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Simple proposal for Great Filter: when human intelligence evolved, it also had negative effects: bigger infant head -> more dangerous, more painful childbirth, also human children are prematurely born compared to other animals, develop slow, require a lot of care. Hence human intelligence directly caused high maternal risk and high maternal investment, and could it be the cause of the runaway intelligence arms race inside the species? Making it more important for men to compete for women since you could easier lose your mate during birth or she is too tied up in maternal care for another one and so on, so more intelligence means more maternal care / risk means more competition means more use for intelligence and more chance of intelligent genes spreading etc. Hominid women running a risk gamble when hooking up with smart men - more chance you die during labor, but if you win, your son may spread your genes effectively.
My point is: perhaps without intelligence having drawbacks i.e. without having to push that big infant brain through a vagina, there is no runaway arms race of intelligence. A species that does not have this danger of intelligence does have intelligence leading to more sexual competition means it does not develop further intelligence.
Was this considered?
Interesting. 1 compared to what animals? 2 what does your theory predict regarding infanticide/cannibalism in intelligent species? 3 at what level of reproduction effort would childbearing remain viable, given a lifespan of twenty max and high children mortality? 4 ...genes effectively - in what way? Polygamy?