Why do you think that this is a biological trait? I don't. It might be, it could be. Certainly religion, income, how you were raised, etc, all play a big role

Why do you think that this is a biological trait? I don't. It might be, it could be. Certainly religion, income, how you were raised, etc, all play a big role in how many children you want. But IF there is a genetic component to how many children you want, then I expect it to spread.