A Thing or Two About Twins

They have the same piercing eyes. The same color hair. One may be shy, while the other loves meeting new people. Discovering why identical twins differ—despite having the same DNA—could reveal a great deal about all of us.

By Peter Miller

Photograph by Martin Schoeller

Every summer, on the first weekend in August, thousands of twins converge on Twinsburg, Ohio, a small town southeast of Cleveland named by identical twin brothers nearly two centuries ago.

They come, two by two, for the Twins Days Festival, a three-day marathon of picnics, talent shows, and look-alike contests that has grown into one of the world's largest gatherings of twins.

Dave and Don Wolf of Fenton, Michigan, have been coming to the festival for years. Like most twins who attend, they enjoy spending time with each other. In fact, during the past 18 years, the 53-year-old truckers, whose identical beards reach down to their chests, have driven more than three million miles together, hauling everything from diapers to canned soup from places like Seattle, Washington, to Camden, New Jersey. While one sits at the wheel of their diesel Freightliner, the other snoozes in the bunk behind him. They listen to the same country gospel stations on satellite radio, share the same Tea Party gripes about big government, and munch on the same road diet of pepperoni, apples, and mild cheddar cheese. On their days off they go hunting or fishing together. It's a way of life that suits them.

"Must be a twins thing," Don says.

This afternoon at the festival the brothers have stopped by a research tent sponsored by the FBI, the University of Notre Dame, and West Virginia University. Inside the big white tent technicians are photographing sets of twins with high-resolution cameras, collecting their fingerprints, and scanning their irises to find out if the latest face-recognition software can tell them apart.

"Although identical twins may look the same to you and me, a digital imaging system can spot minute differences in freckles, skin pores, or the curve of their eyebrows," says Patrick Flynn, a computer scientist from Notre Dame. But so far, he says, even the most advanced commercial systems can be tripped up by changes in lighting, facial expressions, and other complications, whether imaging twins or others.

Because their beards cover half of their faces, the Wolf brothers pose a particular challenge. This seems to amuse them. "After they took my picture," Dave says, "I asked one guy if I went out and committed a crime and then went home and shaved, would they be able to tell it was me? He kind of looked at me and said, 'Probably not. But don't go out and commit a crime.' "

Nature and Nurture
Flynn and his colleagues aren't the only scientists at work here. With the blessing of the event's organizers, a number of others have set up booths in a small parking lot on the edge of the festival grounds. In the tent next to the FBI project, researchers from the Monell Chemical Senses Center in Philadelphia are asking twins to sip tiny cups of alcohol to see if they react the same way to the taste. Next to them, doctors from University Hospitals in Cleveland are quizzing twin sisters about women's health issues. Across the courtyard a dermatologist from Procter & Gamble is interviewing twins about skin damage.

To these scientists, and to biomedical researchers all over the world, twins offer a precious opportunity to untangle the influence of genes and the environment—of nature and nurture. Because identical twins come from a single fertilized egg that splits in two, they share virtually the same genetic code. Any differences between them—one twin having younger looking skin, for example—must be due to environmental factors such as less time spent in the sun.

Alternatively, by comparing the experiences of identical twins with those of fraternal twins, who come from separate eggs and share on average half their DNA, researchers can quantify the extent to which our genes affect our lives. If identical twins are more similar to each other with respect to an ailment than fraternal twins are, then vulnerability to the disease must be rooted at least in part in heredity.

These two lines of research—studying the differences between identical twins to pinpoint the influence of environment, and comparing identical twins with fraternal ones to measure the role of inheritance—have been crucial to understanding the interplay of nature and nurture in determining our personalities, behavior, and vulnerability to disease.

Lately, however, twin studies have helped lead scientists to a radical, almost heretical new conclusion: that nature and nurture are not the only elemental forces at work. According to a recent field called epigenetics, there is a third factor also in play, one that in some cases serves as a bridge between the environment and our genes, and in others operates on its own to shape who we are.

