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“The relationship between fertility and intelligence has been investigated in many demographic studies. There is evidence that, on a population level, intelligence is negatively correlated with fertility rate and positively correlated with survival rate of offspring.[1] The combined net effect of these two conflicting forces on ultimate population intelligence is not well studied and is unclear. It is theorized that if an inverse correlation of IQ with fertility rate were stronger than the correlation of IQ with survival rate (and if heritable factors involved in IQ were consistently expressed in populations with different fertility rates), assuming this continued over a significant number of generations, it could lead to a decrease in population IQ scores.”
― Wikipedia: Fertility and Intelligence