The derivation of heritability from human twin studies involves serious met
hodological flaws. Heritability is consistently overestimated because of bi
ological confounds of twinning, consistent and often gross underestimation
of the environmental variance: and nonadditive genetic influences that can
hugely exaggerate heritability values. Despite this bad research design, be
haviour geneticists continue to publish results implying that their heritab
ility results are valid.