On the Human Condition
Volume XXVIII Number 2
Gavriel, left, and Yonatan Shere, the author's 5-year-old twin sons
Photo courtesy Jeremy Shere
Photo by Chris Meyers, IU Home Pages
So, how do you feel about having identical twins?" said the lab-coated ultrasound technician, shifting her gaze from the monitor to my wife and her belly bloated by 16 weeks of pregnancy. I was struggling to anthropomorphize the grayish blobs and squiggles of the ultrasound images, so it took me a moment to understand that the technician was being rhetorical. No matter how we felt about it, we were going to have identical twins.
How did we feel? My wife immediately burst into tears. As best I can remember now, after four sleep-deprived years, I didn't feel a thing. Maybe it was shock. Perhaps my brain filtered the unexpected information so that the full meaning of the phrase "identical twins" didn't register. The one thought I do remember was, "that's interesting."
While that may strike some as a rather distant, un-fatherly sentiment, it's also true. After all, identical twins are pretty rare--only one out of every 250 births in the United States involves identical multiples. (The frequency for fraternal twins is somewhat higher.)
Beyond the statistical rarity, there's something eerie about twins generally and downright uncanny about identical twins. They are, after all, natural clones, the products of a single fertilized egg that for reasons unknown to science splits at an early stage of embryonic development. But something even beyond the fact that identical twins share the same exact DNA draws our attention. Why else would Doublemint gum base entire marketing campaigns around the cheeky weirdness of identically dressed identical twins riding tandem bicycles? What else could explain the showbiz success of the remarkably untalented Olsen twins?
Simply put, twins are a bit like a car wreck. It's nearly impossible not to stare and even more difficult to come up with what to say upon encountering a pair. From the very first time we ventured out in public with newborn identical twin boys strapped snugly into their double stroller, total strangers thought nothing of stopping us on the street, staring in wonderment and saying things like, "boy, you've got your hands full," or "double trouble, huh?" However much such attention was unwanted and, eventually, pretty annoying, I can't really blame them. Identical twins are fascinating, for reasons that the average person sometimes finds hard to explain.
Scientists, though, have no trouble explaining why twins are interesting, particularly when it comes to studying genetics, environment, and their complex, interdependent impact on human behavior and development. Since the pioneering work of 19th-century British polymath Sir Frances Galton (half-cousin of Charles Darwin and coiner of the phrase "nature versus nurture"), scientists have used identical twins to investigate the genetic basis of intelligence, personality, perception, and practically every other human trait.
What makes twins so valuable for geneticists, psychologists, and any scientist interested in human behavior is their unique combination of sharing not only environments but also genetic material. Using a research design known as the twin model, scientists interested in determining the influence of genes on a given trait, such as depth perception, can gain unparalleled insight by gathering data from large sets of identical and fraternal twin pairs. Both kinds of twins share birth dates and surroundings, but differ in one crucial aspect: fraternal twins, which arise from two separate fertilized eggs, share about half their genes; identical twins, which result from the splitting of a single fertilized egg, have all of their genes in common. So for any given trait with a significant genetic component, identical twins should be more similar than fraternal twins.
Given the recent mapping of the human genome and technological advances that make such gene mapping more efficient, twins have become more valuable than ever to scientists engaged in renewed efforts to untangle the nature/nurture puzzle. Indiana University scientists are no exception.
Since 1992, Deborah Finkel, professor of psychology at IU Southeast, has participated in a twins study focused on aging, using data collected from thousands of sets of Swedish twins. Due to privacy concerns, most American researchers are unable to access American birth records and other information that would allow them to identify and contact large numbers of identical twins at home. Consequently, Finkel, like most researchers using a twin model, has turned to countries and populations overseas whose governments allow scientists wide access to birth records. Working with the Swedish data, Finkel focuses on how cognitive ability changes as people age, and how both genes and environment affect the change. One important insight that working with twins has revealed, Finkel says, is the mutable nature of heredity.
"Many people think that there is ‘a' heritability and don't think of it as a dynamic process, but it's really a proportion of variance," she says. In other words, insofar as our genes help to determine our personalities and behaviors, they do so in tandem with environmental influences. Plus, the extent to which genes influence our abilities changes over time.
"It takes a while for genes to impact behavior; it's a cumulative effect," Finkel says. "So heritability of IQ can be very low, around 20 to 30 percent at infancy, and then it increases to about 60 percent in young adulthood and adolescence. What we found in the Swedish studies is that heritability continues to increase until very late in adulthood. At about age 50 to 60, we find heritability of about 80 percent."
In simple terms, "heritability" refers to how much genes influence the development and expression of a given trait. In the heat of the nature vs. nurture debate, both the mutable nature of how genes function and the interactive relationship between genes and environment is often overlooked. For example, environmental and biological factors can turn genes on and off. Baldness, for instance, is an inherited trait. But just because you carry a baldness gene doesn't mean that you are born bald, or that you'll go bald right away. Instead, the baldness gene kicks in at some later point. When, exactly, the gene begins to assert itself depends partly on your environment. So "heritability" of baldness, intelligence, and other traits increases and decreases over time.
And, as Finkel explains, the extent to which genes influence our development increases as we get older. When we are infants and children, our parents or guardians choose how we dress, what we eat, where we go to school, and how we spend most of our waking hours. In our early years, in other words, our surroundings play an outsized role in shaping our physical, mental, and other attributes. Genes still play a role, of course--as any parent knows, kids arrive into the world with distinct personalities and dispositions. Finkel's point, though, is that as we rely less on others to make life choices for us, our genes begin to assert more influence on who we are, how we behave, and what we do.
