The derivation of heritability from human twin studies involves serious methodological flaws. Heritability is consistently overestimated because of biological 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, behaviour geneticists continue to publish results implying that their heritability results are valid.
By examining single-word reading times (in full sentences read for meaning), we show that (1) function words are accessed faster than content words, independent of perceptual characteristics; (2) previous failures to show this involved problems of frequency range and task used; and (3) these differences in lexical access are related to perceptual fluency. We relate these findings to issues in the literature on event-related potentials (ERPs) and neurolinguistics.