200 years ago you would have ten children, a few would survive until adulthood, and you needed to have children or nobody would look after you in old age, help you out when they're grown or take over your farm when you became infirm - and besides, everyone else has children, so if you don't you're the odd one out.

Now, you have both parents in the workforce - even with generous parental leave the mother loses a lot of opportunity in the prime of her career.

Then you have to pay for childcare if both are working (or lose out on one income if they aren't), food, clothing, schooling, extracurriculars. And you're competing in the workforce against all couples with less children. And then when you get old you aren't relying on your children to look after you - this is frowned upon, and you get paid out from your investments or the government pays you a pension. Basically most of modern life is set up economically against having children, and the main reason to is purely the biological drive.

I think all these factors need to be taken into account when devising economic incentives for people to have more children, and the current levels in any country are too low to have enough of an effect.