THE MULTIFACTORIAL THEORY OF INHERITANCE AND ITS APPLICATION TO INTELLIGENCE 1

Abstract

Abstract. The hypothesis put forward for discussion is that, among normal persons, differences in “intelligence” (i.e., innate general cognitive ability) are determined by a large number of genes, segregating in accordance with Mendelian principles and each producing effects that are small, similar, and cumulative. From these assumptions, with a due allowance for dominance and assortative mating, formulae are derived for the correlations to be expected between siblings, parents and offspring, and remoter relatives. Data from school surveys yield coefficients which are in almost complete agreement with the theoretical figures thus deduced. An attempt is also made to analyse the variance of the assessments obtained, and to estimate the contributions of genetic factors and non‐genetic factors respectively. Appreciable differences are found between the results obtained from raw measurements based directly on intelligence tests and those obtained from revised or adjusted assessments based on all the available evidence. With the former nearly 25 per cent. of the variance is apparently due to non‐genetic factors; with the latter barely half that amount.

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