winter, snow, fertile land, and just enough lack of resources. areas without cold winters don't need to prepare too much for winter and thus the further you go into the tropics, the lower iq (as these people don't need to prepare or think about future crops or winter survival). however, these areas also had great fertile land, unlike the land further north which is mountainous and still fertile, but not like central and northwestern europe. this created increase caloric intake and vitamins, which lead to much more nourishment and higher iqs. finally the sufficient lack of resources necessitated smarter techniques and ideas (which combined with higher iqs) this meant that they had no choice but to attempt to find ways to find coal, oil, iron, bronze, copper, and food preservatives. and when these items were found, they had to be distilled, and/or refined. japan and korea were in this same situation (snow, and just enough lack of resources) and has lead to the same situation as northwestern and central europe (high iqs and successful cultures). side note, did you know that the mainland japanese think of the okinawans as stupid? okinawa being a pretty tropical set of islands.....this is probably the most likely answer.
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