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The Nature of Nurture: Parents or Peers?

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From: Jerome Kagan
To: Judith HarrisPosted Tuesday, Nov. 10, 1998, at 3:30 AM ET

Dear Judy:

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Your last reply makes two points. The first is that a child's heredity always makes a contribution to his or her behavior and mood. The second claim is that similar neighborhoods usually produce similar psychological profiles because of interactions with peers. Both of these claims are broad and remain premises rather than proven facts once one specifies particular profiles. Each is an idea to be tested empirically. I now deal with each of the claims in turn.
The bold proposition that heredity always makes a contribution to a child's psychological profile is as general and, therefore, as empty of significant meaning as the statement that environments always make a contribution to a child's profile. Therefore, your demand that all investigators who study the role of the environment must control for a child's heredity invites the equally reasonable suggestion that all investigators who study the role of heredity must measure the child's environments. As you know, behavioral geneticists do not do so. No report describing the influence of heredity on any psychological trait, whether it be IQ or a personality characteristic, has directly measured the psychological environments of the subjects over the first dozen years of life. The complex influences of the environment are inferred from an equation called the heritability equation. But these inferences are not accurate for, as you know, the equation that estimates the heritability of a trait contains terms not only for genes and environment but also for the interaction of genes with environment and the influence of all the background genes possessed by a particular individual. This process is given the technical name epistasis. Neither of the latter two influences is awarded a reasonable value in the current heritability equations.
Thus, we have a situation in which the equation that estimates the heritability of a trait has four separate terms: genes, environment, interaction of genes and environment, and epistasis, but the latter three are never directly measured, leaving us with unknowns for three of the four terms. For that reason, all current estimates of the heritability of any psychological trait must be suspect.
Equally relevant is the fact that most studies of the heritability of personality traits in children are based on parental report, and in adults on information from self-report using questionnaires. In the few cases where actual observations of children were made, a paradoxical result occurred. When mothers describe their children's degree of shyness on a questionnaire, estimates of the heritability of shyness increase with age. However, when actual observations of children are made, estimates of the heritability of shyness go down with age. Every conclusion concerning the magnitude of heredity's influence is limited in significance for it depends always on the source of the evidence.
One finding on fruit flies should motivate reflection among those who claim that genes exert profound influences on personality. A pair of alleles in fruit flies produce abnormal wings in offspring if the two parents possess the genes for this anatomical anomaly. But this result only occurs if the flies are reared in a laboratory where the temperature is about 20 degrees Centigrade. If the temperature of the laboratory is raised by only 10 degrees, the offspring develop almost normal wings. If a difference of 10 degrees in room temperature can influence a significant anatomical feature of a fruit fly, surely children's personality traits, which are controlled by a much larger number of genes, must be influenced, in a major way, by the environments in which they are reared. (See W.J. Keaton and J.L. Gould, 1993, Biological Science.) Listen to a leading behavioral biologist, Gilbert Gottlieb who writes in the recent edition of the Handbook of Child Psychology, "When there is heredity-environment interaction, the degree of apparent heritability of some characteristic depends on the specific rearing environment."
This means that every study you cite and interpret as showing the influence of genes is seriously dependent on the rearing environments of the individuals. The vast majority of studies of the heritability of personality are conducted on middle-class, white children and adolescents, and we have no idea of the magnitude of heritability of any trait in other groups, for example, economically disadvantaged Vietnamese immigrant children or children from elite families living in Kuwait. Your book is written as if you do not appreciate that the heritability of any trait can never be a fixed quality; it is always at the mercy of the rearing environment.
Finally, the estimates of heritability you quote assume that the effects of genes and environments are additive. For readers who are not familiar with this idea, it means that the distribution of a particular trait, say children's verbal ability, can be predicted by assuming that the influence of the genes for verbal skill are added to the influence of environmental supports for verbal ability.
This assumption raises a serious problem because most biological characteristics are not additive. For example, the electroencephalogram profiles of identical and fraternal twins are best explained as the result of an interaction among the person's genes (epistasis) rather than by assuming that genetic and environmental forces combine additively. (See J.C. Christian, et al., 1996, Psychophysiology, 33: 584-591.) Even the simpler phenomena we call "size of a snowfall" is not predicted very well by adding the influence of the temperature to the influence of humidity.
Thus, your evaluation of the power of genes to influence a child's personality is permissive and not sufficiently self-critical.
Your second claim is that neighborhoods play an important role in children's development. I found this conclusion surprising for I know of no study that has shown, for example, that half of children living in the same neighborhood are essentially similar in their perceptual, memorial, and verbal abilities or in the constellation of personality traits. Consider as a thought experiment a single block on East 75^th Street in Manhattan, where children from different cultural backgrounds live in the same small area. If you sampled 100, 15-year-olds on that block, you would find extraordinary variability in any psychological trait you decided to measure.
There is a study that supports this hypothetical thought experiment. Most of Warsaw was destroyed by bombs during World War II. When the city was rebuilt after the war, authorities decided that families with different educational and vocational backgrounds should live in the same apartment houses so that their children would play on the same playgrounds and attend the same schools. Your position suggests that these children should be very similar in their abilities. But the data revealed a great deal of variability in cognitive qualities, and this variability could be attributed to the values of their families. The children's intellectual talents were a function of their family's influence, not of the peers with whom they interacted everyday.
You state on Page 317 of your book that we do not know why children who are abused by their parents often grow up with psychological problems. You note correctly that this outcome could be due to parental treatment, parental personality, genes, or peers. Your conclusion is correct not only for the outcomes of abuse but also for every trait and talent that psychologists study. We simply do not know the differential contribution of the varied influences that can affect a particular profile. The proper posture to this frustrating state of affairs is reflection and reserve, and not your opening statement in the preface that claims boldly that you wished to dissuade readers of the notion that a child's psychological profile is shaped by parents. It is hard for me to understand how you can be so certain of the minimal influence of parents when you acknowledge on Page 317 that scientists don't know why parental abuse is likely to produce problems in adolescence. Let me be clear: No one, whether a scientist or an educated citizen, understands the complex ways in which genes, prenatal effects, family practices, parental personality, children's interpretations of their environments, historical era, size of city and, yes, even peers come together to produce any psychological trait. Under these conditions, every scholar should resist temptation to cry, "Fire!"