The Fallacy of Estimating "Heritability" in Medicine and the Behavioral Sciences
By Jay Joseph. Psy.D
(Adapted from a 3/13/2021 twitter post https://twitter.com/jayjoseph22/status/1370785425955065860)
1/ Critics argue that “heritability” is a misunderstood and misused concept, and that “heritability estimates,” which range from 0% to 100% (0.0 to 1.0), do not tell us “how much” genes contribute to human behavioral differences, or how “fixed” human behavior is.
2/ Using the example of the disease favism (Glucose-6-phosphate dehydrogenase deficiency), I illustrate the fallacy of using heritability estimates to assess “how much” genes influence behavior (“IQ,” “personality,” psychiatric conditions, and so on).
3/ Favism is caused by an inherited deficiency of glucose-6-phosphate. The predisposing gene is located on the X chromosome. When the carrier eats fava (broad) beans or inhales fava bean pollen, favism appears. The disease is marked by the development of hemolytic anemia.
4/ In other words, “beans and genes” are both necessary for favism to appear. In the human behavior area, critics often dispute claims that "genes for behavior" have been discovered.
5/ Let’s imagine “Country A,” where all citizens (100%) carry the favism gene. In Country A, 15% of the citizens, all of whom of course carry the gene, are exposed to fava beans and subsequently develop favism.
6/ In Country A, because all citizens carry the gene, but only some were exposed to fava beans, all favism variation is caused by environmental factors (fava bean exposure or non-exposure). The “heritability of favism” in Country A, therefore, is 0% (0.0).
7/ At the same time, it would be very mistaken to conclude that genes play no role in developing favism in Country A, or that the genetic influence is weak or irrelevant. A genetic predisposition is, in fact, a prerequisite for developing favism.
8/ Now imagine “Country B,” where all citizens (100%) eat a diet containing fava beans. In Country B, 15% of the citizens, all of whom of course eat a diet containing fava beans, carry the favism gene and subsequently develop favism.
9/ In “Country B,” because all citizens are exposed to fava beans but only some carry the gene, all favism variation is caused by genetic factors (carrying or not carrying the gene). The “heritability of favism” in Country B, therefore, is 100% (1.0).
11/ In the above examples, the “heritability of favism” simultaneously is 0% in Country A, and 100% in Country B, even though the causes of favism are the same in both countries.
12/ As we see, “heritability estimates” assess variation as opposed to cause, and do not at all indicate the strength or weakness of the genetic influence—or by implication the strength or weakness of the environmental influence.
13/ The use of heritability estimates as an indication of "how much" genes influence behavioral characteristics (traits) or diseases should end. Heritability was developed as a breeding statistic in agriculture, and this remains its only valid use.
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