Casey (left) and Kayla Heim, age 9, inhabit a world of twins. Their best friends are twins, and they even have custom-made twin dolls. The sisters are participating in a twin study of family dynamics, cooperation, and other behavioral traits. In childhood, most identical twins display a distinctive cooperative style, unlike fraternal twins who may be just as competitive as other siblings. But Casey and Kayla don’t always agree: “Sometimes I want to play outside, and Kayla wants to play a board game.” And they don’t typically dress alike. “Only on special occasions,” says Casey. “We like different stuff,” adds Kayla. “Her favorite color is pink and
mine is purple.”
All roads lead to me,” says Nancy Segal, as the car turns down a residential street just south of Los Angeles. On her way to visit a favorite set of identical twins, she is not bragging about her status. If you are a scientist or a journalist pursuing the characteristics of twins, sooner or later you will come across Nancy Segal, the doyenne of didymology in America.
The 53-year-old psychology professor is the author of an everything-you-wanted-to-know book called Entwined Lives: Twins and What They Tell Us About Human Behavior. She writes a bimonthly news column about notable twins in academia, the arts, and sports for the journal Twin Research, and she directs her own behavioral studies of twins from the helm of the Twin Studies Center at California State University at Fullerton.
Part of Segal’s authority comes from being a twin herself. Her sister, a lawyer, lives on the other coast, in New York City. The two are fraternal twins and not much alike.
“Ah, this must be the right house,” she says, nodding at a minivan parked in a driveway. “Mothers of twins always have vans.”
Inside the house are 9-year-old Casey and Kayla Heim and their beaming parents. The two girls, who are genetically identical, are dressed alike for the occasion. Monozygotic twins can easily confuse a stranger, but sometimes a clue lies in their mirror-image features, such as hair that whorls in opposite directions or birthmarks on opposite sides of faces. Kayla has a mole on her left cheek, and Casey has the same mole on the right side of her neck. When addressed, the twins speak almost simultaneously, as if there is an echo in the room.
“When’s your birthday?” they are asked.
“November 11th,” they answer. Even the month and day are twins.
Professionals and the parents of twins often remark on the special bond between identical pairs—how as children they act and communicate on the same invisible wavelength. They may even speak to one another in a private idiom. Sitting across the dining room table from the Heim twins, Segal and an assistant are testing the sisters’ cooperation through games called Island Survival and Prisoner’s Dilemma.
In the first game a barrier is placed between them, and each girl, stranded on her “island,” selects three cards from a deck. The cards depict items, like food or matches, that they think they will need for survival. The twins don’t pick the same three items. But in the second part of the game, the negotiation, which is a test to see who dominates whom, Casey and Kayla quickly agree on the three choices.
An extraordinary result occurs in the next game. At Segal’s direction the girls sit with their backs to each other. For each round of the game, they are asked to hold up either a red or a blue flag. The goal is to win the most points. If both display the same color flag, each receives the same number of points. Two red flags are worth one point to each contestant; two blues merit three points each. But a red flag matched against a blue garners the red twin five points while her blue sibling gets none. Thus the game permits either competition, if a red twin pulls the rug out from under a blue, or bland cooperation.
Time after time Casey and Kayla put up red flags. (As they acknowledged in the review session afterward, they intended to tie each other or to let the other twin win.) But suddenly, having raised a dozen red flags in a row, each girl throws up a blue. Jaws drop around the room. How did they do that? When the game ends after 25 rounds, their scores are exactly tied.
Segal doesn’t try to explain such spookily synchronized outcomes, but her studies suggest that the harmony between identical twins stems in large part from shared genetic identity. That is, their cooperation is inherited as much as it’s learned. Other students of twins have found strong genetic components of behavior, personality, and intelligence. Twins also shed light on the heritabilities of more objective conditions like heart disease and the physical capacity for exercise. In short, virtually any human trait, when brought into focus through the dual lenses of twins, can be traced to the hardwiring of genes.
“They are a beautiful experiment in nature,” says Richard Fabsitz of the National Heart, Lung, and Blood Institute. Fabsitz has followed the risks of heart attack, stroke, and now Alzheimer’s disease in a group of twins who served in World War II. “They’re born on the same day,” he says, “and they’re reared in the same family. When you study twins, you’re getting rid of the extraneous factors.”
Alike and different in ways that scientists can dissect, twins provide grist for the long-running debate about nature and nurture. The idea to use them germinated 130 years ago in England, when Sir Francis Galton, a cousin of Darwin’s, wrote, “their history affords means of distinguishing between the effects of tendencies received at birth, and of those that were imposed by the circumstances of their after lives; in other words, between the effects of nature and nurture.”
“Twins tell us what might be genetically controlled and what might not be,” explains Fabsitz. “Before we had tools of molecular biology, twins were the early genetic studies.” However, as a body of work it has a horrible skeleton in its closet. In the 1930s and ’40s the Nazis perverted the science of twins to further their racist agenda. For years geneticists have struggled to overcome the stigma of those grotesque experiments.
Today in the United States there are at least a dozen twin registries that researchers draw upon for both biomedical and behavioral studies. The world’s largest collection is in Sweden, with 140,000 pairs on file, counting both dead and living twins. The Danish national twin registry, which is celebrating its 50th anniversary this summer, includes health records of pairs born as long ago as 1870. In Australia, another leader in twin studies, researchers have surveyed 30,000 pairs, looking at everything from their drinking habits to male-pattern baldness to the incidence of melanoma. Sri Lanka, Italy, Korea, and China are just getting twin registries started.