What is the test for the so-called changed circumstance rule?

This court has declared the so-called 'changed circumstance' rule to be subject to exceptions where the welfare of the child requires it. 'We do not wish to be understood as holding that 'the change of circumstance' rule is an absolutely ironclad rule, and that there can be no possible exception to it. It is perhaps possible to conceive of a case in which, despite the fact that there was apparently no change of circumstances, nevertheless the welfare of the child might require that the previous order of custody be changed. * * *' Foster v. Foster, 8 Cal.2d 719, 728, 68 P.2d 719, 723.

Section 138 of the Civil Code relied upon by the plaintiff provides: 'In awarding the custody the court is to be guided by the following considerations: (1) By what appears to be for the best interest of the child in respect to its temporal and its mental and moral welfare; and if the child is of a sufficient age to form an intelligent preference, the court may consider that preference in determining the question; (2) As between parents adversely claiming the custody, neither parent is entitled to it as of right; but other things being equal, if the child is of tender years, it should be given to the mother; if it is of an age to require education and preparation for labor and business, then to the father.' As stated in Fine v. Denny, 111 Cal.App.2d 402, 403, 244 P.2d 983, 984, 'In the application of this statutory provision the court has a wide discretion and in determining whether other things are equal the primary consideration is what will promote the best interests of the child.' In view of the inferences drawn from the evidence in this case, the claims of both parents are not equal and it was within the discretion of the trial court to change the custody of both children to the defendant.