The historical bias toward boys—which persists in many parts of the world—likely has to do with parents’ expectations that boys would grow up to be more productive economically. But there are several theories about why this favoritism may have faded more recently.

For one thing, women these days are more likely to graduate from college than men, and despite still facing countless disadvantages, might seem to parents to be a better fit for the modern world and its labor market; disaffected, aggressive young men making news with acts of violence might at the same time be a bad advertisement for their sex. Also, research indicates that parents tend to want their kids to partake in the same hobbies as they do—and the cultural stereotypes around what’s acceptable for girls has expanded in a way that might win over dads imagining the activities they’ll be able to participate in with their daughters, while the stereotypes around what hobbies are appropriate for boys are more rigid.