The small West African nation of Benin has a higher concentration of twins than any other country in the developing world.
That's according to a study published in PLOS ONE, which surveyed twinning rates in 75 low and middle income countries from 1987 to 2010.
The researchers found that Benin had a rate of 27.9 twins per 1,000 births. The global average among the countries surveyed was 13.1 twins per 1,000 births.
Factors that can contribute to twinning, according to the study, include the mother's age, height, previous pregnancies, oral contraceptive use, and heredity — which is why twins are said to run in families.
Previous research has suggested that fraternal twins are more common in thriving areas, since there is enough food and opportunity to raise two babies at a time instead of one. If times are tough, a 1998 study in Nature found, the genetic tendency to have fraternal twins becomes less likely to be passed down through families.
Identical twins, meanwhile, are most likely the result of a birth defect very early in development that causes the same embryo to divide — so rates of identical twins are constant worldwide. The difference in twin rates depends almost entirely on fraternal rates, the researchers report in the PLOS ONE study, which appears to be the most comprehensive study of twinning rates in the developing world.
Here's a look at all of the countries sampled in the study, with the darker colors representing higher twin rates:
'The Land of Twins'
Benin and the cluster of countries nearby is an area that has been spotlighted many times before for what appear to be staggeringly high rates of twins.
Just east of Benin, the westernmost regions of Nigeria have some of the highest twin rates in the world. Igbo-Ora, one such community, even has a sign proclaiming that it's "The Land of Twins," according to reports by AFP and the BBC. In 1987, a Los Angeles Times reporter went to a nearby village to explore "the mystery" of all the area's twins.
Scientists have puzzled over this mystery for decades.
A 2008 study in the Indian Journal of Human Genetics looked at 10 years of births in four southwestern Nigerian hospitals and found a twin rate of 40.2 twins per 1,000 births. As far back as the 1970s, researchers reported that twin rates in Nigeria dwarfed those in Europe and the US.
Researchers have long noticed that the Yoruba tribe — whose members live in Nigeria, Benin, Togo, and elsewhere — is a world leader in twins. A 1989 study in the Journal of Reproductive Medicine noted that their twin rate at the time was more than four times higher than that of Caucasians.
And yet, somewhat puzzlingly, this pocket of fraternal twins seems to counter received wisdom on twins occurring more often in prosperous regions: Most of the people in the Nigerian villages that have been closely studied are subsistence farmers, as the BBC World Service reported. The explanation may be largely genetic.
Years ago, researchers suspected the traditional diet of the Yoruba — and the consumption of a yam-like vegetable called cassava in particular — might play a role. But while diet has been linked to twinning rates, "nobody has provided any scientific explanation or evidence that could prove that yam consumption can cause multiple births," Robert Asiedu, a yam specialist, told Science in Africa magazine.
Rising twin rates in Europe and the US
The PLOS ONE study looked at 75 low and middle income countries. If it had included so-called developed countries, the top twin producing nations might have been quite different.
The use of assisted reproduction methods, like in vitro fertilization (IVF), has become increasingly popular in Western countries along with the rising age of the average mother. Both are likely contributing to a significant — and less mysterious — rise in multiple births.
A 2006 study fittingly published in the journal Twin Research and Human Genetics tracked the rise in twin rates in Europe, Canada, and Australia from the 1970s to the 1990s and found the rate was increasing more for older women. Denmark's twinning rate in the 1990s was the highest of the countries studied (17.98 per 1,000 births) while Hong Kong's (8.40) was the lowest.
Since then, fertility treatments and older mothers have become much more common in the US, and multiple births here are climbing:
In the United States, the twin rate in 2013 was 33.7 per 1,000 births, according to the US Centers for Disease Control and Prevention. By contrast, researchers said in the PLOS ONE study that the high twin rates in the countries they sampled were due to natural causes, since fertility treatments are not yet widespread in most of the developing world.
