What Heritability is Not

Because so many people abuse or misunderstand the concept of heritability, I decided that it would be nice to have a list of what heritability is not in one place. If you have questions or if there is a misconception about heritability you’d like me to address here, feel free to comment. This post will serve as an updated reference.

• Heritability is not an indicator of malleability. Entirely genetic disorders such as phenylketonuria can be cured through the proper diet.
• Heritability is not a measure of straightforward genetic effects. For example, genes that affect physical appearance have an effect on personality development.
• Heritability is not independent of the population. It may differ from one group of individuals to the next, because groups differ environmentally and genetically.
• Heritability is not independent of age. The effects of genes or environments may grow in potency through development.
• Heritability is not an indicator of the causes of group differences. A trait can be highly heritable, as in the crop field metaphor, and group differences may still be due to environment. This applies also in the real world situation for humans, where the environmental differences between groups are not as systematic.
• Heritability is not necessarily homogeneous within a population. A heritability of 50% may be hiding the heritabilities of 40% and 60% in subgroups.
• Heritability is not a measure of intergenerational transmission. A trait may be highly heritable but not pass on from one generation to the next. This is because the relevant genes and environments may differ from one generation to the next.
• Heritability is not a statistic for individuals. If you are using your knowledge of heritability to understand a single individual you are a biographer, not a scientist.

So, some of you may be wondering, why is heritability a useful statistic? That’s easy to answer: it’s a measure of how much phenotypic variation in a given population at a given time is due to genetic variation in that population. Measuring heritability allows us to say that, for adults in the modern world, variation on IQ and personality measures is primarily due to genetic variation. That’s a pretty remarkable, and important finding if you ask me.