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 not disproportionately identical. Additionally, as expected there was nothing to the Josef Mengele angle. Now The New York Times is reporting that the researcher who debunked some of the wilder claims, as well as the environmental explanation, is going to present her results:
But a group of scientists now says it can rule out such long-rumored possibilities. Ursula Matte, a geneticist in Porto Alegre, Brazil, said a series of DNA tests conducted on about 30 families since 2009 found that a specific gene in the population of Cândido Godói appears more frequently in mothers of twins than in those without. The phenomenon is compounded by a high level of inbreeding among the population, which is composed almost entirely of German-speaking immigrants, she said.
“We analyzed six genes and found one gene that confirms, in this population, a predisposition to the birth of twins,” Dr. Matte said.
We’ll see. I can totally accept that there’s a genetic basis for this disposition, twinning is heritable. But I’m a touch skeptical that there is such a large effect gene (my skepticism would be modulated if I knew what gene the researcher was talking about, so as I could include into the calculus its functional plausibility). We are not a species which gives birth to multiples habitually, and there’s probably a reason for that. It’s clearly biological possible, but it seems likely that in most pre-modern environments it would not have been feasible to raise two children of exactly the same age simultaneously. So a cultural response would have been to kill one of the newborns (I don’t know whether it’s an urban legend or not that in some societies this was practiced because twins were “bad luck”). This “excess” production is wasteful from an evolutionary perspective, so I presume that there was some lock-down on “hyper-fertility” genes. Singletons has long been our lineage’s ancestral state (only one family of primates produces twins commonly), so a gene of large effect which reorders probabilities of multiples so much seems unlikely to me. I would actually increase the probability that this gene was the real culprit if there was a lot of evidence of negative side effects (e.g., high miscarriage rates), which one might expect when you change the function of an important genetic switch. But who knows what could be happening in highly inbred families, whose genetic backgrounds are special to themselves.