Crusio, Wim E. (1991) Genetic effects on 'environmental'
measures: Consequences for behavior-genetic analysis. Behavioral
and Brain Sciences 14 (3) 393.
Commentary of Plomin, R. & Bergeman, C. S. (1991) The nature of nurture: Genetic influences on "environmental" issues. BBS 14 (3) 373.
Commentary of Plomin, R. & Bergeman, C. S.
(1991) The nature of nurture: Genetic influences on "environmental" issues. BBS
14 (3) 373.
Wim E. CrusioInstitut für Humangenetik und Anthropologie
Electronic mail: J31@DHDURZ2.bitnet
Dr. Plomin is one of the most accomplished human behavior-geneticists of the moment, as evidenced by the many valuable contributions to the field by him and his collaborators. Again, the present target article has some important implications, this time not only for behavior genetics, but also for mainstream psychology. P&B argue quite convincingly that certain measures often used by psychologists to assess environmental influences on a subject contain a genetic component. Hence, at least as long as indirect environmental measures are involved, these variables are not valid as such, but, rather, phenotypes amenable to genetic analysis. Unfortunately, P&B do not phrase their conclusions in this way. Although in the title of their target article the word "environmental" is placed within quotation marks, in the rest of their target article they stick to this terminology. In my opinion, this is unfortunate and promotes confusion between some important concepts of genetic analysis.
In the eight paragraph of their introduction, P&B explain the difference between a so-called environmental measure and environmental influence. I would have preferred that from this point on, they would have distinguished more clearly between the different types of so-called environmental measures. It is evident that many measures labeled as "environmental" are, in fact, variables that at least partly reflect properties of the subjects themselves (such as ratings of the environment). Treating such variables as environmental measures is clearly invalid.
However, another, and more disconcerting, implication of P&B's findings is not elaborated upon in the target article. I would like to illustrate this aspect at the hand of P&B's example (third paragraph of the Introduction), in which number of accidents experienced by a subject is taken as an environmental measure. As stated by P&B, this measure may be influenced by such personality traits as risk-taking behavior. If the latter were influenced genetically, this would mean that persons carrying particular alleles will, in general, experience more accidents than persons carrying other alleles. Unlike other "environmental" measures like parental care (which is a character of a subject's relatives) or perceived parental care (which is the same as the foregoing, but now filtered through the subject's own personality), the number of accidents experienced may indeed be considered as a direct measure of environmental influence. This is therefore a prime example of genotype-environment covariation, cov(G,E): subjects of some genotypes will prefer high-risk environments more often than those of other genotypes. Measures that reflect (possibly genetically-influenced) properties of subjects' relatives (mostly their parents), with whom they share part of their alleles, may represent other cases of cov(G,E).
Insofar as all "environmental" measures discussed by P&B do indeed reflect some properties of a subject's environment, the target article may be interpreted to show that genotype-environment covariation plays a pervasive role in the regulation of human behavior. Whereas in animals, cross-breeding experiments may be designed in ways that minimize cov(G,E), this cannot be done in human populations. Still, almost all quantitative-genetic designs that are applicable to human populations assume that cov(G,E) is absent or negligible. If cov(G,E) is indeed present, the results obtained using such designs will be biased in rather unpredictable ways. Furthermore, even when disregarding this problem, any genetic effect now becomes very difficult to interpret: it may be direct (that is, the phenotype is the direct result of the expression of certain pieces of DNA) or indirect (for instance when a phenotype is influenced mainly by environmental influences, but the subject shows some genetically-influenced preference for certain types of environments). In my opinion, the most important aspect of P&B's findings lies therefore in its consequences for the interpretation of a large body of human behavior-genetic research.