Mind the gap(s)... in theory, method and data: Re-examining Kanazawa (2006)

Kanazawa (2006) has put forward an evolutionarily grounded theory which claims that individuals in wealthier and more egalitarian societies live longer and stay healthier not because they are wealthier or more egalitarian but because they are more intelligent (2006: 637). The claim rests on an argument which asserts that general intelligence is a solution to evolutionarily novel problems and that most dangers to health in contemporary society are evolutionarily novel. Kanazawa also claims that this relationship does not hold in sub-Saharan Africa. These claims are based on a cross-national analysis which finds a positive correlation between 'national' IQ scores and mortality data. The implication is that intelligence is the principal factor determining longevity in the rest of the world, regardless of issues such as adequacy of diet and availability of health care. Kanazawa's theoretical claims about the evolution of general intelligence as a domain-specific adaptation are inconsistent with adaptationist analysis: natural selection does not solve general problems. The assumptions that sub-Saharan Africa is more representative of the evolutionary past than is the rest of the world, and that most hazards to health in contemporary society are evolutionarily novel, are implausible. The methods used are inadequate because Kanazawa argues for causation from correlation and fails to consider alternative explanations. The IQ data are flawed for reasons to do with sample size and sampling, extrapolation and inconsistency across measures. Nor are they temporally compatible with the economic and demographic data.