The Human Beast

Why we do what we do

Sex, violence, and hormones

Why young men are horny and violent

When male robins enter the breeding season, their testosterone level rises. They become aggressive and amorous. Testosterone has remarkably similar effects on men.

This conclusion does not sit well with most psychologists. Psychology professors emphasize that whereas men with low testosterone levels are generally low on aggression and have a low sex drive, men who are high on testosterone may be neither aggressive nor randy.

Why are we so comfortable with assuming that other animals are controlled by their hormones whereas humans are not? The prevailing view is that human behavior is regulated by higher cognitive processes. Reason cools the blood, allowing us to take responsibility for our actions. This is an ancient philosophical formula that was most explicitly developed by French Philosopher Rene Descartes (1596-1650). Yet, even Descartes did not believe the rational soul was always in charge and accepted that there are times when humans are ruled by their passions.

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Evidence that human behavior is partly governed by hormones is mainly restricted to correlations, which are not the same as causes. At least one experiment found that large doses of testosterone increased aggression but only for a small proportion of men. Young men who use anabolic steroids - a synthetic version of testosterone - are also more likely to be involved in violent crime. Taken together, such findings imply that high levels of testosterone can cause aggression in at least some men. Other evidence points in the same direction.

Young men experience peaks in criminal behavior and testosterone production at around the same ages. When they marry, men experience a decline in both testosterone production and criminal offending when compared with single men of the same age.

It seems that marriage has a civilizing effect on men because it reduces their testosterone levels. Of course it would be impossible to randomly assign some men to marry and others to remain single in order to test this hypothesis experimentally.

The next best thing is a natural experiment. When men divorce and begin dating again, their testosterone level rises, just as it does for male robins in the breeding season.

What is more, their involvement in violent crime increases. This is at least partly due to an altered lifestyle with more time spent staying out late at night in clubs and bars where single women are encountered. Increased alcohol consumption is a complicating factor as this clouds reason and impairs judgment.

As correlations go, the link between testosterone and violent crime is arguably as compelling as the link between testosterone and mating aggression for robins and other animals. Indeed, one can argue that most violent crimes occur because of reproductive competition (e.g., male-male assaults and homicides, domestic violence).

What about the link between testosterone and amorousness in the human male? Once again, the evidence is interesting. Male sexual desire is believed to peak in early adulthood at about the same time as testosterone peaks in the lifespan. Recent research also finds that testosterone levels increase when men encounter attractive women and engage in sexual intercourse.

Men are not the same as male robins, of course and you cannot really understand crimes of violence without also studying societal differences (such as the ratio of men to women), contextual factors (such as location relative to a bar), and behavioral issues (such as why someone got pushed while standing in a line). Even so, the correlation between aggression and testosterone has many remarkable similarities across many vertebrate species that happen to include humans and robins. The same applies to sexual motivation, of course. Anyone who leaves out testosterone in their analysis of human male aggression and sexuality can never hope to understand these phenomena in their true complexity which requires comparisons with other species.

Of course, by saying that hormones play a role in human behavior, including violent crime, one inevitably evokes the old canard that testosterone levels are an excuse for antisocial behavior. The fact is that serious crimes of violence occur at remarkably low rates in modern societies. This means that most young men never engage in any criminal violence, however high their testosterone levels. Still, violent crime is largely perpetrated by young men and high testosterone is a factor in their reckless conduct that we ignore at our peril.

Women have passions as well as men of course. In my next post, I demonstrate that womanly passions are also affected by hormones.

 

Nigel Barber, Ph.D., is an evolutionary psychologist as well as the author of Why Parents Matter and The Science of Romance, among other books.

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