Because spouses occupy confidential relations with one another, when an interspousal transaction creates an advantage for one spouse over the other, a presumption of undue influence arises, and the burden of overcoming this presumption rests with the spouse who has obtained the advantage. In re Marriage of Haines (1995) 33 Cal. App. 4th 277, 297.
The weight of authority concludes that the burden of rebutting undue influence in the marital confidential fiduciary relationship is by a preponderance of the evidence.
Whether a transaction gives one spouse an unfair advantage over the other, and thus raises a presumption of undue influence, is a question for the trier of fact. In re Marriage of Burkle (2006) 139 Cal. App. 4th 712, 734.
If a presumption of undue influence arises, the presumption will be rebutted if the disadvantaged spouse is shown to have entered into the transaction freely and voluntarily, with full knowledge of the facts, and with a complete understanding of its legal effect.