Study: Boys Impact the Fertility of Their Female Twins

Girls with male twins have fewer children than girls with female twins, according to a new study in the Proceedings of the National Academy of Science.

Data collected from church records of five parishes in Finland allowed Virpi Lummaa, Jenni Pettay, and Andrew Russell at the University of Sheffield to analyze the rates of marriage and childbirth among 754 sets of twins born between 1734 and 1888. The use of information collected about twins during this pre-industrial time period allowed the researchers to analyze the affect of males on their twin sisters in the absence of advanced health care and contraception.
The study found that adult females with a male twin were 25% less likely to marry than women with female twins. They were also less likely to have children, or rear their children past the age of fifteen years. Additionally, those with children had an average of two fewer children than their counterparts with female twins.

The reproductive success of women with male twins was impacted regardless of whether or not their brother survived more than three months, indicating that their reduction in fertility was a result of prenatal events, and not caused by postnatal social or nutritional effects. The researchers used statistical methods to correct for other potential influences on reproductive success, such as social class and number of siblings. The researchers did not find similar differences in marriage and reproduction rates between males with male and female twins.

How can fetal males affect their twin sisters in the womb? The answer lies in sex hormones. Testosterone produced by males in the womb can diffuse through fetal membranes and amniotic fluid, and into the body of their developing twin. Previous studies have demonstrated that females who shared the womb with a male develop more masculine faces, attitudes, and behaviors, which may affect female attractiveness to males.

Early exposure to testosterone may also increase chances of reproductive cancers and polycystic ovary syndrome in women, which result in lowered fertility. All fetuses-male and female--produce similar levels of estrogen, and are exposed to maternally produced estrogen, which explains why males are not impacted by female twin estrogen production. Similar effects have been found in other mammals and some viviparous (live-bearing) lizards.

Women with male twins may be relieved to know that the study found that having a male twin does not reduce the longevity of women or that of their children.

References:

Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0605875104
(Abstract available at http://www.pnas.org/cgi/content/abstract/0605875104v1)