Most courts that have considered the question require at least clear and convincing evidence in order to prove an equitable adoption. (See Clark, The Law of Domestic Relations in the United States, supra, at p. 927; 37 Vand. L.Rev. at p. 780.) Several good reasons support the rule. The claimant in an equitable adoption case is seeking inheritance outside the ordinary statutory course of intestate succession and without the formalities required by the adoption statutes. As the claim's “strength lies in inherent justice” (Wooley v. Shell Petroleum Corporation, 45 P.2d at p. 932), the need in justice for this “extraordinary equitable intervention” (Rein, supra, 37 Vand. L.Rev. at p. 785) should appear clearly and unequivocally from the facts.
Second, the claim involves a relationship with persons who have died and who can, therefore, no longer testify to their intent. As with an alleged contract to make a will (see Crail v. Blakely (1973) 8 Cal.3d 744, 750, fn. 3 [106 Cal. Rptr. 187, 505 P.2d 1027]), the law, in order to guard against fraudulent claims, should require more than a bare preponderance of evidence. Where “the lips of the alleged adopter have been sealed by ...