Genetic Determinism

The first variety of genetic determinism is the rather unsophisticated doctrine of genetic fixity, which holds that the genes of parents inevitably determine the characteristics of their children. (Dennett [1996] dramatizes this simple form of determinism under the label sphexishness.) Genes, of course, do influence human development. The differences between humans and chimps are almost entirely genetic. However, even the completion of the human genome project will not enable scientists to predict how a child will develop. Indeed, even a complete knowledge of a child's genes and environment would not allow the complete specification of the organism: chance also plays a significant role in development. This can be seen, for example, in the number of bristles under a fruitfly's wing: this number varies from the left wing to the right, despite the fact that the fruitfly has the same genes and environment on both sides of its body (Lewontin, 1991). Such differences may not be due to pure chance, however, but rather to non-linear dynamic (chaotic) deterministic processes (Molenaar, Boomsma, & Dolan, 1993). Such processes have been shown possible using computer simulations, but whether they operate in real organisms remains an empirical question.

The second variety of genetic determinism is the slightly more sophisticated doctrine of innate capacity, according to which people are like buckets waiting to be filled (Lewontin, 1991). In an impoverished environment, all people will end up with similar characteristics (wealth, knowledge, etc.); but in an enriched environment, those who naturally have big buckets will end up with more than those with small buckets could possibly hold. For example, people who are malnurished will show smaller individual differences in height than those who are well nourished.

The third and most sophisticated variety of genetic determinism is the doctrine of statistical variation, according to which all individual differences can be parsed into either genetic determinants or environmental determinants in some proportion (Lewontin, 1991). For example, it might be said that 80 percent of the variance in children's performance on I.Q. tests is due to genetic influences, and only 20 percent is due to environment. The practical implication seems to be that even a radical change in environment will have only a modest effect on performance. However, this is not the case, as can be seen from the following examples. An ordinary student in primary school today can add a column of numbers much faster than even the most intelligent ancient Roman mathematician, who had to deal with cumbersome X's, V's, and I's. The same student, using an inexpensive calculator, can multiply two five-digit numbers faster than even the most intelligent mathematician a century ago (Lewontin, 1991).

Heritability does not imply immutability. This can be seen from the example of PKU (phenylketonuria), a form of retardation. PKU can be cured by keeping people from eating phenylalanine. One hundred years ago the proportion of genetic variation in acquiring PKU was 100 percent; now individual differences in acquiring PKU are almost completely non-genetic (Plomin, DeFries, & McClern, 1990).


Dennett, D. C. (1996). Elbow room: The varieties of free will worth wanting. Cambridge, MA: MIT Press.

Lewontin, R. C. (1991). Biology as ideology: The doctrine of DNA. New York: Harper Collins.

Molenaar, P, C. M., Boomsma, D. I., & Dolan, C. V. (1993). A third source of developmental differences. Behavior Genetics, 23, 519-524.

Plomin, R., DeFries, J. C., & McClern, G. E. (1990). Behavioral genetics: A primer (2nd ed.). New York: Freeman.

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