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Nature vs. Nurture
By Madeline Drexler, Boston Globe, 12/3/89

Is a baby's mind a tabula rasa, a clean slate upon which life's adlibs inscribe themselves? Or is it a piaybook, guiding the new creature through a hidden script set at birth?
The answer, of course, is "both." Nature and nurture work together to shape experience. Still, one school or the other is usually in intellectual fashion. And these days, the geneticists seem to have the ascendancy, propounding the same sober wisdom as old Mr. Lammeter in the novel "Silas Marner": namely, that "Breed was stronger than pasture."
New data about the uncanny similarities between adult identical twins separated at birth have caught the poular imagination. These reports have expanded the range of traits that seem to be roughly blueprinted in the womb: intelligence, temperament, sociability - even such seeming idiosyncracies as job satisfaction, risk of divorce, and style of humor. Some observers say these reports have fine- tuned scientists' understanding of how personality is molded. But within academia, debate continues over the significance of these studies, with nurturists contending that the data gloss over the complex ways environment molds character.
This most recent round of dissension largely stems from an ongoing series of twin studies at the University of Minnesota. In 1979, Thomas Bouchard, a psychology professor there, came across a wire-service story about a pair of identical twin boys in Ohio, separated in infancy and reunited 39 years later. What began for Bouchard as a casual inquiry became a full blown investigation into hereditary influences on personality and medical history. To date, the Minnesota Study of Twins Reared Apart has collected data on 60 pairs of identical twins and 40 pairs of fraternals.
Bouchard's approach was not unique. For more than a century, research on twins has been considered a valuable tool in describing behavioral and physical development. Because identical, or "monozygotic," twins, whose genes are formed from the splitting of a single fertilized egg, have all their genes in common, any differences between them must be due to environment. By contrast, fraternal, or "dizygotic," twins, formed from the fertilization of two separate eggs by two separate sperm, have only half their genes in common, as do any two siblings - and theoretically 50 percent less chance of inheriting the same genetic traits. By studying identical twins raised apart (who theoretically may have only their genes in common) and comparing them with identical twins raised together, fraternal twins raised together, and fraternal twins raised apart, Bouchard says he has been able to statistically tease apart the influences of environment and heredity.
Bouchard knew he would find some strong genetic correlations. "Intuitively," he says, "you'd expect certain things, such as impulsivity, to represent biological influences. And other things, like motivation or extraversion, would be learned."
As it happens, though, some of the most subtle and complex aspects of personality - traits that one would assume could only be shaped by environment - turned up with surprising frequency among identical twins raised apart. These characteristics were measured in 11 independent personality scales. They reflected such diverse qualities as "social potency," the ability to take charge in group situations; "achievement," the desire to be successful at work; "well-being," the capacity to experience joy and excitement; "self- control," the lack of impulsiveness; "harm avoidance," the propensity to steer clear of destructive situations; even "traditionalism," the tendency toward conservative moral values.
Overall, the researchers concluded that at least 50 percent of measurable personality variations could be attributed to genetics."I was hoping that, for some of these personality traits, monozygotic and dizygotic twins would be less similar," notes Auke Tellegen, professor of psychology at the University of Minnesota and one of the scientists of the study group on twins. Such disparities would have illuminated the role of environment in personality.
But his data, defying expectations, showed "no clear environmentally controlled traits," Tellegen says. "Personality is in a sense, optional. You require that a child read and write and at some point drive a car. But you don't require dominance rather than submissiveness, or risk-seeking rather than risk- avoidance. That's more determined by what we come into the world with."
Does this mean every nuance of behavior is set at conception, from oblique wit to unseemly competitiveness to a predilection for Fred Astaire movies?
Not at all. As Thomas Bouchard explains, "We don't make distinctions between genetic and learned behavior because virtually everything is learned. The question is: To what extent are people predisposed to learn?" In other words, while our surroundings offer up problems, resources, and challenges, our genes determine how we respond to these outside forces.
Intelligence is one of the areas where this interactive model of behavior has been applied. Twin studies have suggested that there is a fairly strong correlation between genetics and intellect. According to David Rowe, a psychologist at the University of Arizona at Tuscon, "Genes account for 50 to 70 percent of variation in intelligence." In other words, someone with above average intellectual skills is likely to have been born with above-average intelligence. (The Minnesota researchers have couched this connection in starker statistical terms. They found that identical twins raised in the same family had 86 percent correspondence in IQ; fraternal twins reared together had 60 percent correspondence; and that identical twins reared apart had a greater resemblance - 72 percent - than the fraternal twins raised in the same home.)
Psychologists have also observed that as children age, they are more likely to fulfill their intellectual potential. Though parents may curtail a child's chances to learn new words or concepts, other opportunities for intellectual stimulation - schools, libraries, siblings, friends - crop up. Thus, as Rowe points out, over time "an individual has more chance to reflect his or her genetic makeup."
