Cold winters theory

Cold winters theory is a theory proposed by controversial psychologist J. Phillippe Rushton as an explanation for his belief that humans from Africa are less intelligent than people from Northern latitudes.
Mechanism
According to the model, when some human populations left Africa, they encountered new climates and adapted to them. Colder, the proponents believe that northern climates to be more difficult to survive in and that they would therefore, according to the model, increase or create additional selection pressure for the evolution of higher intelligence.<ref name=":1" /><ref name=":2" />
In support of the model, several studies have correlated latitude, mean annual temperature and similar metrics with national IQs estimated from IQ data and scholastic tests such as PISA, and observed very strong correlations. For instance, a 2006 study correlated mean temperature, national IQ, skin color and per capita income in a sample of 129 countries.<ref name=":3" /> They reported a correlation between mean winter temperature and national IQ.
Criticism
A 2014 study reported a correlation between the average IQ of US states and mean temperature.<ref name=":0" /> This relationship was also present when they controlled for percent Blacks, r = -.53. The authors interpreted this as being evidence against the cold winters theory because it shows that such correlations can arise in a situation where we know that the differences are not due to evolution.

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