Why Some Families Seem to Have Only Boys or Girls

A Harvard University study is challenging the long-held assumption that the odds of having a baby boy or girl are a simple coin toss. Drawing on records from more than 58,000 women in the Nurses' Health Study, researchers uncovered a pattern, per NPR: In families with at least three children, a "weighted coin toss" of sorts comes into play, meaning that the likelihood of each subsequent child being a boy or girl seems influenced by the sex of previous siblings. If a family's first two children are girls, for instance, there's a higher-than-average chance the next will be, too, and vice versa for boys.

The research helmed by lead author Siwen Wang notes that those odds can be significant. For families with three consecutive boys, the chance of a fourth boy jumps to 61%, and for three girls, the chance of a fourth girl rises to 58%. The study published in the journal Science Advances also found that mothers who start families younger than age 23 have a 40% chance to have either all boys or all girls, while those who begin after age 28 see that number jump to 50%.

Researchers sifted out cases that could skew results, such as single-child families, multiple births, and women using infertility treatments. They also skipped data from each mother's final child to minimize bias from parents stopping after reaching their preferred mix. Why these patterns exist isn't fully clear. The team speculates that factors like maternal age, hormonal changes, and possibly the interplay of both parents' genetics might be involved, but more research—especially involving fathers—is needed. Experts caution that the study's sample, mostly white American women, may not reflect global patterns.