5 - IQ similarity in twins reared apart: Findings and responses to critics

Summary

Research on genetic influence on intelligence has a long and contentious history (Brand, 1993; Fancher, 1985; Kamin, 1974). Both the idea of a general factor of cognitive ability, Spearman's g, and the idea that genetic factors might be an important source of variance in cognitive ability have been continuously debated since they were first systematically expounded by Galton (1869, 1876). Reviews of Galton's books published in the London Times at the time of their appearance could, if slight changes were made, be published today. The debate on the nature of mental abilities and the influence of heredity on such abilities (as well as most other psychological traits) initiated by Galton continues unabated.

The current status of g

There should be no doubt that the issues of the measurability of IQ and its usefulness are still controversial issues. Consider the following recommendation regarding the measurement of abilities and other psychological traits:

Make explicit to everyone (pupils, parents, public and professionals of all kinds) that a person's abilities, activities, and attitudes cannot be measured. The public, especially, misperceive that hard data exist, and that test scores constitute these data. The public does not realize how quickly the point is reached where we do not know how to discriminate validly among people, but where data mislead us to think we do. This is what is meant by the myth of measurability

[Tyler & White, 1979, p. 376].