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Parallel Lives

Which of us has not wondered how our life might have turned out had certain circumstances been different? Like characters inhabiting parallel universes in science fiction plots, we all live only a tiny fraction of the lives that might have been.

As a teenager I won an acting award, spent two years learning the Russian language (which I have hardly spoken since), and avidly pursued astrophotography (my first publication, at the age of fourteen, was a photograph of an aurora borealis that appeared in Sky and Telescope). Each of these early interests somehow faded and was overtaken by others that ended up having a more lasting impact on my life. Yet had a teacher or opportunity encouraged me to concentrate on any one of these early preoccupations, I suspect that my life, and perhaps even some aspects of my personality, might be considerably different today.

In a reflection on the mysteries of human development, the behavioral geneticist Robert Plomin and the psychologist Denise Daniels once entertained a similar question about the life of Charles Darwin.1 When Darwin was invited to sail as a naturalist on the voyage of HMS Beagle, his hopes, and perhaps with them the revolution that bears his name, were almost dashed. First, Darwin’s father objected to the voyage as a “useless undertaking” that would divert his son from an intended career in the clergy. It was only through the strenuous intervention of Josiah Wedgwood, Darwin’s enlightened uncle, who saw in young Darwin “a man of enlarged curiosity” and who drove thirty miles to confront his father, that Robert Darwin’s objections were finally overcome.2

Then the captain of the Beagle—twenty-six-year-old Robert FitzRoy—balked at the selection of young Darwin. An ardent disciple of Johann Kaspar Lavater’s theories about physiognomy and its relation to character and personality, FitzRoy was convinced that the shape of Darwin’s nose indicated a lack of sufficient energy and determination for such an undertaking. Darwin eventually succeeded in winning FitzRoy over, and FitzRoy himself later came to the conclusion that Darwin’s nose “had spoken falsely.”3 How might the history of science have differed had Darwin not circumnavigated the globe on HMS Beagle, visited the Galápagos Islands in 1835, and developed his earliest ideas about evolution based on his five-week visit to this veritable laboratory of evolution in action? We can only speculate about such counterfactual historical possibilities.

Nancy Segal’s Indivisible by Two makes use of a particularly powerful research method for answering such vexing questions about why our own and other people’s lives turn out the way they do. Segal studies twins—identical, that is, from a single fertilized egg, and fraternal, from two eggs fertilized by different sperm—as well as pseudotwins, children of the same age who are raised together. She does so with a passion that derives in part from the fact that she is a fraternal twin herself. A Distinguished Professor in Humanities and Social Sciences and director of the Twin Studies Center at California State University, Fullerton, Segal has published extensively on twins, including her 1999 book Entwined Lives: Twins and What They Tell Us about Human Behavior.4 That work has been justifiably praised as one of the most convincing scholarly treatises yet written on the subject.

Segal’s principal aim in her new book is “to bring humanity and science together” through expanded biographical accounts of twelve particularly remarkable sets of twins, triplets, and quadruplets discussed in Entwined Lives. In some cases, these involve life histories so remarkable that the standard methods of science could never do them full justice. Laboratory studies, Segal insists, have a valuable place in science, but they unfortunately “miss the vitality of twins’ lives—and maybe some important reasons twins differ.”

Twins provide a natural experiment in the relative influence of genes and the environment, as twin researchers have appreciated since the work of Darwin’s evolution-minded cousin, Francis Galton. It was Galton, in the 1870s, who posed the influential distinction between “nature and nurture,” using predominantly anecdotal evidence about twins to support his own view that most physical and behavioral characteristics are innate.5 Darwin was greatly impressed by some of Galton’s arguments, which indirectly supported his own views about heritable variation among animals and its critical part in the evolutionary process. “Nothing,” he wrote to Galton in 1875, “seems to me more curious than the similarity and dissimilarity of twins.”6

The methods used in twin research, and the quality of the resulting empirical evidence, have improved dramatically since Galton’s time. In particular, the development of twin registries around the world has provided behavioral geneticists with an immensely useful source of willing subjects, who now number in the tens of thousands and have been studied in almost every conceivable detail.

In addition to measuring physical traits, twin studies typically assess cognitive abilities and behavioral attributes using test instruments such as surveys that have previously demonstrated their reliability (by being internally consistent and also by yielding similar results from one testing session to another) and that have also proven their validity (by measuring what they claim to measure). One common method for assessing the relative contributions of genes versus the environment is to compare test results for identical and fraternal twins. Because fraternal twins share, on average, only half their genes, the influence of genes on any particular trait can be measured as twice the difference between the correlations for the two sets of twins. For example, if identical twins raised together correlate .85 for a particular intellectual ability and fraternal twins correlate only .60 for this same ability, then genes would appear to account for 50 percent of the “variance” in these test outcomes. The remainder of the variance is attributable to environmental influences, including chance events, as well as to errors in measurement. Such twin studies do not tell us about all the nuances and idiosyncrasies of individual personality. Rather, they tell us in broad terms that some people are more predisposed than others to behave in a generally outgoing and self-confident manner, for example, as opposed to a shy and anxious manner.

