It's long been believed that women are less combative than men – more likely to sue for peace than go to war.
But a study reveals queens have been 39 percentage points more likely than kings to wage war over the centuries.
Researchers set out to test whether there was more peace under female or male rule.
US experts analysed mainly European kings and queens between 1480 to 1913 – covering 193 reigns in 18 countries.
The study included Britain's Elizabeth I, who famously defeated the Spanish Armada in 1588, Catherine the Great, who made Russia a formidable power in the 18th century, and Isabella I of Castile who helped Spain dominate the world in the 15th and 16th centuries.
Researchers found that when a state was ruled by a queen it was more likely to participate in conflict than if it was led by a king.
They were also more likely to gain territory – and to be attacked. Experts believe this might be because foreign powers saw women rulers as a 'soft touch' and their countries more vulnerable.
The study was compiled by academics at Chicago University and College of William & Mary in Virginia. They wrote: 'We find that queenly reigns engaged more in inter-state wars relative to kingly reigns. Queens were also more likely to gain territory over the course of their reigns'.
The authors said their findings contradict 'a common perspective that women are less violent than men, and that states led by women will be more peaceful than states led by men'.
One possible reason for the greater aggression of queens than kings is to signal they are not weak.
However, the authors suggested this is unlikely as queens were more warlike throughout their reigns, not just at the beginning, when there would be a greater need to show they are battle-ready.
The study comes after Canadian author Steven Pinker claimed men planned 'almost all the world's wars and genocides'.
US writer Francis Fukuyama insisted that an increase in female rulers has led to a rise in peace around the world.
This article has been adapted from its original source.
© Associated Newspapers Ltd.