Support payments can be assessed retroactively. It is assumed that the custodial parent will support the child with no accountability. Child Support Guidelines are based on unreasonably high estimates of the cost of raising a child. The IRS has a precedent in calculating these figures on a proportional basis. Custody has nothing to do with child support obligation.
Courts make the argument that custody has nothing to do with paying support.
It's hard to imagine this, even with sterile, mechanical items that can be reproduced. Visitation violations are not enforced. Child Support Collections have now become a big business. Children are no longer the focus in CS and Custody issues, because it has now become an industry unto itself. Child support orders are constant, income is not.