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Yet Another Debate on IQ...

Or: "The Same Vinegarized Wine Injected into New Bottles, Part n"

I. Frank Sulloway in
II. The Inevitable Letter War
III. Two Cents
from Yours Truly

I. Frank Sulloway in

Volume 53, Number 19 · November 30, 2006


Parallel Lives

By Frank J. Sulloway

Indivisible by Two: Lives of Extraordinary Twins
by Nancy L. Segal

Harvard University Press, 280 pp., $24.95

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."

The story of the Fireman Twins, however, could not be more different from that of another pair of identical twins described by Segal. Oskar and Jack were separated shortly after their birth when the marriage of their German parents broke up. Jack remained with his Jewish father in Trinidad. Their Catholic mother returned to Hitler's Germany with Oskar. There she changed her son's last name and raised him as a Christian to hide his Jewish identity. Young Oskar became an eager member of the Hitler Youth, "convinced that what the Nazis said about the Jews, war, and country was true."

In 1954, when the twins were twenty-one, they met with an "icy handshake." "We saw each other as enemies," Jack recalled, thinking "neither one of us would change." (See illustration on page 39.) Despite their similarities—they both liked to read books from back to front and both wore rubber bands around their wrists—the ideological gulf between the brothers was something they could never overcome. They disagreed about the causes of World War II, the Israeli–Palestinian issue, and various other political matters. Intensely concerned, however, by their broken twinship and by the love-hate relationship it inspired, they tried for nearly fifty years to surmount these barriers. The brothers took more than a half-dozen vacations together and occasionally visited each other. Governed in substantial part by their equally aggressive, demanding, and critical personalities, the twins' relationship deteriorated bit by bit each time they met.

The story of Jack and Oskar illustrates how the environments in which people are reared can matter much more for some people than for others. Statistics about "genetically explained variance" in attributes such as personality and social attitudes apply only to groups, not to individuals. These statistics also apply only to groups that have been studied in relatively typical environments; other environments may defy these statistical averages. As Oskar himself commented to Jack one day, "If we had been switched, I would have been the Jew and you would have been the Nazi."

In Part Two of her book, Segal turns to what she calls "Variations on Common Themes," a discussion of the differences—sometimes extraordinary— as well as the similarities between identical twins raised together. Her first example involves a pair of twin girls who, at age four, both developed "selective mutism," a social anxiety disorder that affects one in every hundred thousand children. This affliction caused the twins to be silent at school, where they nevertheless communicated with one another by whispering into each other's ears. They were fortunately quite expressive in nonverbal ways, which allowed some degree of communication with their frustrated teacher.

At home, matters were very different: the two twins talked normally, at least with people they knew. The presence of just one stranger at home, however, would induce their mute demeanor and whispering. Oddly, the twins were the most popular members of their kindergarten class, being treated maternally by the other girls, who chattered on with them and were not discouraged by their lack of verbal responses. By the age of eight, the twins began to talk with other children, but only those who had visited them first at their home. Of particular note in Segal's account of this unusual pair is the fact that one of the twins, the shyer of the two, was much more affected by her selective mutism. The more affected twin also exhibited a substantially greater incidence of specific fears and nightmares, as well as episodes of bedwetting.

Segal next introduces us to two other sets of identical siblings whose sexual preferences are not identical. The Marks triplets, Owen, Tom, and Frank, pose the question of how people with the same genes can differ in their sexual preferences. While Owen and Frank are straight, Tom is gay. Identical twins are three times as likely to share homosexual tendencies as are fraternal twins or siblings; from this evidence it seems clear that genes can affect sexual preferences. Why identical twins should nevertheless differ among themselves in this particular attribute is still unclear.

