At a third level of generality, positive correlations between IQ and per capita income have been reported across nations at a magnitude of approximately 0.7 [...]. This finding has been confirmed in studies that have re-examined the data using alternative measures of per capita income [...] and by studies that have used national scores in math, science, and literacy as proxies for intelligence [...]. The positive correlation between IQ and per capita income across populations is to be expected from the correlation among individuals, because populations are aggregates of individuals, and populations with higher IQs can supply goods and services with greater value than those with lower IQs, and hence command higher incomes.
In this paper we examine the possibility that the north–south difference in per capita income in Italy may be due to differences in intelligence. There is some existing evidence suggesting this may be the case. In northern Italy, Prunetti [...] has reported a standardization of the Colored Progressive Matrices on 500 6–11 year olds in Pisa and the surrounding countryside, and Tesi and Young [...] have reported a standardization of the Standard Progressive Matrices on 2.462 11–16 year olds in Florence and the surrounding countryside. Both studies found that the mean IQ in northern Italy is approximately the same as in Britain and other countries of northern and central Europe. It has not proved possible to find normative data for IQs in the south of Italy. However, Peluffo [...] has reported that the cognitive development of children in southern Italy and Sardinia (one of the poorest regions and part of the south) lags behind that of children in Genoa in northern Italy and in Switzerland in the performance of Piagetian tasks of the understanding of conservation and causality. For example, 65% of 9 year olds in Genoa succeeded in the conservation of volume task, compared with only 35% of 9 year olds in the south. Piagetian tasks can be regarded as tests of intelligence. A correlation between the two of 0.49 is reported by Jensen [...] as the average of 14 studies.
