Setting up comfortable limitations might be partly explained by self-handicapping:
So they chose the harmful drug as an excuse: “Oh, I would have passed the test, only the drug was making me stupid.” As the study points out, this is a win-win situation: if they fail, the drug excuses their failure, and if they succeed it’s doubly impressive that they passed even with a handicap.
Setting up comfortable limitations might be partly explained by self-handicapping: