Mind & Brain / Genetics

The 21st-century studies of twins have gotten a shot in the arm from new molecular technologies. By merging national samples into an international database of DNA, twin researchers hope to pinpoint not only the genetic bases of mankind’s common diseases but eventually to locate the sources of traits like the Heims’ unique form of cooperation.

Identical twins are called monozygotic because they spring from a single zygote, the technical name for a fertilized egg. After combining the genes of the mother’s egg with those of the father’s sperm, a zygote starts to develop into an embryo. At some point during the two weeks following conception, a zygote may split into genetically identical halves. These continue growing as twins. Any further splitting leads to monozygotic triplets, quadruplets, and so on.

In fraternal twinning two zygotes occur simultaneously, because a double ovulation has taken place and separate sperm have fertilized the two eggs. Genetically speaking, the relationship between a pair of fraternal twins is the same as between ordinary brothers and sisters. The only difference is that dizygotic (two zygote) twins start life at the same time. Fraternal twins may be of opposite sexes; monozygotic twins are of the opposite sex only in the rarest circumstances.

For nature-nurture researchers, the distinction that matters is that fraternal twins share on average only half the DNA that identical twins do. This permits a simple exercise, known as the classical twin method, in which scientists compare sets of identical twins with sets of fraternal twins. Assuming that the two types of twins display different responses to the same test, a ratio can be calculated between genetic and environmental influences for the trait under study.

The first published demonstration of the method was a study of skin moles. In 1924 Hermann Werner Siemens, a German, theorized that any condition that could be inherited should be more concordant in identical twins than in fraternal twins. Concordance is a measure of the similarity between two individuals. If genes dictated the formation of skin moles, siblings with the same set of genes ought to be more concordant in moles than siblings of different genetic mixtures. Indeed, in Siemens’s survey of twins, the monozygotic pairs—regardless of the number of moles each displayed—were closer in count than the dizygotic pairs. The difference was held to be proof of moles’ heritability.

It is because the environment intervenes that identical twins don’t have precisely the same number of moles. In the broad sense, “environment” is a way of saying that identical twins lead nonidentical lives. Subtle and unequal pressures, starting even in the womb, will cause them to differ.

Since the method for comparing identical and fraternal twins was popularized, both biologists and psychologists have had a field day estimating the relative contributions of genes and environment to the formation of human traits. Because the two forces in the nature-nurture paradigm can be added together to account for the whole, if you determine the one, you can figure the other at the same time. What follows is an arbitrary catalog of results from the literature. It’s important to remember that all numbers are derived from dozens or hundreds of sets of twins and do not describe any one individual.

Height is said to be 90 percent heritable, which means that the environment, such as childhood nutrition, has only a 10 percent effect on whether you’re going to be shorter or taller. General intelligence is about 50 percent heritable, as are various categories of personality. The risk of developing asthma is somewhat genetic, which may surprise those who look for culprits in the environment. The risk of developing autism, according to one study, is more than 90 percent heritable because the concordance for autism is high in monozygotic twins and low in dizygotic twins. The heritable component of cancer is quite low, notwithstanding the discovery of genes for particular tumors. The risk of developing post-traumatic stress disorder after enduring a terrible event has 30 percent to do with your genetic makeup.

Even such subjective traits as job satisfaction are influenced by genes. Whether or not you are happy at work is 70 percent due to the nature of your job and to the aptitudes and attitudes about work that you have acquired since birth. But your DNA may influence as much as 30 percent of your outlook about your job.

To weigh the influence of the environmental half of the nature-nurture equation, twin researchers use a second technique: the co-twin control method. These studies assume that if the genes of identical twins are the same and yet the individuals are somehow different, the environment has to be the decisive factor. Something like a disease or a critical experience has happened in the life of one sibling that didn’t happen in the other’s.

In their decadelong study of chronic fatigue syndrome, an illness of exhaustion and pain affecting mainly women, University of Washington researchers Dedra Buchwald and Jack Goldberg have brought two dozen identical pairs of twins to Seattle for examination. The team previously established through a classical twin study that chronic fatigue has a genetic component. (The identical pairs were more concordant in contracting the illness.) Yet in this particular set of monozygotic twins, one sister had the symptoms and the other did not. Why?

Tami Spangler and Pam Judy, whose maiden name is Nyborg, arrived for testing last February. Forty-two years old, Tami and Pam still live in the same Idaho town where they were raised and see or talk to each other nearly every day. They might have been the Heim girls grown up. On any given topic each woman appears to know what the other is about to say, and currents of emotion, mainly laughter but also tears, break mysteriously through the surface of their conversation. Like other identical pairs, they have a favorite twin story to relate: When Pam went into labor with her first child, Tami, miles away and unaware of her sister’s condition, felt pains in her stomach.