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Jencks' (1972) classical study Inequality reported a correlation of 0.310 between IQ and income for men in the United States. The present study examines whether this result can be replicated in Britain. Data are reported for a national sample whose intelligence was obtained at the age of 8 years and whose income was obtained at the age 43 years. The correlations between IQ and income were 0.368 for men (n = 1280) and 0.317 for women (n=1085).
Key Words: IQ; Income; Inequality; U.S.A.; Great Britain
1. Introduction
The problem of why some people earn more than others, and therefore why there is so much social and economic inequality, has been of major interest in the social sciences of economics, sociology and psychology for decades and even for centuries. Many theories have been advanced to explain income inequality, including the strength of the work ethic, achievement motivation, the socio-economic status of parents, education, luck and intelligence. The contribution to the problem of the last of these - intelligence - is the subject of the present paper.
The classical study of the contribution of intelligence to differences in income is Jencks' (1972) Inequality. In this he gave a correlation of 0.310 (corrected for attenuation to 0.349) between IQ and income for a white male non-farm American sample aged 25-65. From this he concluded that IQ differences make a modest but significant contribution to differences in incomes. He concluded also that IQ has a heritability of about 50 per cent, and therefore that genetic factors contribute to income differences. Jencks' study has three shortcomings. First, his sample is not representative of the American population. second, the sample is for men only. Third, the data for IQ and income were obtained at the same time in a sample with an age range of 25-64 years and a median age of 45 years. The direction of causation can be questioned in the Jencks' data. It could be argued that the causation is from income to IQ, on the grounds that individuals with high incomes have more cognitively demanding jobs and that this may increases their intelligence (the "use it or lose it" hypothesis).
In a subsequent study Jencks (1979) examined IQs obtained by males at age 17 and incomes...