Society is an insane asylum run by the inmates.
–Erving Goffman, sociologist
Situational ethics: The idea that rules for behavior may not be considered rigid but depend on the circumstances involved. This position is based on the notion that any action may be considered good or bad depending on the social circumstances. What is wrong in most situations might be considered right or acceptable if the end is defined as appropriate. See this paper about student cheating and situational ethics.
Moral licensing: Past apparently virtuous behavior may give us license to be less virtuous later, as if we can withdraw from moral bank accounts. One outcome of moral licensing is that small, seemingly moral acts may prevent us from doing further good deeds and may actually increase the odds of us doing immoral deeds. See this Washington Post article for examples.
Decision fatigue: The more choices you make throughout the day, the harder each one becomes for your brain, with the result that the brain begins to look for shortcuts–becoming reckless or doing nothing–in order to ease mental strain. Decision fatigue is based on ego depletion, the theory that willpower is a form of mental energy that can be exhausted. Via this New York Times article.