Is it a federal offense for a spouse to forge the other spouse's signature on income tax returns?

Can you define the circumstances that this has happened under? If they were legally married then often times one spouse might sign documents for another. Its done all the time especially with paychecks.

If they were separated then that makes it different.

Did they do it without permission, are the couple still living together, did they do this to cheat the other out of money.

I doubt I have signed many papers in years, my wife does most of the office and legal paper work.
But I know she does this, she has my permission and the money is always shared equally.

But then if you are not living together and they did this to fraud and cheat someone out of money, yes it is a crime, what level fraud may have to do with many factors