Women exposed to a male co-twin in utero have decreased probability of fertility and socioeconomic success, according to study findings published in PNAS.

For this study, researchers constructed a multivariable linear regression model to study the long-term effects of testosterone transfer in females who are opposite-sex co-twins. The model was designed using a longitudinal data set that consisted of birth, household composition, schooling, and labor market records for all children born in Norway between 1967 and 1978. A total of 13,717 twin births met the following inclusion criteria: one or both twins survived the first year of life, no birth defects were present, and data were available on outcomes at age 32. Of these twin births, 26.7% were female-male pairs.

The results revealed that women with male co-twins were less likely to marry and have children and more likely to experience adverse educational and labor market outcomes compared with women who had female co-twins. In the sample of all twins (n = 6808), female twins with a male co-twin had a 15.2% higher probability of dropping out of high school, a 3.9% lower probability of graduating from college, and were no more likely to pursue degrees in traditionally male-dominated fields such as science, technology, engineering, mathematics, and economics.

Furthermore, the researchers found that female twins had an 11.7% lower probability of ever having been married by age 32 and had an average of 5.8% fewer children. The results of reduced fecundity remained consistent when researchers limited the sample to women who were married by age 32, which demonstrated that the reduced likelihood of having children was not primarily the result of a lower marriage rate. Finally, females with a male co-twin were 3.2% less likely to be in the labor force and had 8.6% lower earnings at age 32.

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The results remained consistent for women whose male co-twin died soon after birth, thereby implicating the role of testosterone exposure as opposed to the methods by which the children were raised.

The researchers noted that their findings “provide empirical evidence, using objectively measured nation-level data, that human females exposed prenatally to a male co-twin experience long-term changes in marriage, fertility, and human capital.”

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Reference

Bütikofer A, Figlio DN, Karbownik K, Kuzawa CW, Salvanes KG. Evidence that prenatal testosterone transfer from male twins reduces the fertility and socioeconomic success of their female co-twins [published online March 18, 2019]. PNAS. doi:10.1073/pnas.1812786116