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May 14, 2004

Genetic, heritable...people talking past each other....

After my exchange with Frank in the The great "IQ hoax" post, I would like to reiterate one point. When I say something is 50% heritable, I do not mean that 'the trait is 50% genetic'. Rather, I mean that 50% of the variation within a population is due to variation in the genotype (I am almost always talking about 'narrow sense' heritability, so you can usually add the qualification about 'additive genetic variance,' that is, excluding dominance effects). To further clarify, the number of fingers you have on your hand is a genetically determined trait. There isn't much variation within the human species, because the fitness cost is high if you are missing fingers, as are having extra digits that interfere with full function. On the other hand, height (or intelligence) is more contextual in its fitness, and there is a continuous spectrum of variation within human populations [1]. Something like height is determined by both environment and genetics, and the amount of variation in the population due to genetics is dependent on the environment that the population expresses its phenotype within....

1. In ideal environments, tall males might have a advantage in sexual competition, but in famine conditions, the smaller caloric intakes of those of short stature might make that a moot point.

Posted by razib at 05:57 PM