This was borne out in the Minnesota study, in which, again and again, identical twins with very different schooling emerged only a few IQ points apart. In a telling example from Britian, a twin was adopted by a wealthy lawyer and attended exclusive private schools; her sister grew up in a lower middle-class section of East London and quit school at 16 to work. Despite their opposite paths, their IQ scores were only a point apart.
To some, this is proof that an impoverished upbringing need not impede intellectual growth. But as Auke Tellegen concedes, the Minnesota studies probably underestimate the effects of growing up in extremely impoverished surroundings, which "could have a tremendous effect."
Other research has, in fact, shown that in some cases upbringing can permanently harm intellectual capacity. At the University of Virginia, for example, psychologist Sandra Scarr studied infants who were given up for adoption and raised by middle-class families. She found that, by early childhood, these adoptees' IQ's averaged 20 points higher than those of their peers in the impoverished areas where they were born. Scarr attributed the advantage to more intellectually stimulating and emotionally stable environments, which allowed the children to attain their innate potential. "These families afforded these children a wide array of opportunities to pursue their interest and talent," she says.
Intelligence aside, other key aspects of personality- such as shyness and extroversion, anxiety and emotional coolness, and dominance and submissiveness appear virtually at birth. In an ongoing study of infants and their mothers, Carrol Izard, professor of psychology at the University of Delaware, is tracking such qualities. He has found that when infants are exposed to various emotion-enducing stimuli, such as being separated and reunited with their mothers, having a balloon popped in front of them, or being approached by a stranger, they show consistent patterns of facial expressions. These patterns, says Izard, seem to be characteristic of each child - It's like observing and experience."
The amount of sadness an infant shows during a short separation from its mother, for example, foreshadows the sadness it exhibits six months later. The degree of anger baby shows when given a painful inoculation between 2 and 7 months of age predicts the anger that flares in a similar situation a year later. Izard has discovered that infants even have emotional expressions and behaviors that anticipate personality traits of a five year old. Babies who stare the longest at human faces, for example , usually end up being the most sociable, while those who do not fix their attention on faces are generally most shy and withdrawn at the age of five.
These early expressions, Izard says, reflect internal hardwiring such as the autonomic nervous system, which mediates involuntary processes such as rage, fear, and anxiety. If such genetically determined functions also shape internal emotions, as Izard believes they do, then they probably contribute to what he calls "the broad dimensions of personality," such as introversion and extroversion.
Still Izard stresses that "everything in behavior that is genetically influenced is also influenced by environment." In the same study, he noted that an infant's first social bond with its mother has a powerful effect on personality. Babies whose mothers were outgoing, empathic, and open in expressing negative emotions tended to be "securely attached" infants - that is, less likely to cling to their mothers or be disturbed by separation.
Other aspects of personality have also been linked to heredity. Some researchers suggest, for example, that many criminals may be born, not made. University of Southern California psychologist Sarnoff Mednick found, in a study of 14,000 adopted children, that those whose biological parents were chronic offenders were three times more likely to be arrested repeatedly as adults than those whose parents had clean records. Other research indicates that serious offenders were more hyperactive and harder to control as children than non- offenders. But critics of this breeding-over-pasture view charge that it egregiously simplifies the issue and ignores the pervasive effects of poverty, childhood abuse, and cultural conditioning.
Alcoholism has also been linked to genetic inheritance, though this, too, is a view that has invited a chorus of objections. One study found that children of alcoholics face four times the risk of becoming alcoholic that other children do, even when raised by adoptive parents. Another showed that, compared with the sons of alcoholics feel less intoxicated, exhibit a weaker hormonal reaction, and sway less after a session with liquor - meaning that they often don't know when to stop drinking. Half of hospitalized alcoholics are known to have an alcoholic relative. Then, too, certain genetic forces actually prevent alcoholism among 30 to 45 percent of Asians, who can't drink because they lack an enzyme crucial to the body's breakdown of alcohol; whenever they do consume liquor they experience facial flushing, queasiness, and other unpleasant reactions. (It should be noted that babies whose biological mothers drank heavily during pregnancy are constitutionally, not genetically, at risk for alcoholism.)
Yet many investigators contend that research data on alcoholism are subject to divergent interpretations. Alcoholism results from a complex interaction of heredity and environment, and external factors such as ethnic background, upbringing, peer pressure, and even cost may turn out to be more dominant influences. One of the most telling statistics of all is that 60 percent of this country's alcohol abusers come from families with no history of the disorder.
That mental illness has a genetic component seems less contestable. Schizophrenia affects 1 percent of the population - but a child or sibling of a schizophrenic has a 12 percent chance of being afflicted with the disease, and an identical twin a 30 to 40 percent risk. Similarly, a fraternal twin of a person suffering bipolar manic depression has a 19 percent chance of showing the disorder, while the risk for an identical twin jumps to 79 percent.
