Beware of testosterone-fuelled leaders and violent men in the summer. Watch out especially in August, the peak time of the year for starting wars and invasions, as well as for individual acts of aggression, from assault and rape to aggravated burglaries.

In the research, a team from Ben-Gurion University in Israel analysed data on violent and non-violent crimes from four continents, and historical records for the starting dates of more than 3,000 wars or acts of hostility.

Results for individual violent offences did show a pronounced annual rhythm. In the Northern Hemisphere, violent crime peaked in July and August, and was at its lowest from December to February. In the Southern Hemisphere, the reverse was true, with a peak in December-January and a low point in June and July.

The researchers found that violent crimes happened two to three times more frequently during high summer in both hemispheres. In contrast, non-violent offences were distributed evenly throughout the year and showed no seasonal rhythm or link with the amount of daylight hours.

Next, the team turned to the timing of the outbreaks of wars, and a similar pattern was found. North of the Equator, there was a peak in August and a nadir in December and January.

The brain-chemical mechanism that could be at work in these instances is not known, but the prime suspect is serotonin. Aggressive soldiers tend to have lower levels of the neurotransmitter, as do murderers and people who commit suicide.