Born That Way

by William Wright

  Also in her address, Scarr commended reared-apart twin studies and referred to the Minnesota findings as “startling”—citing in particular the high concordance rate of .76 in I.Q.s of Thomas Bouchard’s reared-apart twins. She rubbed salt into the now raw sensitivities of her environmentally oriented audience by saying, “The remarkable studies of MZ twins reared in different families challenge many cherished beliefs in developmental psychology, but fit nicely in the genotype-environment theory,” the theory that Scarr was advancing.

  She also referred to four studies (one by herself and Richard Weinberg) that came to the same conclusion: In comparisons of biologically unrelated siblings raised in the same home (adopted brothers and sisters), no correlation at all could be found in their I.Q.s. In spite of such powerful evidence of the rearing environment’s lack of effect on I.Q., a large number of the angry responses to Scarr’s paper fell back on the shopworn criticism that reared-apart twins are often raised in “similar” homes, thus explaining their similarities in I.Q.s. Those using this argument failed to explain how vague similarities of two unconnected homes could have a major equalizing effect on I.Q. scores when the same home (for unrelated siblings) had none.

  Throughout her career Scarr has stressed the importance of coming to understand the sources of individual differences and deplored the environmentalists’ insistence that no innate differences of any importance existed between humans, that we were all made different by our environments. Because of the many years this article of faith ruled psychology, she feels the study of individual differences lags far behind where it should be. This is particularly lamentable, Scarr says, because in “the situations to which we want to generalize—presumably social problems like crime, alcoholism, learning disabilities—humans behave very differently and we must learn why.”

  IN TERMS OF INTELLECTUAL VIGOR and prodigious research output, Robert Plomin is one of the Young Turks of behavioral genetics. He even looks like one—bearded, six foot three, given to cowboy boots. On the other hand, in terms of his scientific caution and measured view of the genes-environment contribution to human behavior, he is, in his late forties, an elder statesman of the field. Throughout his twenty-year career, he has instigated or participated in a number of the most important twin and adoption studies—primarily those emanating from the University of Colorado, where he was on the faculty, and the Swedish Adoption/Twin Study. The latter was a collaboration between the Karolinska Institute in Stockholm and Penn State, where Plomin is currently a professor of psychology.

  Paradoxically, Plomin feels obliged to the Catholic school he attended in Chicago’s inner city for his genetic-evolutionary outlook. “When I was about ten,” he told me during an interview, “I had to do a show-and-tell for school. I had been reading a book on evolution, one of the Time-Life series on biology. It talked about Darwin in the Galapagos and the slightly different variations of one species of bird on the different islands. I felt sure that God didn’t create one bird for one island, a slightly varied bird for another. At school, I merely said, ‘Isn’t this interesting?’ But I barely had the book out of my school bag when the nun whisked me out of class and down to the principal’s office. It was still a mortal sin for a Catholic to believe in evolution. I got kicked out of school. It was a case of show, tell, and get bounced. They didn’t say ‘Here is why you shouldn’t believe in evolution.’ Their strategy then was just to pretend it didn’t exist. It was an important event for me—also informative.1 I thought, if these people are this way about an issue like this, I better be skeptical about everything else. Instead of being brought in line, I went very much the other way.

  “At college, I decided to go into psychology and happened to attend one of the few grad schools in the late sixties that had a course in behavioral genetics—the University of Texas—and it was required. The department was under a very prominent psychologist, Gardner Lindzey, who was later at Harvard. At Texas, Lindzey hired two behavioral geneticists, which may not sound like a lot, but it was probably two more than most universities had then. Lindsay said he felt the future of psychology was in behavioral genetics. Almost no one of his eminence felt that way at that time. I was blown away by the course. I felt it was so powerful.”

  Since graduating, Plomin’s career has soared, making him one of today’s most respected behavioral geneticists. While still enthusiastic about the burgeoning genes-behavior discoveries, Plomin, unlike many behavioral geneticists, is also fascinated by the large degree of environmental influence that remains after genetic influence is factored out. Because the heritability of many behavioral traits is in the modest 30 percent to 50 percent range, Plomin, while acknowledging the importance of these findings, feels much still needs to be learned about the remaining 70 percent to 50 percent, that is, the environmental elements that affect the developing human.

  While other genetic converts have rushed off in pursuit of heritabilities, Plomin has devoted much of his attention to the altered view of the environment that resulted from the new understanding of genetic influences. “Behavioral genetics research,” Plomin said, “has revealed as much about environmental processes as it has about heredity.”

  A book he coauthored with his wife, Judy Dunn, also a psychologist, entitled Why Children Are So Different, was a diligent search for elements of both the environment and genes that caused such wide variation among siblings. Plomin and Dunn reasoned that because of the modest amount of genetic influence on most personality traits, the rearing environment shared by brothers and sisters should be major determinants of their personalities, and these personalities should be far more similar than they generally turned out to be. This anomaly led Plomin and Dunn to the conclusion that the largest influences on personality are the aspects of the environment not shared by siblings.

