Identical twins almost always have the same sex, but can have different genders as they grow. In very rare cases, identical twins can be born with different sexes due to genetic mutations. Fraternal ...

New reproductive technologies have yielded a vast array of novel variations on the two conventional twin types—identical and fraternal—as well as some curious twin-like sibships. When a Canadian ...

The special relationship between twins allows researchers to examine the differences between genetic and environmental influences over both physical and mental health, as well as traits and behaviors.

Fascination with twins and what twin studies tell us about human nature is universal. There is no question that behavioral and medical science advances have happened largely because twins yield a ...

Twins are taking over! These days, around 31 out of every 1,000 babies born in the U.S. are twins, according to the Centers for Disease Control and Prevention (CDC). Back in 1980, only about 19 in ...