Twins boast considerable entertainment value, to judge from the success of Mary-Kate and Ashley Olsen, among other famous pairs. But they have scientific value as well. Because identical twins have the same DNA, studying them can reveal how much of a given trait is determined by genetics and how much is determined by parenting and environment. That is, twins are helping scientists flesh out the nature-versus-nurture debate. That debate began in the 1870s, when British scientist Sir Francis Galton (inventor of fingerprinting and Charles Darwin’s cousin) coined the phrase. Galton surveyed several dozen twins in an attempt to distinguish between ‘the effects of tendencies received at birth’ and ‘those that were imposed by circumstances of their after lives.’ Galton, not very helpfully, documented the confusion that can arise in families with identical twins. He also concluded that twins can share some unusual traits — such as descending stairs slowly and bursting into the same song at the same time. But he was perplexed by the existence of two types of twins: identical twins, who are mirror images of each other, and fraternal twins, who are no more alike than ordinary siblings. The former come from a […]