While it has long been observed that twins are more prevalent in certain parts of the world, a new comprehensive research study establishes some interesting trends about concentrations of multiple birth. In the past, data was not consistently reported, but this new report consolidates birth rates from more than 75 countries between 1987 and 2010.
The average rate of twins is 13.6 twins per 1,000 births. That rate holds fairly true for developed countries in North America and Europe.
So where are the twins?
Benin: The Central African country of Benin produced the highest rate of twin: 27.9/1,000. While all of Central Africa has a higher than average twin rate (18/1,000), Benin reported even more twins! Like Nigeria, long recognized as a hotbed of twinning, Benin is home to many families from the Yoruba tribe. Some attribute their abundance of twins to diet; the cassava, a type of yam, may induce hyperovulation.
Where aren't the twins?
Twinning rates are lowest in Asia and Latin America, with an average rate of 8/1,000 births.
The study doesn't clarify any of the causes of twinning, and it doesn't distinguish between monozygotic (identical) or dizygotic (fraternal) twins. In general, the rate of identical twins remains quite low (3.5/1,000 births) and consistent across populations and throughout time. Fraternal twins, on the other hand, are influenced by many factors, such as heredity, diet, reproductive assistance, and maternal age.