Most of us start out with the best of intentions. Then we walk right past the fruit bowl in search of the devil’s food cake. Or drink one glass of wine too many. Or, after yet another glass, kiss that co-worker at the holiday party. Unfortunately, life constantly presents us with situations that pit our well-reasoned resolutions against the promise of immediate pleasure. As screen legend Mae West once purred, “I generally avoid temptation unless I can’t resist it.” Withstanding temptation takes self-discipline--no easy trick when immediate gratification plumps our sense of well-being. But it is well worth the effort. Self-control saves us and other people from embarrassing or, worse, damaging consequences.

So why do we so often succumb to the siren song and act against our own self-interests? Scientists have tried for decades to understand this all too human conundrum. Sigmund Freud, the founder of psychoanalysis, viewed all behavior as fallout from conflicts among the id, the ego and the superego. In 1986 psychologist Icek Ajzen of the University of Massachusetts Amherst and economist Thomas J. Madden of the University of South Carolina developed a well-known explanation--the theory of planned behavior--in which all our actions derive from our intentions alone. More recently, though, researchers have turned to models that explain self-control--or a lack thereof--as the outcome of a battle between two emotional systems: our impulses and our powers of reflection.