Why did the European population develop such a high concentration of Creative Geniuses growing and peaking sometime late-ish in the span between 1000 and 2000 AD?
Creative genius requires:
1. High general intelligence
2. High creativity - which is correlated with the personality trait of Psychoticism
(as described by HJ Eysenck).
There is a selection pressure for higher general intelligence in various overlapping situations: agricultural societies, complex societies (with specialization of labour and other functions), high latitude societies (with the problem of surviving through winter).
There is a selection pressure for lower Psychoticism in various situations which overlap with the selection factors for high intelligence.
High Psychoticism is in bold font; Low Psychoticism is normal font.
1. Cold - versus warm, charming
2. Aggressive - versus submissive
3. Egocentric - versus follows groups expectations
4. Unempathic - versus sympathetic, feels the emotions of others
5. Tough-minded (i.e. impervious to events) - versus tender-minded, strongly affected by experience
6. Antisocial - versus gregarious, needs other people
7. Impersonal - versus life consists of intense, direct relationships
8. Impulsive (behaviour dominated by current emotions) - versus behaviour dominated by predictions or weaker emotions.
9. Creative - versus applies peer-approved, learned rules and traditions
High trait Psychoticism supports creative genius; low Psychoticism makes a person more assimilable to large scale, complex human society.
Thus, a relatively complex agricultural society will - over time - tend to Increase Intelligence and reduce Psychoticism.
In other words, complex agrarian societies will tend towards a Smart and Tame population.
(In animal terms, perhaps a group of high-P types could be compared with a wild hunting pack of carnivores such as wolves; while a group of low-P types is somewhat like a herd of domesticated herbivores such as cattle; bearing in mind that when coordinated - e.g. in a stampede - cattle can kill a pack of wolves.)
Creative Genius requires both Intelligence and Psychoticism to be high - so eventually a complex agricultural society will become Smart but Tame - highly intelligent but uncreative.
However, it is possible that the selection pressure for increasing intelligence may (under certain circumstances) be stronger than the selection pressure for reducing Psychoticism: thus the smartening may happen faster than taming.
In such a situation, there would be a temporary period when the population was both intelligence and also creative.
This is the 'sweet spot' for Creative Genius.
On this basis, it is plausible that the European population underwent selection both for Higher Intelligence and lower Psychoticism during the medieval period; but that Intelligence increased faster than Psychoticism reduced, and led to a few centuries of Western Creative Genius, before the taming selection reduced creativity.
Other parts of the world had different experiences: for example, East Asia had a much longer history of complex (and peaceful) agrarian society - thus the population became, after many generations, much lower in Psychoticism as well as higher in Intelligence: to generate the Smart and Tame type of population. Presumably, at an earlier period than in Western Europe (after, presumably, a much earlier era when the more rapid selection for Intelligence led them to they hit the 'sweet spot' for Creative Genius).
And then, from about 1800, selection began to work against high intelligence due to a combination of declining child mortality rates differentially affecting most the less intelligent and declining fertility initially and most strongly among the most intelligent.
Probably, from about 1800, in Europe average Intelligence began to fall, and average Psychoticism to rise - and Creative Genius dwindled quickly (to become very rare by the mid-twentieth century).
The Creative Intelligence combination of high Intelligence and high Psychoticism has probably not happened in many populations in the history of the world; and seems likely to be an unstable and transitional state passed-through in moving between the more stable combinations of creative, chaotic, individualistic low-I/ high-P societies on the one hand; and stereotypical, ordered, communalistic high-I/ low-P societies on the other hand.