  Plomin and his wife cited the abundant evidence to back up their hunch that the shared environment, the overall rearing setting, had negligible effect on a child’s development. This was from studies—primarily the Texas adoption project—that showed little correlation of traits among unrelated siblings—adopted brothers and sisters—raised in the same family. Random, unrelated genes growing up in identical rearing environments produced near-zero correlations in I.Q. This held true in most of the traits measured.

  When the degree of genetic variation was calculated among such biological strangers reared in the same home, it was low enough to leave a large amount of variance unexplained. This left but a sole possibility, the one Plomin and Dunn arrived at: aspects of the environment particular to each individual in the family. They might be events or conditions experienced by only one child—a prolonged illness, for instance, or harsh treatment by a parent. It might also be a prenatal event—oxygen or nutrient deprivation, bacterial infections, all sorts of happenings that because they are nongenetic are lumped under the heading “environment.” And because they happen to only one child they are called “nonshared environmental factors.”

  Another variation of the environmental theme struck Plomin and Dunn, as it had Sandra Scarr. Children affect their environments, often producing “customized” environments not shared with siblings. An example of this would be a naturally unruly child who would confront very different parents than his or her well-behaved siblings (angry and scolding as opposed to smiling and loving). Pushing this concept even further, a profoundly spiritual child might choose a monastery or convent, in which case he or she would have altered his environment completely.

  Plomin and his wife were also leaders, along with Scarr, in developing the concept that environments are not separate and distinct entities in which genes express themselves, but rather are to varying degrees products of those genes. A child growing up in a musical home or a religious one might become musical or religious because of his upbringing or because he shared with his parents predisposing genes. A child growing up in a port city might well attribute his or her love for the sea to his upbringing. But the new view raised the possibility that his parents may have
chosen this location because of sea-loving genes that the child inherited. If the latter explanation were true, the seaside was not a genetically neutral environment but a genetically loaded one. No sooner had the field of psychology begun to recognize the importance of genes in the emerging human, it became apparent that the genetic action did not stop at the human but spilled over into the environment as well.

  This genome-phenome-environmental interaction has been developed most fully by the prominent Oxford zoologist and genetic theorist Richard Dawkins in his book The Extended Phenotype. In it, Dawkins views birds’ nests and rabbits’ holes as expressions of those animals’ genomes, all part of their phenotypes or, as he puts it, extensions of their phenotypes, just as birds’ wings or rats’ tails are. In his strongly antibehavioral genetics book Biology as Ideology, Harvard population geneticist Richard Lewontin expresses the same idea: “[Organisms] actually construct their environments out of bits and pieces. In this sense, the environment of organisms is coded in their DNA.…” Lewontin does not make clear how fish create sea water or what bits and pieces humans use to construct the air we need to survive, but he agrees with most observers of environment-organism interactions that the former is affected, sometimes to a large degree, by the latter. (Without earthworms, dirt would not be dirt, and so on.)

  This broader view of the environment has the effect of chiseling out hunks of what had previously been considered environment, pure and simple, and shifting it back to the genetic side of the fence. Or worse, it would declare certain aspects of the environment a complex mix of genes and environment, which would seem to put the matter back where it all began. When the behavioral geneticists speak of 40 percent or 50 percent heritability (derived from MZ-DZ comparisons), they are speaking of that portion of a trait’s variation, for a given group of people, that comes from their DNA; it does not necessarily speak about the contribution made to that trait by effects of the gene-influenced portion of the environment—that is to say, indirect gene influence. Nor does it speak about the effects on the environment by genes shared with other humans in the vicinity, parents for the most part, genetically related humans who may have had large influences on the rearing environment. These are influences that may have stemmed from the adults’ genes, which the child, to a degree, shares. All of these influences are ways in which genes can indirectly act on a developing child, but so far such effects have not been measured. This omission has the effect of making the heritability figures lower than they actually are.

  Yet another point about the environment renders the entire subject even more muddled. When psychologists use the term environment, they are referring to any influence that is not genetic. As mentioned above, that includes prenatal influences that can be devastatingly powerful. Such womb conditions as fetal position, nutrition, hormone infusions, and viruses are all “environmental” influences but can inflict permanent behavioral changes onto the developing fetus.

  Dean Hamer, who made a major breakthrough in 1993 by locating a DNA anomaly in a group of homosexuals he tested, told of the range of effects that could occur in the prenatal environment: “The environment includes whether you were lying on your right side or your left side in the womb, and a whole parade of other things.”

  Now, with the rapid advance in understanding about these many potential prenatal influences, the term environment would seem to have been broadened into near meaninglessness. Traditionally among psychologists, the environment meant family settings, schools, neighborhoods, national culture. Now the term also includes a “parade” of womb events over which no one has much control. Since the environment’s assumed malleability is so central to the critics’ opposition to a biological view of behavior, the broadened understanding would seem to have stripped the term of much political resonance. With the hard-to-reach prenatal influences coupled with the realization that genes affect postnatal environments, the term “environment” is a far more biologically loaded term than realized up till now.

