Why Sex and Violence Go Together: Insights of Other Species

When young men commit most of the violence, are they acting naturally?

Posted Jul 24, 2015

In a recent post, I addressed links between violent crime and male sexual competition. Now, I want to ask whether animal analogies really illumine human aggression. If a young man kills an acquaintance in a Detroit bar, does this have anything to do with elephant seals battering each other to death in the breeding season?

Most people, including most psychologists, would say that murders in Detroit have almost nothing to do with elephant seals slaughtering each other on an island off the coast of California. Yet, to an animal behaviorist, there are striking parallels with respect to context, motive, and biology.

As to the contextual case (outlined in a recent post), young males are at the highest risk of criminal offending, and violent crime is more common in societies where female sexuality is more liberated.

So violent crime occurs in contexts where men compete over sexual access to women. That leaves motive and biology (including effects of hormones and brain masculinization)


Most crimes of violence are in some sense crimes of passion which means that they often lack objective motivation but are committed for emotional reasons. According to police reports of homicides in Detroit, the majority were classified as “trivial altercation” incidents (1). This means that a fight erupted over some trivial matter such as who jostled whom in a crowded bar, or who was next in line to play pool.

So as far as the police are concerned, most murders are essentially over nothing! Evolutionary psychologists believe that the men were really fighting over face, or status. A young man who backs down in such situations is mocked as a coward and loses status amongst peers.

The real problem about walking away from such confrontations is that a youth who loses in this way is less attractive to women.

Research suggested that young men join gangs so as to obtain sexual access to women. Gang members had more sex partners than non gang members (2). Moreover gang leaders, who got their positions through ruthless aggression, had more partners than rank-and-file members.

The context of violent crime is sexual competition, the motivation for violence is also (indirectly) sexual. What of biology?

The impact of biology relates both to brain development and to the impact of sex hormones at puberty and thereafter.

Brain Masculinity

Male animals are generally more physically aggressive throughout life. Thus hikers fear encounters with bulls but are not threatened by cows. Such gender differences are not universal, however, and there are sex-role-reversed species where females are larger, and more aggressive, than males. Examples include hyenas, and jacanas (birds).

Gender differences in aggression are observable soon after birth and reflect brain masculinzation that occurs in the womb. This conclusion is supported by monkey experiments where females exposed to high levels of testosterone before birth played as boisterously as males subsequently.


A spurt in testosterone production at puberty is responsible for masculine secondary sexual traits, such a growth spurt, muscular development, thickening of the vocal cords, lowering of voice pitch, and growth of facial hair. All of these changes are related to increased aggression and physically intimidating rivals.

Sexual maturation of male mammals is generally accompanied by increased aggressiveness, although this is sometimes contingent on the further spurt of testosterone production in the breeding season for animals as different as robins and camels.

Domestic animal aggression can be a real inconvenience for farmers which is why they castrate males turning them from belligerent bulls into easygoing steers.

That testosterone has analogous effects on the bodies of men and other male vertebrates is not in dispute. Yet, psychologists have trouble admitting that human aggression is increased by elevated sex hormones, acknowledging only that low testosterone reduces aggression.

We do know that violent crime has many associations with testosterone involving the gender and age of violent offenders.

High-testosterone men are more likely to get in trouble with the law and more likely to fight with their wives leading to divorce (3). When men divorce, and begin dating again, their testosterone level rises compared to men remaining married. So does their incidence of violent offending.

Testosterone facilitates aggression but it is rarely the only factor to be taken into account. High testosterone facilitates aggression but that occurs in a context of diminished impulse control that is most often linked to serotonin metabolism in the brain where low serotonin in the brain is correlated with increased aggression (4). So young men are both high on sensation-seeking (linked to testosterone) and low on impulse control (linked to serotonin) during their peak years of criminal offending.

Testosterone is not the only biochemical influence on aggression but when men date, their testosterone level rises (5), and so does their risk of violent criminal offending. In that sense, homicide in a bar in Detroit resembles elephant seals butchering each other during the breeding season.


1 Daly, M. and Wilson, M. (1988). Homicide. Hawthorne, NY: Aldine de Gruyter.

2 Palmer, C. T., and Tilley, C. F. (1995). Sexual access to females as a motivation for joining gangs: An evolutionary approach. Journal of Sex Research, 32, 213-217.

3 Mazur, A., & Michalek, J. (1998). Marriage, divorce, and male testosterone. Social Forces, 77, 315-330.

4 Schulman, E. P., Hardon, K. P., Chein, J. M., and Steinberg, L. (2014). Sex differences in the development trajectory of impulse control and sensation-seeking from early adolescence to early adulthood. Journal of Youth and Adolescence, 44, 1-17.

5 Archer, J. (2006). Testosterone and human aggression: an evaluation of the challenge hypothesis. Neuroscience & Biobehavioral Reviews, 30, 319-345.