Twins multiply data on heredity
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 single fertilized egg; the latter from two fertilized eggs.
Galton focused his research on identical twins, but comparing the two types of twins can tell scientists quite a lot about the heritability of all sorts of characteristics. No one proposed such a study, however, until 1924, when the idea was simultaneously published by American professor Curtis Merriman and German dermatologist Hermann Siemens.
The thinking behind the so-called classical twin study is this: If a characteristic is largely inherited, you should expect to see it in both identical twins far more often than you see it in both fraternal twins. A trait determined entirely by genes, such as hair color, will be shared by identical twins 100% of the time. A trait partly determined by genes will be found in identical twins more often than in fraternal twins. The more correlation there is among identical twins, the more genetically determined that trait is.
Following Merriman’s and Siemens’ proposals, scientists began devising small twin studies in the late 1920s. But before long, it became clear that a twin study required hundreds -- if not thousands -- of pairs to yield any valid conclusions.
Decades later, such massive twin cohorts do exist -- and they’re producing some interesting findings.
The Minnesota Twin Registry, which began in the 1980s, now has data on more than 8,000 pairs of identical and fraternal twins. The twins have enabled Minnesota researchers to show, for one, that choice of a spouse or life partner isn’t influenced by genes but that likelihood to divorce is.
British researchers studying their own twin cohort recently found that fear of new or strange foods -- the trait that inclines many children to purse their lips at the sight and smell of Brussels sprouts -- also appears to be at least partially determined by DNA.
Also influenced by genes, according to other studies: political attitudes, voting behavior, television watching and food preferences -- though the heritability of the last appears to vary by food group. Identical twins are more likely to both love protein-rich foods than they are to share a sweet tooth.
Twin researchers have addressed more grave concerns too. Autism, for one, was largely attributed to environmental factors until a 1970s study showed that identical twins were more likely than fraternal twins to share the condition. More recent studies have shown the same for attention deficit hyperactivity disorder and multiple sclerosis.
A few studies have also looked at twins raised separately. The Minnesota Study of Twins Reared Apart found that separated identical twins often chose similar professions and scored similarly on IQ tests.
But if finding identical twins is challenging, finding separated twins is even harder -- which may be one reason a New York City adoption agency collaborated with researchers in the 1950s and 1960s to study twins they deliberately placed in separate homes. (The results of the small study, which caused a scandal, were never published.)
These days, finding twins for study may be getting easier: The rate of twin births is 70% higher than it was in 1980. Because of assisted reproductive technologies, couples are having more fraternal twins. And women who have children later in life -- as more are choosing to do -- are more likely to give birth to identical twins.
