Growing up on a dairy farm isn’t as glamorous as most people think. In between the 5 AM alarm clocks and the backbreaking work there is a need to stay in touch with the cycles of life with the observational rigor of a scientist. A case in point is how farmers have long known of the reproductive characteristics unique to freemartins, which is the term for a female cow that is the twin of a bull. Given the need for farmers to maximize the reproductive fitness of their herds it has long been common knowledge that most freemartins are also sterile.

So imagine my fascination when perusing the Proceedings of the National Academy of Science last month when I stumbled on an article titled “Male twins reduce fitness of female co-twins in humans.” In a brilliantly designed study led by Virpi Lummaa, researchers pored through records meticulously maintained by the Finnish church from 1734 to 1888. Among 754 twins they identified, women who survived to adulthood were 25% less likely to have children if their twin was a male. Those who did have children had two fewer babies on average than women with a twin sister. Women who had a male twin were also 15% less likely to get married.

And the point? Other than giving credibility to the observational powers of farmers this finding points out the importance of the prenatal environment in determining critical aspects of human development. Prenatal hormonal exposures don’t need to be of the magnitude of having a twin who is a testosterone production factory sharing your prenatal environment to have an effect on human development. On a much less dramatic level it has long been known that increased prenatal testosterone exposure results in the index finger (D2) being significantly shorter than the ring finger (D4) in men. The D2/D4 ratio is nearly equal in women. And certainly, there are numerous other examples where prenatal hormonal exposure impacts our development. This study does dramatically demonstrate the possible magnitude of the impact prenatal hormone exposure can have.