I just listened to a story about intelligence on BBC. Some researchers have found that people in wealthier countries are significantly more intelligent than those in poorer countries.
Neither the researchers, BBC, nor anyone they interviewed, pointed to what seems to be the most obvious explanations: Intelligence test measure certain skills that are highly culturally dependent and trainable. In societies with a western style education system, people will naturally score higher on intelligence test created within a western culture and mindset. Of course, these countries also happen to be wealther, in the western definition of the term (narrowly focusing on money).
As a case in point, I have dramatically improved my score on “intelligence” test which supposedly measure some “inherent” ability – through plain old practice (for instance, 40 point improvement on IQ tests). Mostly, it is a matter of figuring out what they are looking for – as is the case with just about any test on any subject (tests are generally to a minimal degree about any inherent ability, or a wider understanding of a particular topic).