Can we change a person's behavior purely by changing their incentives? If we can, it follows that the person was able to act differently all along, but preferred not to; their condition is a matter of preference, not constraint. I will refer to this as the "Gun-to-the-Head Test." If suddenly pointing a gun at alcoholics induces them to stop drinking, then evidently sober behavior was in their choice set all along.

Stephen King (who is, incidentally, a former alcoholic) wrote a short story, "Quitters, Inc." with this as the premise. If they catch you smoking, they'll do horrible things to you and your family members.