Fertility and intelligence

The relationship between fertility and intelligence has been investigated in many demographic studies, with contradicting evidence that on a population level, intelligence is negatively correlated with fertility rate, and positively correlated with survival rate of offspring. The combined net effect of these two 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 survival rate, and if heritable factors involved in IQ were consistently expressed in populations with different fertility rates, and if this continued over a significant number of generations, it could lead to a decrease in population IQ scores. The Flynn effect demonstrates an increase in phenotypic IQ scores over time, but confounding environmental factors during the same period of time preclude any conclusion concerning underlying change in genotypic IQ. Other correlates of IQ include income and educational attainment, which are also fertility factors that are inversely correlated with fertility rate, and are to some degree heritable. It should also be noted that while fertility measures offspring per woman, if one needs to predict population-level changes, the average age of motherhood also needs to be considered, with lower age of motherhood potentially having a greater effect than fertility rate. (For example, a subpopulation with fertility rate of 4 with average age of reproduction at 40 years old, generally speaking, will have relatively less genotypical growth than a subpopulation with fertility rate of 3 but average age of reproduction at 20 years old.)

A negative correlation between fertility and intelligence (as measured by IQ) has been argued to have existed in many parts of the world at various times; however, critical review of these arguments shows these conclusions to be "superficial and illusory" and not supported by empirical data.

Some of the first studies into the subject were carried out on individuals living before the advent of IQ testing, in the late 19th century, by looking at the fertility of men listed in Who's Who, these individuals being presumably of high intelligence. These men, taken as a whole, had few children, implying a correlation.

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