It certainly varies from state to state. As I understand it, in Calif. the state will go and look for non-suportive fathers (which is why they press to have one declared one on the birth certificate; if the mother is unmarried [a child born in a marriage is legally the offspring of the father, regardless of the facts, and getting it corrected to relate the truth can be very sticky), if the mother gets public assistance.

When they find them they tend to threaten them into providing support, by saying the father has a choice, start kicking in now, or get billed for all the state-supplied aid.

I seem to recall at least one case where the actual father was living with the mother, but she'd been married when the child was born. Calif. went after him for non-support and it got very ugly.

TK