Does Having Boys or Girls Run in the Family?

Several years ago, the first author’s sisternoted, without even a trace of doubt,that “Rodgers men produce boys.” Inever take those kinds of statementsfrom my sister lightly. She is a journalistwho writes popular articles about familyand reproduction, and her husband is anM.D. who specializes in infertility. Fur-ther, she had the data on her side. At thatpoint, the eight Rodgers men with chil-dren from the past four generations hadcollaborated with eight different womenin the production of 24 biological chil-dren; 21 were boys and 3 were girls.Although she had not run the analysis,my sister’s statistical intuition was excel-lent. In the United States, approximately51% of the babies born are males. Undera binomial model of sex outcomes, hav-ing three or fewer boys out of 24 chil-dren would happen by chance aroundtwice in 10,000 families.Like my sister, many mothers andfathers believe that a tendency to haveboys or girls runs in a family. Informally,we have noted that belief among manyof our friends. Pregnant women appearparticularly interested in and amenableto the notion that sex composition runsin the family. But research in the statis-tical and cognitive psychology literaturesuggests that humans are notoriouslybad at distinguishing systematic pat-terns from random patterns. Even if thesex selection process is purely bychance, some parents will have all boysin families of size 1, 2, 3, or even 10 or12. For example, mothers of four chil-dren who have all boys or all girls mustnaturally wonder if something system-atic contributed to their “unusual sexcomposition.” Yet, around 1/8th of allfour-child families are expected to be asame-sex family under a chance model,not an especially unusual occurrence.Yet, even parents with Ph.D.’s in statis-tics must be inclined to wonder whethersuch extreme outcomes are caused byan unusual chance event or by a bias incertain fathers or mothers (or combina-tions) to produce one or the other sex.Some dice really are loaded.Many factors have been identifiedthat can potentially affect the human sexratio at birth. A 1972 paper by MichaelTeitelbaum accounted for around 30such influences, including drinkingwater, coital rates, parental age, parentalsocioeconomic status, birth order, andeven some societal-level influences likewars and environmental pathogens. A1997 study in