For those who support the hypothesis that just as certain phenotypical traits are differentially distributed that therefore the same would hold for different populations as a whole. The usual proof offered for this hypothesis is the IQ test and its average scores. But all IQ tests demand behavioral responses to a set of recognised stimili--paper, pencils, recognition of words, patterns, numbers, etc. So the environmental/cultural input would be obviously influential here.

But given intra-cultural[think of obvious things such as regional accents, taste in foods, behavioral gestures, etc.] and inter-cultural environmental variabilities--absent a totally uniform environment the thoughtful researcher would always be stymied by the possible differential gap between an individual's NOMINAL SCORE and his/her potential REAL SCORE.

But back to the main question: is it possible that the environments of frigid and temperate weather Eurasia could have selected for more efficient cognitive abilities in humans. We have the answer already.

Elements of Homo Erectus migrated from East Africa some 500,000 years ago to settle in Frigid weather Eurasia. These elements became full humans in the process with their body plans and other traits adating to the new environments. These elements were given the name Neanderthal by physical anthropologists.

The Neanderthals survived in Eurasia(and Europe of course) for more than 300,000 years and had to face all the challenges that the frigid and seasonal weathers and ecologies threw at them. If the variable-ecology hypothesis were correct then the Neanderthals would have developed technologies that would have been more developed than those of the incoming migrants from tropical and sub-tropical Africa. They would have proven themselves to be more equipped adaptively than the much more recent incomers.

But it didn't work out that way. The tropical ecology adapted migrants from Africa proved to be much more adaptive than the long-resident Neanderthals--who by the way had larger crania than their incoming guests. Ergo, the ecologies of cold-weather Eurasia really was not more challenging than the environments of Africa.

Furthermore, survivability in Eurasia did not require any more cognitive abilities than knowing how to track game-animals, scavenge, pick wild fruit and to find a safe and warm place to live. Note that the ability to create fire artificially was already discovered in Africa. So too the ability to make body clothing against the elements. So too the ability to create lithic tools for for hunting and cutting.

The point is that once ability to think abstractly was developed--as demonstrated in African cave paintings--humans were were more than adequately cognitively equipped to survive anywhere.

Sure there are genetic configurations that produce specific non-adaptive(Tay Sachs) traits in some groups or traits that are adaptive(Malaria) only for specific environments but such affect only a very small proportion of the populations in question. Proof: in the Malaria areas of Africa people still routinely succumb and once in awhile some people die. Some people are chronic malaria sufferers. So where are the supposed prophylatic effects of the Sickle Cell syndrome. Note too that West African variety of Sickle Cell is also found in Greece, Sicily and Turkey.

The same for Tay Sachs and Cystic Fibrosis(Northern Europeans). These non-adaptive syndromes affect only very small percentages of the relevant populations.

In the case of Tay Sachs one may say that the relevant assumed recessive genes manifested themselves as dominant because of close kin assorted mating. But close-kin assorted mating was/is the norm for a significant percentage of the world's populations. In many parts of Africa and Asia it is routine for for first cousins to marry.

But again the world's peoples did not learn to read and write until a few hundred years ago. The capacity was there all along with the first homo sapiens populations in Africa. So whatever differences there are in POPULATION IQ scores world wide are due only to environmenal/culture effects.

After all, Greece which serves as the "civilizing"--as is claimed--template for post-Barbarian Western culture scores 92 on IQ tests. Westerners also claim that the earliest inspiration for their intellectual culture derives from Mesopotamia should also know that Iraq(modern Mesopotamia) scores 87 on IQ tests.

The simple explanation for group differentials in IQ is that industrialized societies with its specific kinds of cultures and sociological environments score higher than those that are not. At present India has an IQ of 81 but as it continues to industrialize its score will increase. The same for places like Thailand(91), Indonesia(89), Nepal(79) and Burma(86). As they industrialize their scores will change.