Can a person sign for somebody else on a document without their written authorization, e.g. on a deed? The answer is yes. The legal term for this is "amanuensis." See Estate of Stephens (2002) 28 C4th 665.
In Estate of Stephens, the decedent orally instructed his daughter to sign his name on a grant deed which vested title in the decedent and the daughter as joint tenants. When the decedent passed away, the daughter argued that the property was hers because she was the surviving joint tenant. The son argued that the property should be divided equally between the son and daughter, as stated in decedent's will.
The California Supreme Court ultimately ruled that the conveyance was valid, citing the amanuensis rule. It provides "that where the signing of a grantor's name is done with the grantor's express authority, the person signing the grantor's name is not deemed an agent but is instead regarded as a mere instrument or amanuensis of the grantor, and that signature is deemed to be that of the grantor." In this case, the daughter's signature "was a mere mechanical act, and not an exercise of judgment or discretion." The decedent told the daughter to sign the deed on his behalf and she did so.
Still, the opinion noted that "the signing of a grantor's name by an interested amanuensis must be presumed invalid. In such a case, the interested amanuensis bears the burden to show that his or her signing of the grantor's name was a mechanical act in that the grantor intended to sign the document using the instrumentality of the amanuensis."
The rationale for such a law is evident. A person who stands to benefit from signing a document on somebody's behalf will naturally do so under just about any circumstance. Thus to eliminate fraud, the signatory needs to show that they signed the document at the person's dictation, instead of signing the document out of self-interest.
The California Supreme Court concluded the opinion by agreeing with the trial court's determination. "Given that Shirley was an interested party to the deed, it is presumed that her signing of Austin's name was invalid. However, this presumption has been successfully rebutted in this case. The trial court found, based on overwhelming evidence, that Shirley acted as a mere amanuensis, signing the deed at Austin's direct request, albeit not in his immediate presence. Because her signature was a mere mechanical act, and not an exercise of judgment or discretion, Austin's oral instruction to Shirley was sufficient. 'It is perfectly natural for a parent to be more bountiful to one of his children who has assumed the greatest burden of care and lavished the highest degree of solicitude upon him.' Camperi v. Chiechi, supra, '134 Cal.App.2d at p. 505."