Intelligent people are more likely to think on the consequences when deciding to have a child. But there is a prisoner's dilemma type of situation here:

One reason smart people forego reproduction is because they might feel children make them more unhappy overall for at least the first few years (a not unreasonable assumption). Or simply because they are not religious (smart religious people do still have lots of children) As a consequence, in 20 years, the average IQ of that society will fall (bar some policy reversals encouraging eugenic breeding, or advances in genetic engineering), as only the less intelligent breed. Since, all other things equal, smarter people perform better on their jobs, the average quality of services provided in that society (both public and private) goes down. So in the end everyone becomes more unhappy (even though unhappiness of a childless smart person resulting from societal dysgenics may not outweigh the temporary unhappiness from having a child)