CA Civ Code Section 39

(a)

A conveyance or other contract of a person of unsound mind, but not entirely without understanding, made before the incapacity of the person has been judicially determined, is subject to rescission, as provided in Chapter 2 (commencing with Section 1688) of Title 5 of Part 2 of Division 3.

(b)

A rebuttable presumption affecting the burden of proof that a person is of unsound mind shall exist for purposes of this section if the person is substantially unable to manage his or her own financial resources or resist fraud or undue influence. Substantial inability may not be proved solely by isolated incidents of negligence or improvidence.