What is the Philbin rule on child support where the ability to earn exceeds actual earnings?

"Thus, the rationale for the Philbin rule is that a fair and logical deduction that ability to earn exceeds actual income can only be made where there is evidence that the party to be charged has failed to make a reasonable effort to obtain employment that would generate a higher income. The proponent of the claim for past support has the burden of persuasion and the burden of production of evidence to establish the parent's ability to earn an income sufficient to defray support. [Citation.] The proponent of support may employ discovery to ascertain the facts to satisfy these burdens. Accordingly, it is appropriate that in the absence of evidence affording an inference that earning capacity exceeds actual earnings the capacity to earn is equivalent to earnings." (County of Yolo v. Garcia, supra, 20 Cal.App.4th at p. 1782, 25 Cal.Rptr.2d 681.)