The Jim Twins
The idea of using twins to measure the influence of heredity dates back to 1875, when the English scientist Francis Galton first suggested the approach (and coined the phrase "nature and nurture"). But twin studies took a surprising twist in the 1980s, following the discovery of numerous identical twins who'd been separated at birth.

The story began with the much publicized case of two brothers, both named Jim. Born in Piqua, Ohio, in 1939, Jim Springer and Jim Lewis were put up for adoption as babies and raised by different couples, who happened to give them the same first name. When Jim Springer reconnected with his brother at age 39 in 1979, they uncovered a string of other similarities and coincidences. Both men were six feet tall and weighed 180 pounds. Growing up, they'd both had dogs named Toy and taken family vacations in St. Pete Beach in Florida. As young men, they'd both married women named Linda, and then divorced them. Their second wives were both named Betty. They named their sons James Alan and James Allan. They'd both served as part-time sheriffs, enjoyed home carpentry projects, suffered severe headaches, smoked Salem cigarettes, and drank Miller Lite beer. Although they wore their hair differently—Jim Springer had bangs, while Jim Lewis combed his hair straight back—they had the same crooked smile, their voices were indistinguishable, and they both admitted to leaving love notes around the house for their wives.

As soon as he heard about the two Jims, Thomas Bouchard, Jr., a psychologist at the University of Minnesota, invited them to his lab in Minneapolis. There he and his team gave the brothers a series of tests that confirmed their similarities. Although each had charted his own course in life, the Jim twins, as they came to be known, seemed to have followed the same paths.

"I remember sitting at a table with them when they first arrived," Bouchard says. "They both had fingernails that were nibbled down to the end. And I thought, No psychologist asks about that, but here it is, staring you in the face."

Skeptics later claimed that such details were exaggerated or that coincidences were just coincidences. But Nancy Segal, a professor of psychology at California State University, Fullerton, attests to the Jim twins' striking similarity. "I met them maybe a year after they were reunited, and they were absolutely on the level," says Segal, who joined Bouchard's team in 1982. "Even though their hair was different, I couldn't remember who was who."

By then, researchers had discovered other twins who'd been separated as infants and reunited as adults. Over two decades 137 sets of twins eventually visited Bouchard's lab in what became known as the Minnesota Study of Twins Reared Apart. The twins were tested for mental skills, such as vocabulary, visual memory, arithmetic, and spatial rotation. They were given lung-function tests and heart exams and had their brain waves measured. They took personality tests and IQ tests and were quizzed about their sexual histories. Altogether, each twin was bombarded with more than 15,000 questions. "We threw the kitchen sink at them," Bou­chard says.

Armed with this mountain of data, Bouchard, Segal, and their colleagues set out to unravel some of the knottiest mysteries of human nature: Why are some people happy and others sad? Why are some outgoing and others shy? Where does general intelligence come from? The key to their approach was a statistical concept called heritability. In broad terms, the heritability of a trait measures the extent to which differences among members of a population can be explained by differences in their genetics. By comparing the likelihood that identical twins share a given trait with the likelihood that fraternal twins share the same trait, researchers can calculate how much of the difference is due to genetic variation. A person's height, for example, is often estimated at 0.8, meaning that 80 percent of the differences in height among individuals in a specific population are due to differences in their genetic makeups.

When they looked at the data on twins' intelligence, Bouchard's team reached a controversial conclusion: For people raised in the same culture with the same opportunities, differences in IQ reflected largely differences in inheritance rather than in training or education. Using data from four different tests, they came up with a heritability score of 0.75 for intelligence, suggesting the strong influence of heredity. This ran counter to the prevailing belief of behaviorists that our brains were blank slates waiting to be inscribed by experience. More alarming to some, the suggestion that intelligence was linked to heredity evoked the disgraced theories of the eugenics movements of the early 20th century in England and the United States, which had promoted improvement of the collective gene pool through selective breeding.

Date: 2015-02-28; view: 887