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Does the sex of the first child affect the probability of the sex of the second child?
Like my sister, many mothers and fathers believe that a tendency to have boys or girls runs in a family. Informally, we have noted that belief among many of our friends. Pregnant women appear particularly interested in and amenable to the notion that sex composition runs in the family. But research in the statistical and cognitive psychology literature suggests that humans are notoriously bad at distinguishing systematic patterns from random patterns. Even if the sex selection process is purely by chance, some parents will have all boys in families of size 1, 2, 3, or even 10 or 12. For example, mothers of four children who have all boys or all girls must naturally wonder if something systematic contributed to their “unusual sex composition.” Yet, around 1/8th of all four-child families are expected to be a same-sex family under a chance model, not an especially unusual occurrence.