When discussing the biological differences between men and women, one substance stands out: testosterone. Men have about ten times more of this hormone in their blood than women. Testosterone causes boys to grow beards, develop deeper voices, and build more muscle mass. It also drives more risk-taking and competitive behaviour in men.
Testosterone plays a key role in reproduction: it ensures that men are interested in sex and motivates them to compete for the attention of women. Without testosterone, humanity might have gone extinct long ago, but the hormone also causes society quite a bit of trouble.
Assertive Birds
Juvenile delinquents with higher testosterone levels are more likely to be incarcerated for violent crimes, according to research. Stock traders with high testosterone levels take more risks, and according to the author of this study (a former trader himself), testosterone was a major contributing factor to the financial crisis.
Testosterone also explains behavioural differences in animals. For example, mice that were sandwiched in the womb between two brothers provide less parental care to their offspring later on than mice that were situated between two sisters, due to higher prenatal testosterone exposure, says VU biologist Joris Koene.
In bird species that lay eggs over several days, the eggs that are laid later in the sequence, receive more testosterone. Koene explains: “The chicks from the later-laid eggs are at an evolutionary disadvantage because they’re smaller than their older siblings, but that’s compensated for by an increased drive to beg for food – thanks to the extra testosterone.”
Placebo effect
Still, the relationship between testosterone and behaviour is far from straightforward. While some studies show that men with higher testosterone levels are more aggressive and competitive, whether this manifests, depends heavily on context. “If you administer testosterone to subjects but don’t give them any specific task, nothing happens. It only has an effect when the social environment calls for it”, said neuroscience professor Jack van Honk in an article for De Correspondent.
There’s also a placebo effect: participants who believed they had received testosterone behaved more aggressively – even if they hadn’t. Conversely, participants who had received testosterone but believed they had not, behaved less competitively.
Making More Noise
Not only do testosterone levels vary greatly between individuals, so does the brain’s sensitivity to testosterone. The receptors in the brain that respond to testosterone are formed in boys while still in the womb and continue developing in early childhood. These sensitivity differences are partly genetic, but the mother’s hormone balance also plays a role. If a mother experiences high levels of stress during pregnancy – and therefore high cortisol levels – this can disrupt the development of testosterone receptors.
And of course, social factors also significantly shape male behaviour. Boys are encouraged to be tough and less to be empathetic. Boys are more likely to receive negative attention when they behave outside expectations, while girls are more often praised for conforming. These kinds of factors also help explain why men, later in life, tend to make more noise in all sorts of ways.
Back to testosterone. The hormone doesn’t only cause social trouble. It also drives men to protect members of their group, motivates them to act in the group’s interest, and encourages them to put group safety above personal safety.
Preference for spicy food
It almost seems like a hobby for scientists to study the sometimes comical effects of testosterone. The research on the male hormone is so abundant that it could fill libraries. One study, for example, found that testosterone influences men’s self-perception: when given testosterone, participants thought they looked tougher than they actually did.
Another study showed that in a game setting, men given testosterone were quicker to punish fellow players for unfair behaviour but also more generous when treated fairly themselves. Men with elevated testosterone levels also tend to be more honest in games—but worse at collaborating.
They behave more flirtatiously toward women and are more likely to interpret female friendliness as sexual interest. Finally, men with higher testosterone levels have a stronger preference for spicy food.
In short, testosterone influences how a person interprets and approaches the world, shaping their preferences and irritations. Our non-binary illustrator Dido Drachman once described the effect of testosterone gel as: “Suddenly, nothing seemed to matter to me anymore. In a way, it was pleasant – but it was also bizarre.”
Evolutionary one-way street
Why do some animal species have males and females, while others are hermaphroditic and can reproduce with any member of their species – or even by themselves? Evolutionary biologist Joris Koene explores this question.
Once a species has gone down the path of separate sexes in evolution, it’s not easily reversed, Koene explains. In that sense, all mammals and birds are in an evolutionary one-way street.
Many snails and worms, on the other hand, are hermaphrodites – they are both male and female at the same time – just like some fish. They can reproduce with any partner. Additionally, insects like beetles and bees can reproduce asexually: unfertilized eggs become males, fertilized eggs become females.
In certain circumstances, crocodiles and snakes can reproduce without the involvement of males. Some fish can even change their sex during their lifetime, says Koene. All these adaptations can be useful when mates are scarce.