Genetic Determinism

The American Psychological Association (APA) defines genetic determinism as:

“the doctrine that human and nonhuman animal behavior and mental activity are largely (or completely) controlled by the genetic constitution of the individual and that responses to environmental influences are for the most part innately determined.”

While the APA definition is a useful starting point, it is also unsatisfyingly circular, as the word to be defined (determinism) is part of the definition (“innately determined.”) Genetic determinism is also defined, inappropriately in my view, as a “doctrine,” that is, as a matter of faith or ideology, rather than as an empirically-falsifiable proposition.

In order to refine the definition of “genetic determinism”, we can go back a step, and consider the definition of a more general term, “determinism” (Hoefer, 2016):

“The world is governed by determinism if and only if, given a specified way things are at a time t, the way things go thereafter is fixed as a matter of natural law.”

Applying that definition to the narrower case of genetic determinism, one potential definition would be:

A phenotype is genetically determined if and only if, given a specified genotype at time t, the way the phenotype develops thereafter is fixed as a matter of natural law.

A genetically determined phenotype, according to this definition, is predictable: If one knows the genotype at time t, then one can predict the phenotype at subsequent times with complete (or nearly complete) certainty. Using Ned Block’s classic example of number of fingers, having five fingers on each hand is genetically determined, because, if one has a particular genotype at time t, then one will (almost) invariably develop five fingers on each hand (Block, 1996). This certainty about the organism’s future phenotype is warranted because of the stability of the genotype-phenotype relationship across environmental contexts (“fixed as a matter of natural law”).

I say “almost” invariably, because, there are environmental exposures that might alter morphological development beyond one’s genotype, such as exposure to thalidomide in utero. Yet the existence of this exception does not do much to shake our intuition that number-of-fingers is genetically determined, as exposure to teratogenic drugs is a rare event that is not part of the “expectable environment” for humans, regardless of time or place. Thus, as the philosopher of science James Woodward (2010) pointed out, “stability comes in degrees.”

Like Woodward (2010), the psychiatrist Ken Kendler (2005) has used the phrase “non-contingency” as a synonym for “stability”. A non-contingent relationship between genotype and phenotype is:

“not dependent on other factors, particularly exposure to a specific environment or on the presence of other genes.”

Tabb and colleagues (2019) similarly emphasize the stability / non-contingency of the genotype-phenotype relationship in their definition of genetic determinism as applied to human behavior:

“the notion that behavior genetics identifies genes that restrict human freedom by guaranteeing certain behavioral outcomes.” (my emphasis added)

That is, given the genotype, the phenotype (in this case human behavior) could not be otherwise.

Putting these different elements together, our definition of genetic determinism could be further refined as:

“A phenotype is genetically determined if and only if, given a specified genotype at time t, the way the phenotype develops thereafter is stable and non-contingent across all or nearly all environmental conditions.”

The astute reader will have noticed by now that genetic determinism, as a concept, does not map to heritability. There are highly heritable phenotypes that are not genetically determined, as the heritability is unstable and contingent across environmental contexts. And, there are genetically determined phenotypes that are not at all heritable, because there is little to no variation in the relevant genes. Attempts to ground definitions of “genetic determinism” in heritability, and to infer a scientist’s beliefs about genetic determinism from their discussion of heritability, rely on a fallacious interpretation of heritability, determinism, or both.

References

Hoefer, C. (2016). Causal Determinism. In E. N. Zalta (Ed.), The Stanford Encyclopedia of Philosophy (Spring 2016). Metaphysics Research Lab, Stanford University. https://plato.stanford.edu/archives/spr2016/entries/determinism-causal/

Kendler, K. S. (2005). “A Gene for…”: The Nature of Gene Action in Psychiatric Disorders. American Journal of Psychiatry, 162(7), 1243–1252. https://doi.org/10.1176/appi.ajp.162.7.1243

Ned Block. (1996, January). How Heritability Misleads about Race. The Boston Review, 30–35.

Tabb, K., Lebowitz, M. S., & Appelbaum, P. S. (2019). Behavioral Genetics and Attributions of Moral Responsibility. Behavior Genetics, 49(2), 128–135. https://doi.org/10.1007/s10519-018-9916-0

Woodward, J. (2010). Causation in biology: Stability, specificity, and the choice of levels of explanation. Biology & Philosophy, 25(3), 287–318. https://doi.org/10.1007/s10539-010-9200-z