In any one individual, genetic and environmental influences are inextricably mixed. This does not mean, however, that it is pointless to try to separate them. Behavioral genetic methods can provide information that may be useful to investigators in other areas of psychology.
2. Nor do I have any quarrel with Caporael's assertion that the outcome of development at each stage is contingent upon the results of development at previous stages, and that, at each stage, genes and environment "interact" (I assume she is using this word in the colloquial rather than the statistical sense). But at this point we part company. Because each stage of development arises from a previous stage in which genetic and environmental influences were thoroughly intermingled, Caporael concludes that trying to separate them is impossible, pointless, and maybe a trifle uncouth. I disagree: I continue to believe that Caporael's attacks on "nature-nurture dualism" are counterproductive.
3. Trying to separate genetic from environmental influences is a job description for the field of behavioral genetics. The work being done in this field is rigorous and interesting, the results have a robustness seldom seen in psychology, and some of the findings are unexpected. It was one of these unexpected findings that provided the impetus for my own recent work. Caporael (1996) has favorable things to say about group socialization theory (Harris, 1995a) and the ethological data I used to support it; she just wishes I had left out the nature-nurture stuff. But without the nature-nurture stuff, the article she refers to would never have been written. Group socialization theory is my attempt to solve a puzzle turned up by behavioral geneticists. Without the data produced by the nature-nurture methodology Caporael rejects, no one would have known there was a puzzle.
4. Let me demonstrate, using Caporael's own hypothetical example, how behavioral genetic methods can provide us with valuable and useful information that cannot be obtained any other way. The example (Caporael, 1996) involves a pair of identical twins who both have a gene that makes them sensitive to a certain chemical. During childhood, one of the twins is exposed to the chemical; the other is not. Later in development, the exposed twin develops some "phenotypic effect" -- a rare disorder, perhaps -- as a result of exposure to the chemical. Caporael uses this example to demonstrate the fruitlessness of trying to separate environmental influences from genetic influences:
The gene had to be present for the chemical to have an effect; similarly, the chemical had to be present for the gene to have an effect. Is it true that 100% of the phenotypic variance is genetic? Yes. And it is equally true that 100% of the phenotypic variance is environmental. Given that relevant epigenetic factors enable the activity of genes, it makes little sense to declare genes or environments as "determinants" of behavior. The "determinants" are the developmental process itself. (Caporael, 1996, par. 5)
5. In giving us this example, Caporael presents us with a fait accompli: we are provided at the outset with the information that it was a gene that made both twins susceptible and a chemical that triggered the disorder in the affected twin. In real life, this information is not known at the outset: we see a disorder and do not know what caused it. Was it caused by a defective gene, by exposure to some environmental toxin, or by a combination of genetic and environmental factors? Most medical and psychological disorders, like Caporael's hypothetical example, are caused by a combination of genetic and environmental factors. However, a disorder will occasionally turn up that is entirely genetic (e.g., certain growth defects) or entirely environmental (e.g., radiation sickness).
6. If all we had to go on was a single affected individual, it would be virtually impossible to determine the cause of the disorder. In any one individual, as Caporael points out, genetic and environmental influences are inextricably mixed. However, there is a way to disentangle them. It involves looking at a number of individuals, using the sort of paired-comparisons procedure invented by behavioral geneticists. Looking at many pairs of individuals who vary in degree of genetic similarity gives us information about genetic influences; looking at many pairs who vary in environmental similarity gives us information about environmental influences. Analyses of variance and numerical estimates of heritability are just frills (and controversial ones at that); the real essence of behavioral genetic methodology is the simple correlation.
7. With a rare disorder of unknown etiology, a good way to begin is with a twin study. If the results of the twin study indicate that the disorder is entirely genetic (identical twins are always concordant for the disorder, fraternal twins only occasionally so), researchers could go on to search for the defective gene, determine what protein is produced by the normal version of this gene, and perhaps find a way of inserting either the protein or the gene itself into affected individuals. If the disorder proves to be entirely environmental (identical and fraternal twins are equally likely to be concordant), we can search for the environmental cause and try to eliminate it. If, as in Caporael's example, both genetic and environmental factors are involved (identical twins are not always concordant but they are concordant more often than fraternal twins), we can use genetic testing to identify individuals who are susceptible to the disorder and guard them against exposure to the chemical that triggers it.
8. Although Caporael's example involves a single gene and a single environmental factor, the same methods can be used when there are multiple genes and multiple environmental factors, and when our goal is not to cure a rare disease but to understand the determinants of human behavior. We will not make much progress toward that goal if we limit ourselves to a few old familiar methods. A better strategy is to use as many different methods as possible, in order to attack each problem from many sides at once. The work Caporael is doing is valuable and I urge her to continue it. But I hope that she, and others who share her views, will not let their prejudice against "nature-nurture dualism" prevent them from making use of the information provided by the methods of behavioral genetics.
Caporael, L.R. (1995) Sociality: Coordinating Bodies, Minds and Groups. PSYCOLOQUY 6(1) group-selection.1.caporael.
Caporael, L.R. (1996) Repeated Assembly. PSYCOLOQUY 7(4) group-selection.6.caporael.
Harris, J.R. (1995a) Where is the child's environment? A group socialization theory of development. Psychological Review, 102, 458-489.
Harris, J.R. (1995b) Individual Differences Within Human Groups. PSYCOLOQUY 6(40) group-selection.4.harris.