The identical Marks triplets differed in another noteworthy manner. All three of them suffered from multiple sclerosis, but only one of them was seriously affected. A second triplet had mild symptoms and the third was nearly symptom-free. The nearly symptom-free triplet, Tom, realized during childhood that he somehow diverged from his brothers in ways that gradually revealed his differing sexual identity. He was more "emotional and sensitive" than his brothers, took less interest in sports, and "was fascinated by the urinals in the [public] bathrooms because of all the varieties [of male organs]" that were on display. By sixteen, Tom knew he was gay.

Segal follows the story of the Marks triplets with that of identical twin sisters Agnes and Audrey. Even in childhood Agnes knew she wanted to be a boy. When she was five she prayed to God to make her into a boy for Christmas. At age thirty-three, Agnes became Andru through a series of operations and hormone treatments that transformed her from female to male. "I am a better man than I was a woman," Andru commented to Segal in one interview. Her twin sister would probably agree and has said she envies Andru's happy marriage, remarking at one point, "I wish that my husband was so nice and affectionate."

How can we explain such remarkable instances in which identical twins, who share the same genes, nevertheless end up differing in something so central to selfhood as sexual preference and sexual identity? The seeming puzzle is substantially lessened when we appreciate that shared genes do not mean shared bodies or minds. Considerable research has shown that twins, who compete with one another in the uterus for their mother's resources, do not develop in exactly the same manner. For example, various biological factors, including whether twins share the same placenta, can cause differences in their exposure to hormones in the womb and in the nutrition available to them during gestation.

As a consequence, twins generally differ at birth in their weight and degree of physical development, which helps account for their overall differences since their shared genetic blueprints—gene by gene and molecule by molecule—have been biochemically translated into different physiological realities. Imagine several different architects being given the same detailed blueprint for a huge city and being told to build that city from scratch in different locations. Were we to examine these urban constructions upon their completion, we would inevitably find that local contingencies had required construction crews to deviate here and there from the basic blueprint. Like these imagined cities, identical twins are identical only in their blueprints. By the time they are born, they are already disparate in countless neurological and physiological ways that mostly we cannot see.

In her third section, Segal gives moving accounts of twins whose unusual circumstances in life illuminate the remarkable strength of the bond between them. So intense is this bond that many twins are more attached to one another than they are to their own spouses or children. Two twins who were subjected to Josef Mengele's ghoulish experiments at Auschwitz— and who survived Mengele's medical whims against all odds—epitomize this altruistic sense of identity. "We are two bodies and one soul," they repeatedly told Segal, who traveled to Australia to interview them. Other twins in Segal's account reinforce this portrait of the remarkable intimacy among identical twins, through stories of twinships broken by the death of one twin, followed by the enhanced sense of loss that often engulfs the surviving twin. Segal also writes of a woman who bore two artificially inseminated children for her sister, an identical twin who turned out to be infertile. She also describes a pair of identical twins who specifically sought mates "who would allow their twinship to flourish" and who solved their problem by marrying another pair of identical twins. Segal's book is a rich source of answered and still unanswered questions about twins and twinships, and it leaves us wanting to know more.

Returning to the question I posed earlier about Darwin's remarkable life and his fortuitous invitation to become the Beagle's naturalist, I am inclined to respond to such questions about chance events rather differently in the light of twin research. It helps us to appreciate that most people, driven in part by their genetic blueprints, chart adaptive routes during their long voyages through life by creating their own environments—a phenomenon that has come to be known as "niche picking." An identical twin who is adopted at birth, who turns out to love reading, and who is raised in a household with many books may tend to read those books one after another. This person's co-twin, if also adopted away but raised in a household that contains very few books, is more likely to discover the satisfactions of public libraries. In Indivisible by Two and elsewhere, Segal has chronicled many such striking concordances in the lives and careers of identical twins. Examples include the skiers Phil and Steve Mahre, who in 1984 won gold and silver medals in the Olympic slalom; Harold and Bernard Shapiro, who became presidents of Princeton University and McGill University, respectively; and Judith and Julie Swain, who have both achieved distinction in medicine—one in cardiology research and the other in cardiac surgery.[9] In short, Galton's old distinction between "nature and nurture" has been substantially replaced in recent years by a view of life as a continuous process of genetically influenced interactions with the environment—a process that encompasses what has been termed "nature via nurture."[10]

