There are some confounding factors, like education and income also affecting fertility, and correlated with most measures of intelligence, but that doesn't actually affect the diagnosis - regardless of if intelligence directly lowers fertility, or if the effect is mediated by e.g. education, the net effect is that smarter people have fewer children (in our current environment).
They are: https://en.wikipedia.org/wiki/Fertility_and_intelligence
There are some confounding factors, like education and income also affecting fertility, and correlated with most measures of intelligence, but that doesn't actually affect the diagnosis - regardless of if intelligence directly lowers fertility, or if the effect is mediated by e.g. education, the net effect is that smarter people have fewer children (in our current environment).