IRC §6013(a) requires that both spouses must sign a joint return. The signature (actual or electronic) evidences affirmative consent.

If one spouse does not sign or explicitly consent, the return is not a valid joint return. See Bearden v. Commissioner, T.C. Memo. 1980-121, and Heim v. Commissioner, 251 F.2d 44 (8th Cir. 1958).