  The tidy idea of nature on one side and nurture on the other is finished, just as is the idea that one or the other of them calls all the shots. Fortunately, Plomin, Scarr, Kagan, and others are undaunted by this new complexity. They feel that the clearer understanding now enables them to weigh an environment with greater accuracy than before, to break it down with greater precision than had been possible when its biological component was ignored. Their goal is to zero in on the environmental variables that most affect developing humans and to ascertain the degree of influence. This, in turn, will enable them to assess more accurately prospects for beneficial interventions.

  Many psychologists, on acceding to the point about genetic effects on the environment, would throw up their hands and declare that the new perception rendered the entire genes-environment interaction too tangled for productive study. Robert Plomin, on the other hand, is the sort of scientist who is excited by new complexities and the new challenges they bring to his field.

  Like other psychologists who have focused on the DNA underlying behavior, Plomin has had to educate himself in Mendelian genetics and molecular biology. With two of the most distinguished pioneers in the field, J. C. Defries and G. E. McClearn, Plomin has written a basic textbook, Behavioral Genetics: A Primer, which lays out for students the fundamentals of molecular genetics, as well as the advances in recent years made by the observational studies of twins and adoptees, the area they refer to as quantitative genetics. While still involved in studies of the latter, the more traditional domain of psychologists, Plomin has joined many of his colleagues in turning his attention to DNA itself.

  He is convinced that rapid advances in molecular-biological screening techniques will revolutionize behavioral genetics, perhaps in the same way that behavioral genetics revolutionized psychology. “Just fifteen years ago,” he wrote in 1990, “the idea of genetic influence on complex human behavior was anathema to many behavioral scientists. Now, however, the role of inheritance has become widely accepted, even for sensitive domains such as I.Q.” Plomin worries that the pendulum may have swung too far toward biological determinism and that people will overlook the important role the environment still plays in many aspects of personality and behavior. He cites the 40 percent schizophrenia concordance in identical twins, a figure that leaves a hefty 60 percent of the cause from the environment.

  He is greatly encouraged by the many prominent developmental psychologists like Jerome Kagan who are now using genetic strategies in their research, men and women who hold no theory-grounded expectations for either biology or the environment. They root for neither but believe the answer lies in a fascinating, little understood, interaction between both. Plomin’s greatest hope, he says, is that “the next generation of developmental psychologists will wonder what the nature-nurture fuss was all about.”

  UNTIL THE MINNESOTA separated-twins study issued its papers, most of the findings of genetic links to behavior had been derived from fraternal-identical comparisons of raised-together twins. The assumption of these studies was that if identical twins have more personality similarities than fraternal twins, their rearing settings being equal, the explanation must lie in the genes. To date, there have been large number of these studies involving large numbers of twins, and they all came to the same conclusion: Behavior is influenced by genes. Because of this, the antigenes critics have expended much energy on discrediting the study model itself.

  Their main criticism of this approach, voiced repeatedly to dismiss the genetic evidence, is that identical twins are brought up to be more alike so, in effect, become more alike. Greater degrees of similarity for identical twins measured by the tests do not, as claimed, indicate genetic influence but rather indicate the greater social pressure on them to be more similar. Since this was a plausible criticism, at least until Bouchard’s reared-apart twins produced the same results, beleaguered behavioral geneticists went to great pains to test it empirically. With every test they have found the proposition to be untrue.

  When Kenne
th Kendler was at work on a massive study seeking a genetic component to schizophrenia based on identical-fraternal comparisons, he tried to preempt the usual debunking effort by publishing a paper in the American Journal of Psychiatry that summarized a number of earlier experiments that tested the hypothesis of MZs and DZs having different upbringings even though they were reared in the same home. All the studies found this phenomenon of different treatment given MZs than DZs, to the small extent it existed, had no significant effect. In spite of this battery of disproof, the argument continued to be raised, and is to this day.

  The studies are interesting for two reasons. First, they demonstrate the enormous pains behavioral geneticists have repeatedly taken to answer their critics and prove the soundness of their research. Second, because of the studies’ effectiveness in answering criticisms, and because of their ineffectiveness in putting to rest this particular objection to DZ-MZ comparisons, they reveal the critics’ imperviousness to empirical proof, or even worse, their obliviousness to it. For this reason, I feel it is worth examining in some detail the rebuttal studies Kendler cited. They also happen to be quite ingeniously constructed.

  Before presenting the data on different treatment of identical and fraternal twins, Kendler examined the possibility that there was an element of twinness itself, or perhaps just of identical twinness, that might lead to the trait he was interested in, schizophrenia. He cited several studies that compared the rate of schizophrenia in twins to that of the general population. None found any significant difference.