In this paper it is proposed that regional differences in intelligence are the major factor responsible for the regional differences in Italy in per capita income and in the related variables of stature, infant mortality, and education. This hypothesis is derived from the extensive research showing that intelligence is positively related to income. This relationship holds at three levels of generality: among individuals, across regions within nations, and across nations. At the level of individuals, the classical study of the contribution of intelligence to differences in income is Jencks' Inequality. In this he estimated a correlation of 0.31 (corrected for attenuation to 0.35) between IQ and income for a white male non-farm American sample aged 25–65. He concluded that this is a causal relationship such that IQ differences make a significant contribution to differences in incomes. He also concluded that IQ has a heritability of about 50%, and therefore that genetic factors contribute to income differences.
Jencks' conclusions have been confirmed by a number of subsequent studies in the United States [...] and also in Sweden [...]. An analysis of eight studies of the relation between intelligence and income concluded that the correlation is 0.27 [...]. In arecent meta-analysis of 85 data sets drawn from the United States, the United Kingdom, Norway, Australia, New Zealand, Estonia, Netherlands and Sweden, Strenze (2007) concluded that in all studies the correlation between intelligence and income is 0.20, in the best studies the correlation is 0.23, and in 35–78 year olds the correlation weighted by sample size is 0.25.
This meta-analysis did not include a recent study of a national sample in Britain in which a correlation of 0.37 between IQ obtained at the age of 8 years and income at the age 43 years was found for men, and for women the correlation was 0.32 [...].
The positive correlation between IQs in childhood and income in middle age suggests that IQ is causal to subsequent income. This has been confirmed by studies of sibling pairs that have shown that siblings with higher IQs have higher earnings than their lower IQ brothers and sisters [...]. The use of sibling pairs controls for possible family and neighborhood effects that might affect both IQ and income. The likely explanation for the positive correlation between IQ and income is that those with higher IQs work more efficiently [...] and can supply goods and services with greater value than those with lower IQs, and consequently can command higher incomes.
