Evidence of heterosis in man

In animal species the principle of balanced polymorphism due to heterosis has been accepted for many years, but the serious application of the same idea to man has been slow. Human experimental breeding is not possible, and effects, demonstrable in laboratory animals, may be concealed in a randomly breeding population. On the other hand, populations under natural selection may be suitable for certain kinds of studies not possible in experimental work. Evidence for the existence of some kind of hybrid vigour in man comes from both direct and indirect measurement. Most of the evidence is indirect, but the few examples of direct measurements on known or supposed heterozygotes may be considered initially.