A new analysis of sibling records from more than 300,000 individuals suggests that some parents continue to reproduce until they have children of both sexes.

The practice, which the two University of Michigan biologists who conducted the study dubbed “coupon-collection behavior” in human reproduction, appears to have increased in popularity in recent decades and reduces the amount of sex-ratio variation among families.

In a study published Aug. 6, 2020 in the journal Current Biology, Jianzhi Zhang and Erping Long report that significantly more families than expected had children of the same sex except for the last-born child. The trend is more pronounced in the most recent data.

“We believe that coupon-collection behavior becomes popular only when daughters and sons are considered to have similar utility to families, which would require society-wide improvement in gender equality and appreciation of gender diversity,” said Zhang, a professor in the U-M Department of Ecology and Evolutionary Biology.

The coupon collector’s problem comes from probability theory, where it refers to someone who continues to buy a particular brand of cereal, each package of which contains one randomly placed coupon, until he or she collects all the different types of coupons in the set.

Zhang and Long propose that the concept also applies to reproductive behaviors that reflect a preference for having children of both sexes. At the time of this work, Long was a U-M visiting scientist from Sun Yat-sen University in Guangzhou, China.

Their conclusions are based on an analysis of sibling records from the UK Biobank database, which contains genetic, health and family information from volunteer participants in the United Kingdom, the vast majority of whom were born between 1940 and 1970.

EEB recognizes that sex and gender are not binary, and are not necessarily aligned.

Read Michigan News press release