"Twinning" is genetic and the central African country of Benin has the highest twin birthrate, while Asia and Latin America have much lower rates of twinning. Benin has almost 30 twins for every 1,000 births. Europe, the US and Australia–where most maternal genes are the same–average nine to 16 sets of twins for every 1,000 births.
The high twins rate in Benin is most likely tied to the Yoruba ethnic group, which can be found in Nigeria and Togo as well. the number of twin pregnancies rises substantially with the age of the mother, then decreases after age 38. The number of pregnancies a woman had before a twin pregnancy also appears to play a role, as might things like smoking, contraceptive use and even a woman’s height: the taller you are, the greater your chances seem to be of giving birth to fraternal twins.
In LiveScience.com, Charles Choi quotes sociologist Jeroen Smits as saying, "The twin database allows us to look at mortality difference between boys and girls. If they are born at the same time in a family, the parents directly have to choose between investing resources in one or the other–hence if twin girls suffer more from child mortality or have less chances to go to school than their twin brothers, this is a strong sign of gender discrimination."
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