Generally, you pay for the time you are not actively supporting the child. Example; if each parent has 50/50, then neither parent pays child support. If one parent has 75% and the other has 25% custody, then the parent with 25% pays for 25% of the time they are not with the child. If a parent has 100% custody, the other parent pays for 100% of the time. It's hard to get a parent to have 0% custody. Generally, this is only in cases of abuse. The system creates an incentive for parents to fight about money using the child as a tool. It's unfortunate.