What is heritability? The heritability of a trait in a given population tells us what proportion of differences in that trait is due to genetic differences. It provides an answer to the main question in the nature–nurture controversy.

Heritability can be calculated both for physical and psychological characteristics but it generates controversy mainly in the context of human behavioral traits like intelligence, personality differences, criminality, etc. For this reason I will focus here only on discussions of heritability in psychology (i.e., human behavior genetics). Criticisms of heritability claims in this area are frequently based on a curious mixture of methodological objections and warnings about political motives and/or implications of this research. A systematic study that would try to disentangle all the argumentative threads that are often run together in this contentious debate was long overdue. So this book was simply waiting to be written.

I was introduced to the nature–nurture debate by reading Ned Block and Gerald Dworkin's well-known and widely cited anthology about the IQ controversy (Block & Dworkin 1976a). This collection of articles has long been the main source of information about the heredity–environment problem for a great number of scientists, philosophers, and other academics. It is not an exaggeration to say that the book has been the major influence on thinking about this question for many years.

A book on heritability without a chapter on political issues would be a bit like Hamlet without the ghost of Hamlet's father. Even when the ghost of politics is not addressed at all, it always lurks in the background, haunting the protagonists and influencing both the tone and dynamics of the heritability controversy.

But why not exorcise the ghost from the debate once and for all? For, if we want to understand heritability as a scientific concept, is it not advisable to isolate it from vagaries of political storms that only create confusion, distrust, and anger? There are two problems with this idea. First, since in the discussions about heritability, politics has occupied center stage so forcefully and for so long, we can hardly make sense of what went on if we neglect such an important element in the story, however irrelevant it may “objectively” be for the issue at hand. And second, before looking into these matters more carefully, we cannot actually be sure that heritability research is indeed devoid of political implications, as many people keep telling us.

A good way to start this chapter is to consider two frequently used arguments to cross the barrier between science and politics: first, the claim that a scientific belief is mistaken because it is politically motivated, and second, the claim that a scientific belief is politically motivated because it is mistaken.

TWO CONCEPTS OF INTERACTION

A widespread conviction that heritability claims are devoid of almost any interesting explanatory content is often based on an argument that genes and environments interact, and that for this reason their causal contributions to phenotype cannot be separated and measured independently. An immediate problem with this argument is that there are two very different meanings of “interaction”: commonsense and statistical. According to the commonsense notion (interactionc), to say that two causes A and B interact means that neither can produce the effect without the presence of the other. To use a standard example, striking a match and the presence of oxygen interact to produce fire. According to the statistical notion (interactions), however, to say that two variables A and B interact means that a change in one variable does not always have an effect of the same magnitude: its effect varies, depending on the value of the other variable. For instance, the very same life event, such as parental divorce, may affect children with different personality characteristics quite differently.

Already a half a century ago Waddington proposed that, in order to avoid confusion, the term “gene-environment interaction” should be reserved for the statistical concept:

The main source of political nervousness in discussions about heritability is its possible implications for race differences. John Searle echoed a widespread view: “once you believe that there are innate human mental structures it is only a short step to argue that the innate mental structures differ from one race to another” (Searle 1976).

The fact that the step from individual heritability to group heritability is perceived as “short” may explain the occasionally acrimonious opposition to claims about individual heritability: better to stop the inference at an early stage than to find oneself in the position where later, after conceding too much, one no longer has a good strategy to resist the abominable conclusion. But whatever resistance could be expected at the level of general discussions about heritability, the situation dramatically changes when race differences are addressed explicitly. Linda Gottfredson conveys the mood well: “One can feel the gradient of collective alarm and disapproval like a deepening chill as one approaches the forbidden area” (Gottfredson 1994: 56).

The most alarming hypothesis, of course, is the explosive mix of three ideas: race, IQ, and heritability.

The fact that organisms with genotype G1 may have phenotype P1 more often than organisms with genotype G2 is not necessarily a good indication that a G-difference is directly responsible for a P-difference. It may well be that (for whatever reason) G1 organisms simply find themselves more frequently in environment E1 than G2 organisms, and that it is E1 which causally leads to phenotype P1 (while E2 produces P2). This phenomenon is called “genotype–environment correlation.” It has been extensively discussed in the behavior genetics literature, but it has also been widely used in methodological criticisms of heritability.

The expression “genotype–environment correlation” usually refers to the situations where two separate sources of phenotypic variance (genetic and environmental) happen to be correlated. Sometimes, however, it is interpreted more broadly to cover also the cases where there is a correlation between a genetic and environmental characteristic, even when the genetic characteristic is not directly influencing the phenotype.

A CONVENIENT JINGLE OF WORDS

Heritability is basically a measure of the strength of genetic influence on phenotypic differences. The emphasis is on the word “differences.” It was Francis Galton who initiated a systematic study of human variation in the nineteenth century. He chided statisticians of his time for being only interested in the mean values and never in differences:

Heritability is usually defined as the proportion of phenotypic variation that is due to genetic differences. So giving a particular value to a heritability estimate of a given trait is solving the nature-nurture equation in that specific context. There is no general answer to the nature-nurture question. As J. B. S. Haldane said: “The important point is to realize that the question of the relative importance of nature and nurture has no general answer, but that it has a very large number of particular answers” (Haldane1938: 34). Dobzhansky concurs: “There is not one nature-nurture problem but many” (Dobzhansky1956: 21).

GENETIC AND ENVIRONMENTAL CAUSATION

Can phenotypic differences arising from genetic differences be eliminated as easily as environmentally caused differences? Those who answer this question in the affirmative like to point out that being caused by genes does not entail being unchangeable, fixed, or predestined. This trivial truth is easily granted. But after we concede that, indeed, “heritable” does not mean “unchangeable,” there is a temptation to make another step from this truism to a much stronger claim, namely that there is no difference at all between the ways genetic and environmental effects are modifiable. This is a step from truth to falsity.

Let us begin with quotations from Jencks, Dawkins, and Lewontin, which make the same point and initially sound very plausible:

At the end of this book many a reader may feel that my approach should have been more balanced. Even if I am right in pointing out many weaknesses of environmentalist criticisms of heritability, doesn't fair scholarship require that problematic aspects of hereditarianism be addressed as well? Surely, bad arguments cannot be a “privilege” of one side in the debate.

In my defense, let me remind you that my goal was not to offer a comprehensive discussion of the nature–nurture problem. I focused just on a small segment of that controversy. As a philosopher of science, I found it interesting to scrutinize very general methodological arguments that are often used to short-circuit the debate in the attempt to undermine one of the rival positions, without going into empirical details at all. And precisely here is the source of the disparity. It is only environmentalists who want to use this kind of methodological shortcut. Hereditarians are quite happy to let the empirical evidence decide the matter. So the imbalance of my approach is the result of an existing asymmetry, not of my partiality.

Whereas methodological arguments purported to prove that heritability claims are meaningless, confusing, or causally uninterpretable, a completely opposite criticism is that they are trivial.