How does this interactionist view of human development affect the way we might think about the life of Galton's famous cousin? It is often overlooked that young Darwin, who had dreamed of visiting exotic places since childhood, had already begun to organize his own oceanic voyage before receiving the invitation to sail on HMS Beagle. In fact, Darwin had diligently investigated passage on ships bound for the Canary Islands. He had even invited his beloved teacher, John Stevens Henslow, to join him on this planned natural history expedition. It was only a few months later that the British Admiralty asked Henslow to nominate someone to sail as naturalist on the Beagle. Had this chance invitation come just six months later, Darwin—in his efforts to satisfy his longstanding wanderlust—would probably have been on his way to the Canaries.[11]

Like the Galápagos Islands, the Canaries would have been an excellent place for Darwin to have begun questioning the reigning dogma of the immutability of species. After visiting the Canary Islands in 1815, the German naturalist Leopold von Buch developed a theory of evolution based on the effects of geographic isolation within this island group, just as Darwin himself did following his visit to the Galápagos in 1835. Two years after his Galápagos visit, Darwin came across von Buch's evolutionary insights about the Canaries and, in one of his research notebooks, praised von Buch's "admirable discussion." Von Buch's views, Darwin later acknowledged to botanist Joseph Hooker, represented "the nearest approach" to his own theory about speciation through geographic isolation.[12]

I like to think that Darwin, who believed he was a born naturalist, would have been comforted by the thought that some of the alternative lives he might have lived, had he not received that unexpected offer to sail with the Beagle, included a potential for some of the same revolutionary science that grew out of his famous voyage. Like Segal's twins, Darwin and the rest of us are substantially the masters of our own vessels, sailing through our life courses, buffeted by the frequent waves and occasionally larger swells of environmental influences, but still persisting in journeys that complement our genetically based predispositions. For those of us who desire to understand why lives turn out the way they do, Segal's book inclines us to stop and reconsider, in fresh ways, whether the shape of Darwin's nose and his uncle's fortuitous intervention really could have changed the course of intellectual history.


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

[3] Darwin, Autobiography, p. 72.

[4] Dutton.

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

[7] Segal, Entwined Lives, pp. 213, 215, 314.

[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.

[9] Segal, Entwined Lives, pp. 210, 247– 257, 270–272.

[10] Matt Ridley, Nature via Nurture: Genes, Experience, and What Makes Us Human (HarperCollins, 2003).

[11] The Correspondence of Charles Darwin, Volume 1, 1821–1836, edited by Frederick Burkhardt et al. (Cambridge University Press, 1985), pp. 120–127.

[12] Charles Darwin's Notebooks, 1836– 1844: Geology, Transmutation of Species, Metaphysical Enquiries, transcribed and edited by Paul H. Barrett, Peter J. Gautrey, Sandra Herbert, David Kohn, and Sydney Smith (Cornell University Press, 1987), p. 210; More Letters of Charles Darwin, Vol. 2, p. 28.

II. The Inevitable Letter War

Volume 54, Number 4 · March 15, 2007


How to Inherit IQ: An Exchange

By Jack Kaplan, Reply by Frank J. Sulloway

In response to Parallel Lives (November 30, 2006)

To the Editors:

Frank Sulloway writes ["Parallel Lives," NYR, November 30, 2006] that "genes are responsible for...about 60 percent of individual differences in general intelligence." Although this claim is frequently made by researchers in the field of behavioral genetics, I do not believe it stands up to careful scrutiny.

Estimates of the proportion of variability in general intelligence that is explained by genetics are based on three types of research—studies of identical twins raised apart, studies of biologically unrelated siblings raised in the same home, and studies comparing identical to fraternal twins. Each of these types of studies is based on one or more highly questionable assumptions.