But even here, statistics don't tell the whole story. As Donald Koshland Jr., editor of Science, writes; "Some individuals who have normal genes become overwhelmed by adversity in their environment, sink into depression, and attempt suicide. At the other extreme, some who have loving parents, ideal schooling; and a stress-free life are overwhelmed by their internal chemistry and also succumb to depression and suicidal intentions. Still others are pushed into depression by stresses that are easily surmounted by individuals with different genetic components."
It's impossible to predict when, or how, genes become blueprints. But the very notion of biological determinism evokes a cultural history of racism, sexism, educational inequality, and eugenics. Ideology alone is reason enough for some observers to criticize the results of the twin studies. As Minnesota's David Lyldken dryly remarks, "You've heard about the people like us who do this research: we're fascists, incompetents, racists, and liars. Well, it's not true."
Some scientists criticize the twin studies on purely objective grounds. They maintain that the data reflect statistical legerdemain more than authentic similarities, and they blame the Minnesota researchers for overstating the role of genetics.
"When a field is young, it tends to go toward extremes instead of a balanced position," says Jerome Kagan, professor of psychology at Harvard University. "In the late'60s and 70s, we denied the influence of psychological processes. When scientists discovered that inherent physiology played a role, they gave it more power than it deserved."
To balance the picture once again, says Kagan, "it would take the realization that genetics do give an initial bias to a child. And then the environment acts on that bias and can do a lot to change it." For instance, two identical twin children may be born with a propensity for becoming anxious and fearful. If one is raised in a home that is stressful and fear-provoking, that child will probably grow into an anxious adult. If the other is raised in a family where the parents recognize their child's fears and gently work to instill confidence, there's a good chance that the child will be a calmer adult - at least externally.
What the twin studies fail to reveal, critics say, are the ways in which genes interact with the environment. Inborn temperament prompts us to choose and structure environments in order to play out our skills and predispositions. It also elicits certain reactions from relatives, friends, and authorities - a common influence that may contribute to the striking resemblances between separated twins. Unfortunately, these environmental effects can't be qualified as readily as genetic differences and therefore, tend to be eclipsed in the nature/nurture discussion.
The Minnesota researchers are acutely aware of these gaps in their work. "What's the mechanism for genetic heritability? The short answer is: We don't know," says University of Minnesota psychologist David Lykken. "The genes code for enzymes, and we have no way to go up the stairway from the manufacture of proteins to the basic mechanism of the brain."
Environmental experience helps stamp personality, as the Minnesota group recognizes. But according to Lykken, that fact does not negate genetic legacy: "It's like asking,'What's more important in making this building: the bricks or the blueprint? I think of it as 'nature via nurture': Genes have their influence via experimental learning."
Intelligence, for example, is not a single entity but a mix of verbal fluency, spatial coordination, perceptual speed, memory and accuracy. One doesn't inherit the ability to penetrate a Rilke poem or troubleshoot a fuel injected engine, but one does acquire the neurological skeins that make these pursuits easier or harder, more or less attractive.
If there's one thing scientists agree on, it's the need for more long-term research into causes and effects in personality development. In particular, psychologists want to know how personality unfolds over time - and studies of monozygotic twins reared apart would be perfect vehicles for examining this question. The Minnesota scientists believe that twin studies would also illuminate how different inborn temperaments seek out different peer groups, intellectual environments, and leisure styles over time. According to Lykken, such knowledge "would allow us to explain genetic results without being nihilistic or helpless." Even now, twin studies "should make people more sensitive to the fact that kids are different from each other," says Auke Tellegen."If you have a kid who's high strung or cautious, you can adapt your treatment to him." Adoptive parents, in particular, "should know about genetic sources of behavior. For parents to think it is all up to them would be a terrible mistake."
To a casual observer, the anecdotal data from the Minnesota study may be reason enough to convert to the prenature argument. Many identical twins separated at birth have the same mannerisms and gestures, pace themselves alike when walking or talking, even decorate themselves the same. Frequently they share the same phobias, hobbies, jokes, style of clothes and vocational interests.
There was a pair of identical twin women, raised apart, whose sons had each won a statewide mathematics contest, one in Wyoming and the other in Texas. And a pair of identical twin men, separated when they were five days old and reunited 31 years later who felt as if they were staring into a looking glass when they met. Both were tall, burley firefighters who had considered a forestry career when younger; both had droopy mustaches and aviator glasses; both were bachelors, attracted to the same type of women; they enjoyed hunting and fishing, going to the beach and old John Wayne movies. Both drank their beer in the same fashion with their pinkie curled under the glass and even crushed the can in the same manner.
But beyond these eerie coincidences lies deeper meaning. For reunited twins, meeting their genetic double strengthened their perception of individuality. Confronting their mirror image, they gained uniqueness - and possibility. "They feel enriched. Their sense of self has expanded and their horizons have enlarged," says David Lykken. "The human genome contains a lot of potential, some for good, some for not-so-good. One way to find out more things about yourself, or what you could have been, is to meet your twin.

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