The University of Texas estimates that 32 out of every 1,000 people in this world are a twin or 16 pairs in 1,000, which translates to 3 percent of the population. This figure appears to vary by region, race and ethnicity. Certain ethnic groups produce more twins than usual. Others produce less.

While researchers agree that identical twins happen by sheer chance, having fraternal twins is hereditary. If someone else in your family had fraternal twins, then you are more likely to give birth to fraternal twins. If a mother herself is a fraternal twin, the chance of conceiving twins increases four-fold. These factors cause twins to usually run in families and in certain ethnic groups as a whole. In fact, race and ethnicity seems to play a major role in twinning. For instance, in the U.S., twins are most common among African-Americans (36.8 per 1000) and least common in Hispanic/Latino Americans (21.8 per 1000).

A pair of twin girls in the “twin town” of Kodinhi, India. Photo credit