A genetical formula for the inheritance of intelligence in man

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Abstract
The inheritance of natural intelligence in man was studied from (1) data collected by the author on 194 Leicestershire families consisting of 388 parents and their 812 offspring; and (2) the data presented by Woods (1906) on the intelligence of the 424 parents and their 558 offspring in the royal familiea of Europe. Intelligence was rated for each individual, the general mental achievements of the individual being taken as the criterion, into 10 equal grades: 10, illustrious; 9, eminent; 8, brilliant; 7, talented; 6, able; 5, mediocre; 4, dull; 3, subnormal; 2, moron; 1, imbecile; (0, idiot). Equivalent juvenile gradings in terms of I.Q. are estimated at 20 I.Q. per grade, e.g., 200 I.Q. = grade 10, 100 I.Q. = grade 5, 20 I.Q. = grade 1. Judging from the segregation or non-segregation of intelligence in their offspring, it would seem that the most frequent (grade 5) parents are of 3 genetical types, NN, Nn and nn, in which N is a dominant gene for normal mediocre intelligence (grade 5) and nn permits variability. The 10 grades of intelligence are provisionally referred to the action of 5 pairs of minor genes Aa...Ee in the presence of nn. The dominant minor genes A... E act as equal and cumulative increasers of intelligence while the recessive minor genes a...e act as decreasers.

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