Studies of identical twins raised apart assume that the twins were separated soon enough after birth that their common upbringing prior to separation had no appreciable effect on their IQ scores when measured later in life. But many of the subjects of these studies either lived together for a substantial period of time prior to separation or were reunited long before adulthood. In the largest study, published by J. Shields in 1962, 13 of 44 pairs lived together during at least the first year of life. Of these 13, 8 lived together until at least the age of two and 5 until at least the age of four. One pair lived together until the age of eight. Of the 31 pairs separated prior to their first birthday, 9 were reunited by the age of twelve or earlier, including 2 pairs who were reunited at the age of five.

Just as importantly, these studies assume that there is no correlation between the post-separation environments of pairs of twins—that their environments differ just as much on average as they would if the twins had been randomly assigned to households in the general population. But in reality many, if not most, pairs of twins in these studies were raised by different members of their families. A common arrangement in the Shields study was that one child would be raised by the mother or father and the other by either a grandparent, an aunt, or an uncle.

Adoption studies are equally problematic. These studies assume that adoptive families do not differ from conventional ones in any way that has a significant effect on measured intelligence. But this is by no means self-evident, since adoptive parents are both self-selected and screened by agencies. It would seem plausible that the environments provided by adoptive families are better on average and less variable than those provided by families generally. It is hard to imagine, for example, a child being adopted by an unmarried teenage girl, or by a seriously dysfunctional or impoverished married couple.

Furthermore, there is statistical evidence supporting this hypothesis. Most studies of adopted children do show much less variability in IQ scores than in the general population, indicating that adoptive children have either less genetic or less environmental variability than biological children.

That leaves studies comparing identical to fraternal twins. The main problem here is that identical twins may, on average, experience environments that are more similar than do fraternal twins. They may, for example, be more likely to share the same friends. Or they may on average spend more time together. Or they may be treated in a more similar way by parents, teachers, and others. It is known unambiguously that fraternal twins experience more similar environments than do ordinary siblings. Many studies have shown a higher correlation for fraternal twins than for ordinary siblings on intelligence and other variables, and the only possible explanation for this is environmental, since the genetic resemblance between fraternal twins is the same as that between ordinary siblings. If fraternal twins experience environments that are substantially more similar than do ordinary siblings, isn't it plausible that the same thing is true for identical twins compared to fraternal ones?

Jack Kaplan
Professor of Statistics
Quinnipiac University
Hamden, Connecticut

Frank J. Sulloway replies:

Professor Kaplan has criticized several key assumptions that underlie heritability estimates for IQ. He seems unaware, however, that these issues have all been addressed empirically during the last two decades and are no longer regarded as "highly questionable." For example, Thomas Bouchard and colleagues, in 1990, analyzed 56 sets of identical twins reared apart and later reunited in adulthood.[1] What is particularly instructive about Bouchard et al.'s study is their extensive analysis of data on the rearing environments of the twins included in their sample. Bouchard's team assessed nine different aspects of the twins' rearing environments, including the adoptive parents' levels of education, the adoptive father's socioeconomic status, parental treatment, and various cultural and intellectual resources available within the home.

Bouchard et al. found that the rearing environments of the twins were indeed moderately correlated (r = .22), confirming a potential bias in the estimation of heritability. Bouchard's team then assessed the direct influence of each of these environmental factors on IQ, from which they were able to determine the total contribution to heritability. The maximum contribution for any one of these nine individual environmental measures turned out to be a minuscule .03, and the mean contribution for the nine measures was just .006. In other words, the heritability estimate for this set of reared-apart twins, which was .69, would be reduced to about .66 based on environmental similarities among the adoptive homes of the twins.

Bouchard and his colleagues also tested the possible contribution made to IQ by pre-separation and post-reunion contact between the twins, another of the key issues raised by Professor Kaplan. Total contact time between the twins averaged 5.1 months prior to separation and 20.3 months following first reunion, when the twins were mostly in their late twenties and early thirties. Degree of contact accounted for almost none of the similarity in the twins' IQs (r = .14, which was not significantly different from zero).

