A few years ago a story came out about a town populated by Germans in Brazil which exhibited a tendency toward twinning. The combination of Germans, Brazil, and twins, naturally meant that Josef Mengele came into the picture. A more prosaic explanation for the twinning, favored by locals, was that it was something environmental, like their water. The oddity warranted coverage by National Geographic, and you can imagine what the British press did with the story. At first I thought I saw references to elevated frequencies of identical (monozygotic) twins, which would have have been strange indeed. Twinning varies across populations and families, but that variance tends to be of the fraternal (dizygotic) variety. Some of this is heritable, but some of it is clearly due to environment. Specifically, nutritional inputs that increase levels of insulin-like growth factor, which is found in milk and meat (I suspect this explains the higher twinning rate in Northern Europe vis-a-vis Southern Europe). This doesn’t even go into other factors brought on by modernity, such as delayed childbearing and fertility technology.

But in any case, it turned out that the Brazilian twins were ...