I just realized today that we don’t seem to have any identical twins in the NH. Thinking about this a little bit more, I realized that I could look up a list of 1,000 geniuses from various fields and find them distinctly under-represented (should be around 2%). Hmm…
The Yoruba have the highest rate of twinning in the world, at 45–50 twin sets (or 90–100 twins) per 1,000 live births, possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may stimulate the ovaries to release an egg from each side.
In Central Africa there are 18–30 twin sets (or 36–60 twins) per 1,000 live births. In Latin America, South Asia, and Southeast Asia, the lowest rates are found; only 6–9 twin sets per 1,000 live births. North America and Europe have intermediate rates of 9–16 twin sets per 1,000 live births.
Multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting only 37 weeks (3 weeks less than full term) on average.
Women who have a family history of fraternal twins have a higher chance of producing fraternal twins themselves, as there is a genetically linked tendency to hyper-ovulate. There is no known genetic link for identical twinning. Other factors that increase the odds of having fraternal twins include maternal age, fertility drugs and other fertility treatments, nutrition, and prior births.
This is remarkable, given our reliance on twin studies in genetic studies of intelligence (et al). I’ll be the first to point out that genius is not strictly intelligence, but it also seems to be the case that twins do not often have extraordinary intelligence either.
A brief scan through Terman’s study of 1,000 gifted children suggests only one pair of twins meeting the selection criteria. Two more might have had twins with low enough ability that they weren’t selected with their siblings, suggesting that (given high heritability) the selected sibling was probably in the 135-145 cadre.
There are a number of possible explanations, all of which seem to suggest that twin births are an R-selected trait.