[WarmWash]

You also have the state which pays for most of the top line expenses of having kids. Once you start making money, those benefits don't fade, they instantly disappear entirely.

[bombcar]

If you're at a certain income, additional kids is additional income as more and more gets taken care of.

And that certain income can be surprisingly high, as it's often a multiple of the poverty level which is calculated based on family size.

eg: https://www.dhs.wisconsin.gov/badgercareplus/fpl.htm

[Spooky23]

Yup. And it tends to cluster around rural and sectarian lines.

When people start having 6+, the formulas get weird and vary by state and sometimes county.