During the cooling periods we experience lower crop yields, less predictable precipitation, and more deaths by plague. Forget all the caterwauling the political class does about global warming. We should be glad we live in the Modern Warming Period. During the warm periods the cities become more populous because a relatively small acreage of land can feed a large city. During the cooling periods the cities depopulate because people have to spread out to grow crops (at least before modern transportation).
During the Modern Warm Period we've had a few major wars and atrocities because of people like Hitler and Stalin and Mao. During the cooling periods there are more atrocities because virtually everybody is Hitler -- even the most noble leaders will do anything it takes for his people to survive the harsher conditions of life in the cooling periods. My Anglo ancestors were kicked out of their homeland (Angli in modern day Germany) because the Germanic goths forced them out (call it our version of the Trail of Tears). We know this is because the Huns themselves migrated to where the Goths were, forcing the goths out.
Some say it was because the Dark Age cooling period reduced crop yields on the Huns, forcing them to try to find better land west. Likewise, later when many European descendent Americans were starving in the eastern portion of the U.S. during the Little Ice Age, it was all the more tempting to take over land and expand (more crop land = more food). During the same Little Ice Age Africa experienced mega-droughts. Thus it was more tempting for black leaders to take over neighboring lands to feed their own folks -- with the blacks living on those lands taken to the coasts and sold as slaves.
When stuff like that's going on during the cooling periods, there's not much idle time to be spent on philosophy and such. I can see why historians look at those times and assume, wrongly in my opinion, that IQ was lowered.
