Is forging another's signature prosecutable if forger had permission?

Question

After reading this post, I realized that a significant percentage of my friends and family could go to prison for forgery. Many couples I know sign one another's names all the time, including on the backs of checks. My wife and I have given one another blanket permission to sign for one another, and I know many other people with a similar understanding.

In the United States, is it in fact illegal to sign someone else's signature on the back of a check if one has permission to do so? How about signing my wife's signature on our IRS forms?

Answer 1

Take the laws of Washington to be typical. RCW 9A.60.010 defines crucial terms:

To "falsely complete" a written instrument means to transform an incomplete written instrument into a complete one by adding or inserting matter, without the authority of anyone entitled to grant it;

and:

"Forged instrument" means a written instrument which has been falsely made, completed, or altered

Forgery then requires a bad intent:

(1) A person is guilty of forgery if, with intent to injure or defraud:

where

(a) He or she falsely makes, completes, or alters a written instrument or;

(b) He or she possesses, utters, offers, disposes of, or puts off as true a written instrument which he or she knows to be forged.

In the scenario described, there is no intent to injure of defraud, and furthermore the document is not forged, because a forged document is one made / completed without authorization.