The last of the questionable assumptions about heritability estimates involves the issue of whether identical twins experience environments that are more similar than do fraternal twins. This issue is important because heritability is sometimes calculated as twice the difference between the correlations for IQ scores among identical as opposed to fraternal twins. Behavioral geneticists themselves have long acknowledged the potentially problematic nature of this method, although some research suggests that fraternal twins are not substantially different from identical twins in this regard. In any event, an obvious way around this potential confounding influence is to compare the IQ scores of identical twins reared apart with those of fraternal twins reared apart. With this approach, a shared "twin environment" is no longer a confounding factor for either class of twins.

Five previous studies of identical twins reared apart have yielded a mean-weighted heritability estimate of .75 (based on 158 twin pairs)—that is, 75 percent of individual differences in IQ scores are accounted for by heredity.[2] Although conducted in different countries and time periods, and with different measurement instruments, these results have been remarkably consistent (.64 to .78). Two previous studies of fraternal twins reared apart have yielded a mean-weighted heritability estimate of .38 (based on 73 twin pairs). These collective data yield a heritability estimate—unconfounded by shared twin environments—of .74 (.75 minus .38, multiplied by 2). It is worth noting that these 7 studies of twins reared apart actually yield a higher heritability estimate than do the 75 studies involving identical and fraternal twins reared together, which is only .52 (based on 4,672 identical and 5,546 fraternal twin pairs).[3] The difference between these two heritability estimates is likely due to the relatively younger age of the participants in studies where twins have been raised together, as heritability estimates increase with age.

In sum, for twins, heritability estimates for IQ appear to be between .50 and .70, depending on the particular method by which IQ is calculated, the age of the study participants, and measurement error. The possible confounding influences mentioned by Professor Kaplan appear to make almost no difference in any of these findings. This general conclusion does not mean that environmental influences on IQ are unimportant. On the contrary, abundant evidence has shown that family environments make a substantial contribution to intelligence, especially before children reach adulthood and especially in impoverished environments that do not allow for the full development of genetic predispositions.[4] The increasingly evident portrait of human development that has emerged from these twin studies is one of nature via nurture, as people growing up are drawn to environments that provide the best outlets for their inborn dispositions and abilities.


[1] Thomas J. Bouchard, Jr., David T. Lykken, Matthew McGue, Nancy L. Segal, and Auke Tellegen, "Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart," Science, Vol. 250 (1990), pp. 223–228.

[2] Nancy L. Segal, Entwined Lives: Twins and What They Tell Us about Human Behavior (Dutton, 1999), pp. 135–136.

[3] Thomas J. Bouchard, Jr., and Matthew McGue, "Familial Studies of Intelligence: A Review," Science, Vol. 212 (1981), pp. 1055–1059.

[4] Kathleen McCartney, Monica J. Harris, and Frank Bernieri, "Growing Up and Growing Apart: A Developmental Meta-Analysis of Twin Studies," Psychological Bulletin, Vol. 107 (1990), pp. 226–237; Eric Turkheimer, Andreana Haley, Mary Waldron, Brian D'Onofrio, and Irving I. Gottesman, "Socioeconomic Status Modifies Heritability of IQ in Young Children," Psychological Science, Vol. 14 (2003), pp. 623–628.

III. Two Cents from Yours Truly

First, it might be interesting to note that Bouchard has been funded by the Pioneer Institute, which has a dicey history, to say the least. Bouchard also signed on to support The Bell Curve years ago with several other scientists, over 50% of whom were at one point or another supported by Pioneer.

Second, I'd like to point out that regardless of the political background of the research, one shouldn't assume that "they are wrong." In fact, it's more profitable to assume the opposite. Let's assume "they" are right. Let's go even further than Sulloway: 100% of intelligence (which we'll assume to be as quantifiable as height) is determined by genes.

