Why are there more same-sex than opposite-sex dizygotic twins?

Study Question: Are there more same-sex (SS) than opposite-sex (OS) pairs among dizygotic twins?

Summary Answer: In violation of Weinberg's Differential Rule, there are significantly more SS than OS pairs among dizygotic twins in nationally representative samples both in the UK and the USA.

What Is Known Already: Since at least 1874, twin researchers have assumed that dizygotic twins were equally likely to be SS or OS (later formalized as Weinberg's Differential Rule). However, recent research on the sex-specific nature of breastmilk suggests that OS twins might be at a developmental disadvantage relative to SS twins. There may therefore be evolutionary selection for the tendency to have SS twins and against the tendency to have OS twins.

Study Design, Size, Duration: We analyze two population samples (not subject to Lykken's rule of two-thirds): the National Child Development Study in the UK (n = 17 419) from 1958 to present, and the National Longitudinal Study of Adolescent Health in the USA (n = 20 745) from 1994 to present.

Participants/materials, Setting, Methods: We performed a one-sample t-test of the proportion of SS dizygotic twins against the theoretical null value of 0.5022.

Main Results And The Role Of Chance: The proportion of SS pairs among dizygotic twins was 0.6043 (t(325) = 3.838, P = 0.00015) in the UK and 0.5739 (t(520) = 3.398, P = 0.00073) in the USA.

Limitations, Reasons For Caution: Twin type (monozygotic versus dizygotic) was classified by the mother, not by DNA typing, although supplementary data suggest that misclassification was negligible and supplementary analysis shows that potential misclassifications do not change our main conclusions.

Wider Implications Of The Findings: Weinberg's Differential Rule may be incorrect in its assumption of independence of sexes within dizygotic pairs, and there may be evolutionary selection for SS, and against OS, dizygotic pairs.

Study Funding/competing Interests: None.