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Prior studies have shown twinning rates can vary considerably over time in developed nations. However, this new research on the developing world did not show any large changes either up or down. The findings may help answer questions about the causes of twinning, which may range from a mother's age, height and diet to genetic factors that are passed down through the maternal line, as well as mortality differences between boys and girls in certain regions. Until now, scientists had a very incomplete picture of the number of twins around the world. Past research suggests that identical twins occur at a relatively constant rate of 3. For instance, in the United States, Australia and many European countries, they decreased from about 12 or higher per 1, births in the s to fewer than 10 per 1, births in the s and then increased again to values of 13 to 16 in about The Science of Twins ].