So? What kind of social policies does this lead to? None, logically, because the fact of nature (granted here for argument's sake) has absolutely no bearing on what to do with that range of intelligences. One could as easily determine that a social-welfare state that accepted "full genetic determination" of intelligence would set as a goal the progressive help for the less intelligent, of whatever kind. Note how you never hear such a suggestion from any hereditarian, whether old-school or "nature-via-nurture." What you do hear is the usual conservative (to put it politely) argument on not wasting resources on a permanent biological underclass.

Now, to return to the real world, to really know in a rigorous sense to what extent intelligence is heritable, we would need:
  1. A completely rigorous definition of "intelligence." I refer you to Gould's The Mismeasure of Man for a demolition of the notion of a "real" IQ (2nd ed.)
  2. The ability to carry out breeding experiments with total control. I mean, like we do with fruit flies, mice, etc. Then we could truly rule out any environmental "noise" (if it is considered such).
Now, the problem is, #1 doesn't exist, as far as I can tell, and #2 requires a fascist state, as we have seen. Twin studies are all well and good, but they would never count as proof in a non-human organism. And they do seem to be fraught with controversy, such as that noted by Kaplan above.

Here's a relevant, common-sense question: Who cares how "intelligence," whatever that is, is determined? I can't really see that it makes a difference, which is why I thoroughly distrust these hereditarian studies -- not the results, necessarily (although those, too), but the impetus. Does greater intelligence lead to greater happiness? Are the policy moves, as noted above, clear or obvious even if such questions could be determined? Is there any correlation between "intelligence" and ethical behavior? And so on.

Finally, note that there is actually another way to determine the heritability of intelligence, assuming #1 could be developed: instead of holding the environment constant by Nazi-like methods, hold it as constant as possible by equalizing access to food, education, social services, and suchlike. This would require some kind of libertarian, democratic (or representative-democratic) socialism. Funny, I never read a single hereditarian suggesting that.

Sulloway is an interesting writer, but he's essentially a "Darwinologian" -- that is, a worshipper of Darwin in a religious sense. He learned his Mayr-oid lessons well, and while he is truly an interesting writer, he's suffering from the anxiety of The Master's influence (Darwin, not Mayr). Sulloway is a system-builder, which is probably why he tried to demolish Freud back in the day, whatever you think of Freud. A competing system, you see.

He says more than he means to, I think, in this paragraph:
Like Segal's twins, Darwin and the rest of us are substantially the masters of our own vessels, sailing through our life courses, buffeted by the frequent waves and occasionally larger swells of environmental influences, but still persisting in journeys that complement our genetically based predispositions.
How, exactly, are we "substantially the masters of our own vessels" if our "genetically based predispositions" are charting the -- the -- course through the never-too-choppy seas of "environmental influences." This is self-contradictory. What he probably meant was that, according to his notions, we are passengers on a ship run by our genes; we'll arrive at roughly the same port no matter what, statistically speaking. Interesting that he doesn't draw this obvious logical conclusion.

In any event, the love affair with Darwin -- whom I consider to be one of the greatest geniuses in human history, by the way -- is a bit embarrassing. The counterfactual hypothesizing on Darwin's life is ludicrous. What is comforting about it, particularly, to Sulloway, ridiculousness aside? I'm not quite sure, but I'll guess below. More importantly, it highlights how seemingly measured statements of "nature via nurture" shade very quickly into hardcore hereditarian theology, encapsulated in the oceanic metaphor that leaves me (due to my genes?) quite seasick.

My guess about Sulloway's need to reassure himself that we're all captains of our ships, despite the obvious antithetical thrust of his argument, is based in this passage:
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.
First, his introductory claim that "we all live only a tiny fraction of the lives that might have been" completely conflicts with his conclusion, cited above, that we are "substantially the masters of our own vessels" with our "genetically based predispositions" charting the course through the never-too-choppy seas of "environmental influences." Thus, his oceanic metaphor is not only internally self-contradictory, but it also contradicts the introductory claim. Sulloway is obviously intelligent (whatever that means); this isn't sloppy thinking. I'm assuming that it's evidence of some deeper, more interesting confusion.

