Two, the second hypothesis of this study is that the north–south gradient of IQs in Italy may explain much of the difference in economic development between the north and southof Italy. This hypothesis is confirmed by the correlation of 0.937 between regional IQs obtained in 2006 and per capita incomes in 2003. This correlation indicates that IQ differences explain 88% of the variance in per capita incomes across Italian regions. The 14 IQ point difference between the most northerly region of Friuli-Venezia (IQ=103) and the most southerly region of Sicily (IQ=89) can be compared with the 10.1 IQ point difference in the United States between the highest and lowest states (Massachusetts IQ=104.3; Mississippi IQ=94.2) [...], and the 8.1 IQ point difference in the British Isles between the highest (London and the south east IQ=102.1) and the Republic of Ireland (IQ=96.0) [...]. The large regional IQ differences in Italy help to explain the large regional differences in per capita income. It is proposed that these correlations between population IQ and per capita income arise through a positive feedback loop in which the population IQ is a determinant of per capita income, and per capita incomeis a determinant of the population IQ. Thus, the population's IQ is both a cause and a result of its per capita income. The population's IQ is a cause of its per capita income because individuals and populations with high IQs are able to work more efficiently than those with low IQs and consequently command higher incomes. The population's IQ is a result of its per capita income because populations with high IQs provide a better environment (better nutrition, health care and education) for the development of the intelligence of their children. This positive feedback loop is known in behavior genetics as genotype–environment correlation [...].
