Although the banker stated that he did not explain to the Decedent the effect the joint accounts would have on her will, this lack of evidence is insufficient to rebut the presumption of validity afforded the joint bank accounts. While we recognize that in some cases an individual may open a joint bank account "because of necessity and/or convenience" even though his or her "true desire is to retain ownership" of the account, McCullough v. Wasserback, 30 Utah 2d 398, 518 P.2d 691, 693 (1974), there is no evidence to rebut the presumption of validity here. The Decedent was mentally competent when she opened the joint bank accounts, and the banker explained to her that Respondent Jess would be a co-owner of the funds in the accounts.