Twins attract double takes.
They appear extraordinary, maybe even a little strange. Just how alike are they, we wonder? How different?
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Twins gathered to watch a parade during the annual Twins Days Festival in Twinsburg, Ohio. Studying identical and fraternal twins allows scientists to assess the influence of genetics and environment on many aspects of human health and behavior.
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Twins challenge our beliefs about individuality and the idea that every person is unique, says Nancy Segal, a professor of psychology at California State University Fullerton and the author of two books on the subject, “Entwined Lives” and “Indivisible by Two.”
“Their lives are hard to imagine,” she says.
But scientists like Segal, who directs Fullerton's Twin Studies Center, are trying to understand what makes twins alike – and different. These studies, in turn, reveal much about the rest of us.
In their genetic novelty, twins present a singular research opportunity. The first proposal to use them as living laboratories dates to A.D. 415, but it was Charles Darwin's cousin – a 19th-century English polymath named Francis Galton – who firmly established the concept that studying twins could answer profound questions about all of humanity.
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On the Web: For information on the California Twin Project, visit: twins.usc.edu/
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Writing in 1875, Galton said the stories of twins afforded the “means of distinguishing between the effects of tendencies received at birth, and those that were imposed by the special circumstances of their after lives.”
In other words, nature versus nurture. By examining and comparing identical and fraternal twins, Galton theorized, it should be possible to determine how much of human behavior is genetically based and how much derives from learning and the environment.
Twin studies are now the oldest research method used by behavioral geneticists. “Twin research has given us virtually irrefutable evidence that genes affect most of our behaviors,” said Segal.
But more than that, said Dr. William S. Kremen, an adjunct professor of psychiatry at UCSD who is conducting three longitudinal twin studies of the aging process, “Twin studies provide the strongest evidence for the influence of the environment.”
Twin studies may not resolve conclusively the nature-versus-nurture debate, but they are arguably the single best tool for the job – by a factor of two.
The eggs and I
No one knows exactly why twins exist, what causes a fertilized egg (called a zygote) to split in the first two weeks after conception into two separate entities. But it happens, about once in every 250 births.
The result is identical twins, or what researchers prefer to call “monozygotic” or MZ twins. Such twins aren't really identical. They can be affected differently by genes, the environment or both, before and after birth. But generally speaking, MZ twins share 100 percent of the same DNA.
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The annual festival in Twinsburg attracts hundreds of twins - and also many of the scientists who study them. Dozens of twin registries exist around the world to collect data for researchers.
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One in three twins is an identical twin. The other two are “dizygotic” (DZ) twins, or fraternal twins – something of a misnomer, since fraternal twins can be either male or female.
DZ twins are produced when a woman simultaneously releases two eggs, both fertilized. Fraternal twins share half of their genes, just as ordinary siblings do.
It is the genetic difference between identical and fraternal twins that is fundamental to learning from them. When a researcher investigating a specific trait compares similarities between sets of identical twins to similarities between sets of fraternal twins, any excess likeness between the identical twins can be attributed to genes rather than the environment.
In recent decades, twin studies have been used to disentangle the environmental and genetic components of an astounding array of behaviors and traits, from aggression to intelligence to schizophrenia to alcohol and tobacco dependence.
They have been used to probe more unusual phenomena: the inheritability of divorce, humor, odor perception, even love. In the last case, for example, researchers at the Minnesota Twin Family Study, created in 1983, wondered whether identical twins shared criteria for mate selection just as they shared genes.
The scientists found that MZ twins are no more alike in their mate preferences than DZ twins, and hardly more alike than random pairs of people. Indeed, two-thirds of the surveyed twins said they were either indifferent to or actively disliked their twin's mate.
Wrote the somewhat surprised scientists: “Our choice of mate is one of the most important choices we ever make, and yet it seems to be a random affair, not strongly influenced by genes or upbringing. The person that we end up with is determined by a kind of lottery, by whom we happen to be near when Cupid's arrow strikes.”
More often, though, twin studies have revealed that genes measurably affect behavior, from food preferences to sports participation to religious beliefs. The degree of that influence varies by trait. Intelligence appears to be strongly heritable; feelings of job satisfaction do not.
There's an important caveat here, researchers caution. The science of heritability is often misunderstood. Studies indicating genetic differences in the IQs of races, for example, have been criticized as racist and condemned for undermining the ideal that all humans are created equal.
But twin research, scientists counter, is strictly statistical, a measure of the variance of a trait in a group. It does not assess individual genetic effects or genetic risk. A heritability estimate of 70 percent for obesity doesn't mean 70 percent of a person's obesity is due to genes. Nor does it mean a person has a 70 percent risk of disease. It merely shows the relative influence of genes on that trait.
A twin study cannot speak to the intelligence of a single person. Genes may be the dominant factor, but they do not work alone.
For example, a 1996 French study of twins separated and adopted at birth into different families found that twins who went to poorer families had lower average IQs than their twins adopted by wealthier families. In this case, for poor children at least, environment mattered.
Checkered history
Despite years of use and influence, twin studies remain controversial. In part this is due to their checkered history. Twin studies helped spur the eugenics movement at the turn of the last century. They were used by Josef Mengele, the notorious medical director at the Auschwitz-Birkenau concentration camp during World War II.
Mengele believed twins could unlock the secrets of heredity and help him create an Aryan super-race. During the war, he experimented upon an estimated 3,000 twins, almost all of them children.
Only 157 survived.
Modern-day twin studies are a far cry from Mengele, of course. Dozens of well-respected and scientifically based twin registries now exist around the world, most of them in the United States and Europe. They pursue a wide range of research topics, not just behavioral but also medical and health-related.
Dr. Thomas Mack, an epidemiologist at the University of Southern California, directs several twin registries and studies, including the California Twin Program, which encompasses more than 20,000 twins born in the state.
He says twin studies focused on health may be even more useful than those used in behavioral genetics.
“Many behavioral twin studies must make certain basic assumptions, like supposing that twins grow up in the same environment with the same influences,” Mack said. “That may or may not be true.
“More to the point, they're looking at behaviors, and anything behavioral is confounded. There are factors and variables you just can't know or account for, which means the results tend to be somewhat ambiguous.”
However, Mack added, “twin studies are still a very good starting point for genetic questions and probably the cleanest. Other research methods are likely to be even more confounded.”
Advances in other scientific technologies and methodologies are helping to eliminate some of these issues and generally improve the efficacy of twin studies. Genomics (the study of genes) and molecular biology, for example, allow researchers to look for specific genes and assess how they interact with each other and with the environment.
Such improvements are a boon to researchers. But even without them, there's no disputing that twins are inherently intriguing and endlessly fascinating. Twins alone won't ever fully resolve the conundrum of nature versus nurture, but clearly they raise profound questions of multiple worth.
