What is joint and several liability?

Joint and several liability is to a joint return what tenancy by the entirety is to property: it means "both spouses are each entirely responsible for the return and the tax liability and its tax obligations." In general, the I.R.S. ignores any agreements between the spouses about responsibility for taxes; if a deficiency is found, the I.R.S. can pursue either or both spouses for the back taxes and any penalties.

A joint return unsigned by one spouse does not absolve the non-signing spouse unless he or she can demonstrate that "there is ... an implied intention not to sign the return," and unless a non-signing spouse can demonstrate duress, fraud, forgery, misrepresentation, "or a demonstrated refusal to sign the return" he or she may be treated as if he or she signed the return.