During the last several decades the resulting accumulation of often surprising findings in twin research has had a dramatic influence on thinking about human behavior. Genetic influence is now known to account for between 80 and 90 percent of individual differences in height, which is why identical twins usually differ in stature by less than an inch. Similarly, genes are responsible for about 70 percent of individual differences in weight, about 60 percent of individual differences in general intelligence, and contribute less, but still substantial amounts of variance, to most behavioral traits. According to Segal and other twin researchers, as much as 50 percent of the variance in personality traits appears to be explained by genes, with somewhat smaller degrees of variance accounting for occupational interests (40 percent), social attitudes (30–40 percent), and job satisfaction (30 percent).7

Twin research is not just about proving the substantial contribution of genes to human development and behavior. Such studies can also tell us a lot about the effects of the environment. In one of the biggest surprises in behavioral genetics to date, one set of psychological attributes turns out to be almost entirely determined by the environment, namely, “love styles.” Whether we fall in love gradually or are swept off our feet, for example, is not predetermined by our genes, although no one knows exactly why this is the case since, in statistical surveys of large groups of twins, much else apparently is, including the expression of emotions.8

Segal’s fascinating explanation of the lives and experiences of twins involves her consideration of four kinds of natural experiment: (1) twins who have been separated at birth and have later encountered one another in adulthood; (2) twins who differ in unusual ways, such as sexual preference; (3) twins who have lived through extraordinary circumstances, including two sisters who survived Josef Mengele’s notorious experiments at Auschwitz; and (4) what Segal calls “everyday wonders,” a general category that includes identical twins who have married identical twins, and the challenges faced by a family with six children, including quadruplets, one of whom was stricken with cerebral palsy. Each of these four sections of Segal’s book has a specific purpose—namely, to acquaint us, by describing the kinds of remarkable life events that cannot normally be quantified and assessed in scientific research, with what it is like to go through life as a twin.

Stories about identical twins separated at birth who have later chanced upon one another in adulthood are sources of much fascination and often make their way into the news. Such instances are also vital to twin research, allowing behavioral geneticists to assess the impact of the differing environments in which the separated twins were raised, thus facilitating a controlled glimpse into the relative influences of nature and nurture.

As a member of Thomas Bouchard’s pioneering Minnesota Study of Twins Reared Apart, Segal began her career with just this kind of research. In Bouchard’s laboratory during the 1980s, she studied half of the 135 reared-apart twins who were involved in this research project, conducting interviews and administering batteries of tests during week-long assessments that included chest X-rays, heart examinations, stress tests, and answers to roughly 15,000 questions, including those designed to probe numerous aspects of personality. Segal observed the interactions between twins, and was “riveted by their stories of separation and reunion.” Her aim in the first section of her book is to introduce her readers to the extraordinary similarities so often observed among identical twins who have been reared apart—thereby underscoring the power that genes have in shaping our lives—while also questioning the limits of this genetic blueprint by exploring the lives of twins who were reared in radically different environments.

Bouchard’s research team in Minnesota often gave nicknames to their twins brought up apart. There were the “Giggle Twins,” named for their frequent and spontaneous laughter; the “Jim Twins,” both given the same first name by their adoptive parents; and the “Fireman Twins,” who were adopted by separate families living just thirty miles apart and who both grew up to become volunteer firemen. The last two twins were reunited in their mid-thirties after one of them was mistaken for the other at a firemen’s convention. The resemblances were uncanny. Besides both being volunteer firemen, they each had a loud, staccato-like laugh; liked to issue one-word responses to questions; enjoyed hunting, fishing, and forestry; hated bad cooking; drank the same brand of beer; and held their beer cans in the same peculiar manner, supported by a pinky finger underneath. Their IQs differed by only two points. Although one might be tempted to ascribe these and other similarities to chance, they occur with much greater frequency among identical than fraternal twins, indicating a role for genetic factors. When the twins first met, they found themselves to be so alike that, in the words of one of the twins, “there was no need to get acquainted.”

  1. 1

    Robert Plomin and Denise Daniels, “Why Are Children in the Same Family So Different from One Another?” Behavioral and Brain Sciences, Vol. 10 (1987), pp. 1–16.

  2. 2

    Charles Darwin, The Autobiography of Charles Darwin, 1809–1882, edited by Nora Barlow (Norton, 1958), pp. 228, 230.

  3. 3

    Darwin, Autobiography, p. 72.

  4. 4


  5. 5

    Francis Galton, “The History of Twins, as a Criterion of the Relative Powers of Nature and Nurture,” Fraser’s Magazine, Vol. 12 (1875), pp. 566–576.

  6. 6

    More Letters of Charles Darwin, edited by Francis Darwin and A.C. Seward (Appleton, 1903), Vol. 1, p. 361.

  7. 7

    Segal, Entwined Lives, pp. 213, 215, 314.

  8. 8

    Niels G. Waller and Phillip R. Shaver, “The Importance of Nongenetic Influences on Romantic Love Styles: A Twin-Family Study,” Psychological Science, Vol. 5 (1994), pp. 268–274.

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