The Testosterone Cycle

Male sexual heat is a common feature of wild animals like deer, elephants, and many bird species. It is related to annual increases in testosterone production that increases aggression and sex drive. What about humans?

Although our species is very weakly seasonal, research finds that there is a distinct peak in testosterone production during the summer months. What, if anything, does that do to human psychology and behavior?

Aggression

Some insight into the behavioral effects of testosterone is provided by seasonally-breeding animals, such as blackbirds and many other birds. In spring, testosterone levels rise and males begin to sing as a warning to competitors to stay away from their territory. Male birds fight with each other to settle territorial disputes.

Anyone who has walked near a purple martin nest box knows how aggressive these little birds can be in mounting repeated “dive-bombing” attacks on the head of the intruder.

Testosterone increases aggression in birds. Among humans, there is a strong correlation between age and acts of violence, including violent crime, such that most violence is perpetrated by males between the ages of 15-35 years when testosterone levels are highest.

Psychologists are fond of claiming that this relationship is not causal based on experiments with rats but there is little doubt that surging testosterone levels contribute to aggression in males, including male mammals whatever other factors may be implicated. Moreover, testosterone has many different associations apart from aggression and sex drive, including sensation-seeking and curiosity.

Sex Drive

The phenomenon of seasonal increases in aggression is widely observed in mammals as diverse as elephant seals, red deer, elephants, and camels. Male elephants in sexual heat, or musth, are extremely dangerous (pictured). The same is true of camels as memorably depicted in the movie Tracks that depicted Australian wild camels literally frothing at the mouth with rage.

Like other male vertebrates, men's sex drive is increased by high testosterone levels in youth and declines with age.

Using a large data set, scientists described an annual cycle in human testosterone with a distinct peak in summer, although earlier research, using inadequate samples, was inconsistent. Is there an associated annual cycle of aggression and sex drive? One approach to this question is to use data on crime.

All violent crimes, including sexual assaults, are more common in summer. In the past, criminologists attributed higher crime rates in summer to the fact that people go out more in fine weather. This would mean that there is more opportunity for criminals to target the public. Similarly, people who leave the relative security of their homes are more likely to become victims of crime and the fact that people spend more time outdoors in summer increases the statistical likelihood of being the target of crime.

Reasonable as this interpretation may seem, it is probably wrong. The main evidence for this is that there is no summer peak in nonviolent crimes, such as petty theft and vandalism and in the US.

The fact that sexual crimes of violence increase in summer suggests that there may be a peak in sexual desire corresponding to the summer peak in testosterone levels.

If this translates into an increased frequency of sexual intercourse, one might reasonably expect to find a peak in births in spring, approximately nine months later.

Yet, that is not what happens. More babies are actually born in the summer months in the US. This would mean that the peak of conceptions is during the short dark days of winter rather than the long bright days of summer. This pattern differs from seasonally-breeding mammals whose births are clumped in spring and lacks a satisfactory explanation either in terms of causal biology, or function. It has been suggested, however, that summer newborns — who have a weak immune response — benefit from avoiding the colds and flus associated with winter.

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Men Are Cyclical

The influence of monthly hormonal cycles on human female psychology and behavior are widely studied and it is clear that women dress more attractively, and behave more flirtatiously, around the time of ovulation (1).

These findings are generally not very popular because psychologists and the general public like to exaggerate the role of choice and freedom in human activity and psychology, and to downplay biology and hormones.

Now, we are learning that men have pronounced annual cycles in their sex hormones that affect their conduct and psychology. In addition to the yearly cycle, there is also a daily one that peaks in the morning. It is not clear what this means but expect self-help publications that help men get through their testosterone day!