The moment a Court says anything about equity or equitable defenses, you will notice that the attorneys in the room perk up and really pay attention. Equity is a wildcard in the courtroom. A Court will invoke equity to ensure that an unfair result does not ensue. The one area where you see it used with child support is when the parties violate a custody order.
Suppose Father is paying $500 a month for the support of his son, Melvin, who lives with Mom. When Mom and Melvin have a fight, Melvin goes and lives with his Dad and Dad stops paying Mom child support. After two years of Mom living with Melvin, Mom gets desperate for cash and sues Father for back child support. The courts in this situation will say that Mom cannot retroactively obtain these funds because Father had custody of the child, which is an equitable result. Dad should get an Order modifying custody as soon as Melvin moves in to ensure that his rights are protected, which is to say a Court might not feel like using the equitable defense.
Notice also, though, the Courts will not award Father retroactive child support. New Mexico's Child Support Statutes simply do not allow for this.
Common Arguments that are not Acceptable to the Courts
Occasionally, someone will argue that their support for a month should be reduced because they bought items for the children, like clothes, or, as I once saw argued, took the children to Cliff's Amusement Park. This will not work. The Court will tell you that this "in-kind" support is not appropriate and that the statute defines child support as money paid to the custodial parent. Notice that I do not think child support is even defined in the statute, but it is effectively cash paid to the custodial parent.
More coming soon.