Intelligence and Fitness: The Mediating Role of Educational Level. 2017. Research found that higher IQ individuals tended to delay reproduction (choosing to have children later in life), which affected their total reproduction success.
Do income and marriage mediate the relationship between cognitive ability and fertility? Data from Swedish taxation and conscriptions registers for men born 1951-1967. 2021. Researchers found that men with higher cognitive ability had higher fertility, while men who were of low cognitive ability and low income had low fertility and high rates of childlessness. The researchers conclude that there is strong evidence that men with lower cognitive ability had lower access to marriage and reproduction.
No evidence for a relationship between intelligence and ejaculate quality. 2020. This research did not find a relationship between intelligence and ejaculate quality. Why would someone look at this? Well, there is some research that suggests that genetic quality overall might be expressed across many traits, which the researchers also believed could have been across ejaculate quality as well. In fact, a previously published study in 2009 did find a positive correlation between intelligence and semen quality. We may need a few more studies on this one before we know the answer!
Cognitive Ecology in Humans: The Role of Intelligence in Reproductive Ecology. 2020. Researchers found that intelligence was positively associated with both planned and observed age at first reproduction and negatively with expected total fertility. They also found that the childhood environment played a role in fertility and reproduction as well. They conclude with support for the hypothesis that individuals with higher intelligence may have a slow life history behavioral trait (basically that smarter folks wait longer to have kids).
The decline of the world's IQ. 2008. Researchers argue that the world's IQ is dropping. They believe that there is dysgenic fertility occurring, which means there is a negative correlation between intelligence and number of children. Smarter people are having fewer kids.