There is an article here that shows strong ecological correlations between measures of IQ and the mean temperatures of American States.
(Note: "Ecological correlation" is a technical term in statistics that has nothing to do with environmentalism. It refers to correlations between grouped data -- and such correlations must not be confused with correlations among individuals. Ecological correlations are characteristically much higher than individual correlations.)
What the data purport to show is, broadly, that the inhabitants of warm Southern States are dumber than the inhabitants of cool Northern States. There have been previous politically motivated "studies" that purported to show that but they have been shown to be fraudulent or very poorly founded. This study, however, is a scholarly one which uses a reasonable (though still inferential) measure of IQ.
There are however reasons why I believe that the conclusions of the study cannot be accepted. The simplest and strongest objection is the old adage that correlation is not causation. It might not be the warmth in Southern States that causes the lower IQs but some other characteristic of those States. He is very diplomatic about criticizing a co-blogger but Razib points out cultural differences that could be involved, for instance. Different parts of the USA tended to be settled from different areas of the British Isles and cultural differences can be surprisingly persistent over time. And a culture that is highly reverent of education (for instance) might tend to attract high IQ people and repel low IQ people. So New England with its disproportionate number of America's elite universities would draw unto itself the high IQ people from all over and thus raise average IQs there.
I myself once did a lot of research on another aspect of warmer climate: The effect on ideology. My home State of Queensland (in Australia) at one time had a reputation as being particularly conservative so I did survey research to test that. Initially, I found the hypothesis confirmed. Surveys carried out while Queensland was ruled by the very conservative Sir Johannes Bjelke-Petersen confirmed that Queenslanders were more conservative in general social attitudes than were inhabitants of the more Southerly (and hence cooler) State of New South Wales. I have however never been one to found conclusions on a single piece of research so I kept doing more surveys on the question. And I eventually found that the differences had run their course and were no longer to be found. See here.
So a difference that appeared to be characteristic turned out to be temporary. That could be equally true of the American IQ results reported above. Both conservatism and IQ are highly hereditary so there is no reason to differentiate the two variables for that reason. Heredity may be the dominant influence in both cases but environment does still play a part, a part quite sufficient to explain small geographical differences.
And I want to suggest what one of those environmental differences could be: Dumbed-down education. And a major reason for educational dumbing down is to cover up the extraordinarily poor performance of blacks at educational tasks. So where there are more blacks there will be more pressure to dumb education down. And where are there more blacks? The difference is certainly much less that it was but it is still the Southern States where blacks are proportionately more present. So it is in the South that one is most likely to find down dumbed down education.
And education DOES have a small effect on IQ scores. A widely accepted explanation for the Flynn effect (rising IQ scores over time), for instance, is the increasing test-sophistication that accompanied the rise in average years of education in the second half of the 20th century. So I suspect that the North/South differences that we are looking at above are an effect of poorer education in the South rather than an effect of lower IQs in the South. And it should be noted that the IQ scores used in the research above were in fact inferred from educational attainment.
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