Sulloway's many childhood interests may very well have been taken up. Counterfactuals are about as reliable as predictions of the future for complex systems. Maybe even less so: they are possible futures already in the past, as Sulloway rightly points out. Whether any of them have any validity is literally unknowable.

And I would think a Darwinologian -- or at least an evolutionist -- would understand this. Chance and necessity, the old Scylla and Charibdis of philosophy, co-operate in the evolutionary process. Some processes, like natural selection, are at least "locally" deterministic (never "globally" -- fitness is due to local events, which are ever-changing, at least on some timescale). Some, like genetic drift, are wholly random (stochastic). The built-in random error of meiosis is actually what allows sexually reproducing creatures to massively increase variability from one generation to the next. (Asexual clones must rely on mutations alone; sexual creatures have recombination events of various kinds to rely upon.) This variation is the raw material of selection, as every 10-year-old outside of America knows.

On the macro scale, no matter what the exact pattern of evolution's tempo is for all knowable lineages, there is no doubt that massive extinctions periodically "clear the slate," so to speak. An asteroid slams into the earth, or some other "exogeneous" event so radically changes the rules of survivability in so many ecosystems at once (at least geologically speaking) that even though the difference between a mass extinction and "normal evolution" is ultimately probably a matter of gigantic degree, and not of kind, the history of life is probably permanently deflected into "unforeseen" pathways. These "unforeseen pathways" have little, if anything, to do with the cumulative evolution that had accumulated up to that mass extinction. This deduction is, admittedly, probably also in the "unknowable" zone in the rigorous sense, but it is not a counterfactual. It's the fact about the history of life on earth, arguably.

How does all this theory of history relate to Sulloway's confusion? I think that like most people, he has a lot of trouble with doubt. How much easier to assume that "he" (his genes?) sailed himself into port than to consider that had he been born in, say, West Africa, or had he, say, been felled by cancer forty years ago -- or any of the other thousand natural shocks which flesh is heir to -- he simply would not have been the idiosyncratic scholar he most certainly is (idiosyncratic and scholarly). People like Sulloway should be given lots of time and money to do what they want, and, luckily, he has. Should people not like Sulloway be given lots of time and money to do what they want? Should this be based on luck -- genetic and/or environmental -- or upon the essential dignity of man? (Effort on the part of Sulloway, and any hypothetical other recipient of time and money, is assumed here.)

However, for all his brilliance -- much greater than mine, be sure I assume -- I think Sulloway's twin failings of system-building and an inability to know which questions are just not yet (or possibly ever) answerable -- or is that one failing? -- cause him to make errors as obvious as those I've cited.

In a society that truly valued diversity, the entire question of the provenance of intelligence, etc., would be meaningless. Moreover, in a society that valued each person simply because that person is a human being, the question might be partially answered through a levelling of social opportunity.

My wife and I saw David Lynch's The Elephant Man last night. I think Sulloway needs to watch that film and both understand and feel the power of Merrick's defiant cry amongst the urinals to which he was driven: "I am not an animal! I am a human being!"

For all those who are less intellectually gifted than at least Lynch's Merrick, the same applies. Is an autistic person "less valuable"? How? Why? How does one rank human value? Should one? If all people are equally valuable, at least at birth (to avoid the thorny issue of crime and punishment), then where does that lead to, politically? Logically?

These are crucial questions, none of which requires knowing the unknowable: the provenance of "intelligence," and in what it consists.

I wish a mind like Sulloway's would be more concerned with whether Darwin would have discovered natural selection in a coal mine than in the Canaries.


  1. I like the observations here about social anxiety disorder, although I think the statistics are way off (way, way too low-- selective mutism is far more common than that).

    I've been helped as an adult by Social Anxiety Anonymous and their free social anxiety support groups--


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