Because of the contextual nature of measured heritabilities, paradoxes often arise. For example, the heritability of a trait could be near 100% in one study and close to zero in another. In one study, e.g., a group of unrelated army recruits may be given identical training and nutrition and then their muscular strength may be measured. The variation in strength observed after the (identical) training will translate into a high heritability estimate. In another study, whose purpose might be to assess the efficacy of various workout regimes or nutritional programs, study subjects may be first chosen to match each other as closely as possible in prior physical characteristics before some of them are put onto Program A and others onto Program B, and this will lead to a low heritability estimate.
In the case of scholastic ability, how well one does in the final school exams depends on both what and how well one was taught, how hard one has studied, how ‘naturally’ smart one is and, of course, on a fair bit on luck. The actual heritability estimate will depend on the subjects used (reflecting genetic variation) and the testing conditions (reflecting environmental variation).
