We are all the way we are because of a mixture of inherited characteristics and environmental factors. The importance of heredity was obvious to common observation for thousand of years before genetic theory produced a convincing explanation of why, for example, some looks, skills, quiddities, deficiencies, and diseases run in families.

Plato's proposals in The Republic (380BC) for a perfect society were based, in part, on the assumption that it should be composed of perfect individuals: the best citizens should be encouraged to reproduce. The children of the dim and deformed should be exterminated to stop them from breeding. This was a glaringly bad idea: quite apart from its moral repugnance, it would be unlikely to work. Mental or physical qualities, even if they could be satisfactorily identified, constitute a small part of character and make only a partial contribution to an individual's worth.