If some families have more boys and others more girls, it would not be a coincidence.

The researchers started from an observation: they all have a colleague, a friend, or a family member who has had only girls or only boys. Sibling groups of three or four children of the same sex defy the odds: worldwide, the distribution of sexes at birth is almost equal, with, on average, 105 boys for 100 girls in 2020 according to the National Institute of Demographic Studies. How can we explain that these statistics are not verifiable at the couple level? This is what a team from Harvard University sought to understand in a study published on July 18 in the journal Science Advances .

The basic scenario is one that follows a binomial statistical distribution: fertilization is similar to tossing a two-sided coin, with each time there being a 50/50 chance of getting the "girl" side and a 50/50 chance of getting the "boy" side. The researchers analyzed another possibility, the beta-binomial distribution, where each person does indeed toss a coin, but the latter is unique for each individual: " The coin is not weighted the same way depending on your personal traits or environmental factors ," explains Siwen Wang, first author of the study and a doctoral student at Harvard University. Each individual would therefore have a personal probability of having a girl or a boy, more or less far from 50%.

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