryptophan) and exercise

Also, useful data is very often deterministic in that it provides us with a prediction about what factors will cause people to turn to crime.  Daly and Wilson found that gender is a major determining factor in causing people to turn to crime due to males taking more risks. This suggests that males will always be at significant risk of turning to crime as they are inherently more likely to take greater risks than females, so data is more useful to society as it allows for practical solutions to be created to help train males to avoid high-risk situations.  On the other hand, this ignores individual differences due to factors such as personality and the social situation in which a person is in. This means the data is less useful as it may not apply to all males.  This means that many factors may contribute to why people turn to crime, so research focusing on only one factor, like gender, will not always be useful.