During a meeting of her war council in June 1482, as she plotted the siege of a town in Granada, Queen Isabella went into labor. After a brutal 36 hours, only one of the twins she bore lived, and, days later, her troops returned bloodied and defeated. But the 31-year-old queen proved as tenacious in war as in labor. In less than a decade, Isabella had taken Granada from the Moors, unifying Spain, and launching the rise of history’s first global superpower.

Nowadays Isabella is less known for her conquests than for having paid Christopher Columbus to sail the oceans—maybe because we don’t often think of queens as warmongers.

But apparently, they were. In fact, between 1480 and 1913, Europe’s queens were 27% more likely than its kings to wage war, according to a National Bureau of Economics working paper (paywall). And like Isabella, queens were also more likely to amass new territory during their reigns, found the paper’s authors, economists Oeindrila Dube and S.P. Harish.

But why? A lot of it comes down to the queenly management style—and how radically it differed from that of kings.

The first clue comes from the fact that, of all European sovereigns, married queens were the most bellicose, launching more wars than unmarried queens, and kings of all types. This might be because, thanks to gender norms, women rulers tended to benefit more from marriage alliances than kings. Married queens were likelier than kings to wage war alongside allies, often their spouses’ nations. And queens frequently roped their husbands into helping rule—something that kings hardly ever did with their wives.

Gender norms of the day edged even queens as powerful as Isabella out of the military sphere; though she planned military campaigns and sometimes rallied her troops decked out in armor, it was Ferdinand who led them into battle. Since male royalty tended to hold positions in their home militaries and had experience with state affairs, a queen often gained a husband and a trusted supporter to lead her most important institution. This sharing of duties sometimes made a queendom run more effectively.

For some queens, this cooperation was official. In 16 of the sample’s 34 female reigns, a queen and her husband ruled jointly—as Isabella and Ferdinand did over Leon and Castile, and Suzanne and Charles I did in the Duchy of Bourbonnais between 1505 and 1521. Even without this explicit formality, the husband advantage was useful, though. Think Prince Albert, who served as Queen Victoria’s closest advisor and heavily influenced her management of Britain’s colonies.

Plus, especially in earlier days, it could be hard for a queen to find people to trust. Family members were usually out of the question. (In those days, you never knew who among your kin was plotting a bloody overthrow.) As for husbands, most states banned them from succeeding their spouse unless they had already been named co-monarch. Because they weren’t in competition with each other, queens and their husbands could cut through the intrigue and trust each other. This often brought greater stability to a queen’s reign, while broader alliances and collaborative rule expanded her capacity to organize and finance war.

And starting in the 1500s, that became increasingly clutch. Leaps in weapons technology were making other factors besides the symbolism of leading an army far more critical to conquest—namely, money, men, and resources. Consider that between 1550 and 1780, Austria’s armed forces grew 28-fold.

Centralized states that could collect lots of taxes and marshal the most resources were best positioned to wage war. Paradoxically, though, by forcing women to organize their rule differently from men, gender norms may have strengthened the financial resources and alliances necessary to attack other countries, the authors argue. As it happens, queen-led wars were likelier to result in territorial gain. However, the authors emphasize that larger geopolitical policies—and not just war—were also responsible for these gains.

But given the high number of co-ruling queens, how do we know it’s not the husband making the decisions? To test this, the researchers looked at what they call “solo queens,” women who were either unmarried or whose spouses didn’t hold the title of co-regent. Turns out, solo queens were just as aggressive warmongers as the overall group.

Unmarried queens, however, were more frequently attacked than other types of rulers. This may have had something to do with perceived weakness of female sovereigns. King Frederick II of Prussia, for instance, declared “no woman should be allowed to govern anything” and, after Maria Theresa took the Austrian throne in 1745, promptly seized a chunk of her country. (She fought fiercely but never won it back.)

Even with the days of ruling monarchy long behind us, the dynamics Dube and Harish describe might hold relevance today. The authors note that gendered leadership tends to emerge where institutions are weak such that families “play a role in solving the challenge of who to trust in leading”—a description of dynastic drama that doesn’t sound so very remote to anyone following events in the US.