Maybe a policy like this might work: for every child you have, you get a lifetime 10 percentage point reduction in income tax rates. Suppose the top rate is 37

Maybe a policy like this might work: for every child you have, you get a lifetime 10 percentage point reduction in income tax rates. Suppose the top rate is 37%. Have one child, now for the rest of your life, your top rate is capped at 27%. Have a second child, now your lifetime top rate is capped at 17%. Third child, capped at 7%. Fourth child, never pay income tax ever again. But, a lot of people will object to such a policy. Childless people will end up having to pay higher taxes as people with kids pay lower or no taxes–some may accept that as their personal sacrifice for the greater good, many likely won't. Others will object to the fact that the biggest benefit is going to be for the middle and upper classes – who are most likely to pay the higher tax rates – while having far less benefit for those on low incomes, who pay little or no tax already. There is the (difficult to quantify) risk that unsuitable parents may end up having kids just for the tax benefits, and then mistreat/abuse those kids. Given all these objections, I'd be surprised if it ever actually gets adopted. But, in terms of increasing TFR in wealthy countries, it might actually work.