"You choose things that are interesting to you based on your genetic predispositions," Finkel says. "You might choose chess club, or you might choose football. And then those choices impact your intelligence or whatever the trait might be."
Essentially, there's a feedback loop at work. The choices we make as young, and later older, adults are informed, but not determined by, our genetic dispositions. ("Informed" does not mean the same thing as "determined." For example, I cannot, in good faith, blame my genes for my habit of watching Jean-Claude Van Damme action movies at night instead of doing something constructive.)
Genes, as scientists now know, can turn on and off. Puberty and menopause--periods during which we often feel as though we are at the mercy of physical and psychological upheavals beyond our control--are two good examples of particular genes flexing their muscles. But, eventually, after around age 60, the percentage of heritable influences on our behavior and cognitive ability begins to drop off, from its peak of 80 percent to about 60 percent. As we transition from middle to old age, Finkel explains, the accumulations of environmental factors begin to make themselves felt to a greater degree.
"It's a sort of tipping point when things, like how we've abused or treated our bodies well, begin to really affect our cognitive abilities later in life," Finkel says. "Environmental stuff accumulates to the point that, when we're most susceptible in late adulthood, it has a larger impact." This is reassuring, according to Finkel, because it means that we have at least some control over what happens to our mental faculties as we age. A lifetime of exercise, healthy eating, and intellectual stimulation can make a difference, no matter how your personal genome falls out.
It's a Twins Thing
Insights such as the ones Finkel has garnered about the complex interactions of genes and environment would not be possible without the willingness of twins around the world to participate in research. It takes data from thousands of identical and fraternal pairs gathered over long periods of time to determine the heritability of a given trait.
Yet it's entirely too easy to get the wrong impression of twins as individuals. Stories are legion of twins with ESP-like and other extraordinary mental powers--stories that say more about cultural attitudes towards twins than about the reality of being a twin. While it is true that some twins invent their own language, it generally disappears after a short while. And despite sharing the same DNA, identical twins can differ not only in personality but also sometimes in appearance, due to environmental influences in the womb. For example, one of my twins was born with craniosynistosis--a premature closure of the front fontanel (the soft spot near the front of his skull), while the other's bone structure was normal. Surgery fixed the problem, but as a result one twin's head is shaped slightly differently than his brother's.
So twins may be special, but as Richard Rose, IU professor emeritus of psychology and medical and molecular genetics, points out, they are not essentially different from other people.
"Are twins different in terms of intelligence or likelihood of getting genetic diseases than non-twins?" asks Rose, who has participated in studies aimed at determining the genetic component of alcoholism using a Finnish twins model study. "The answer is no--twins are not different in this respect than non-twins. With very rare exceptions, twins are not different in any kind of outcome."
But while twins may not be inherently different from other people, the experience of being a twin sets them apart, as parents of multiples well know. In Entwined Lives: Twins and What They Tell Us About Human Behavior, psychiatrist Nancy Segal recounts the story of identical twins separated at birth who nevertheless made startlingly similar life choices. Both have married and divorced women named Linda, married second wives named Betty, and named their sons James. What can account for this? What's it like being a twin?
For the past four and a half years I've had a ringside seat at the ongoing fighting match that best characterizes my sons' daily existence. As far as I can tell, they are at once the closest of companions and the fiercest of competitors for everything, from who gets the last fruit roll-up to who gets his teeth brushed first. Most of the time, I feel as though I'm on the outside looking in at a relationship I can't really understand, much less control in any systematic way. Plus, at 4 1/2, my boys are too young to let me know what it's really like being a twin.
So, seeking wisdom, last summer I attended the TwinsDays festival in the aptly named town of Twinsburg, Ohio. I went there ostensibly to interview scientists and researchers who had come to the festival from Harvard, MIT, and many other universities to recruit participants from the thousands of twins celebrating the three-day festival. My attention soon turned, however, from the researchers (who were there to explore the relative genetic and environmental influences on various traits) to the twins themselves.
They were everywhere--more than 3,000 pairs dressed in identical outfits. Used to being around my kids and, I thought, no longer capable of being captivated by twins, I was astonished by the sight of teenage twins eating identical corn dogs, middle aged twins with identical beards, and elderly twins hobbling along on identical canes. Even more astonishing was how individual pairs had aged in sync with identically protruding paunches, identically receding hairlines, and in some cases, missing the same teeth.
After several hours wandering among the twins, I began to realize what it is that really fascinates people, scientists included, about twins. They challenge some of our most cherished notions of what it means to be an individual, to be unique and essentially unlike anyone else in the world.
And at the same time, twins reaffirm our individuality. Amy and Lorraine (not their real names), 40-year-old twins from Connecticut who I met at the festival, confirmed as much. They had gone through elementary and high school together but attended different colleges, as Amy explained, to find out who they were when apart.
"Growing up as a twin you share the same birthday, the same clothes, the same everything," she said. "And when you get older you want those things for yourself. But after being in college we realized that even though we were two different people, we really missed the special relationship we had being together."
Amy and Lorraine got me thinking about my kids. Will their identical genes drive them to opposite corners of the world to find themselves? Or will they crave identical environments, end up both marrying people named Jane, and share a duplex? What can I, as a parent, do to better understand the mysterious, genetically symbiotic relationship that I see deepening each day between my sons?
"Human beings are complex," says Deborah Finkel, as though in answer to my question. "The feedback loop between genes and environment is an intricate system, but as long as we have twins to study, we'll keep getting closer to understanding how genes and environment interact."
Jeremy Shere is a freelance science writer in Bloomington, Ind.