Look at what happens in cultures where they regularly eat crops which decrease fertility. The fuck like rabbits, having sex with anyone and everyone without any cultural inhibitions like shame or marriage, starting at puberty, so they can get their birth rates back up.

The only proven way to get birth rates down is to get death rates down; cultures will do whatever it takes to reproduce themselves -- the solution is to make reproduction easier, so they no longer have to err on the side of caution.