5 Variation and Modification

What has been said about means also extends to making claims about intervention. The causes of variation that emerge from twin and family studies relate to a particular population of genotypes at a specific time in its evolutionary and cultural history. Factors that change the gene frequencies, the expression of gene effects, or the frequencies of the different kinds of environment may all affect the outcome of our studies. Furthermore, if we show that genetic effects are important, the possibility that a rare but highly potent environmental agent is present cannot entirely be discounted. Similarly, a rare gene of major effect may hold the key to understanding cognitive development but, because of its rarity, accounts for relatively little of the total variation in cognitive ability. In either case, it would be foolhardy to claim too much for the power of genetic studies of human differences. This does not mean, however, that such studies are without value, as we shall show. Our task is to make clear what conclusions are justified on the basis of the data and what are not.