Once the first task is completed, both groups are asked to complete a second task that requires some effort  solving complex anagrams, for example, or turning down free beer knowing that a driving test will follow.

The results? The group that had to resist temptation did not perform as well on the second task as the group that was allowed to give in to temptation, said Timothy A. Pychyl, an associate professor of psychology at Carleton University in Ottawa.

The conclusion was that those who had to exert more willpower in the first task “exhausted their self-regulatory strength, at least temporarily, and therefore are unable to muster the self-regulation needed for the second task,” Professor Pychyl said.

One practical example, he noted, is that after a stressful day at work, studies show, people are less likely to exercise and more likely to watch television.

Of course, as with all personality traits, some people are born with a greater ability to exert willpower. But that doesn’t mean there aren’t ways to increase it.

One method is to put in place external controls. If you feel you check your e-mail too frequently, install a program to shut it down temporarily. If you have trouble saving for retirement, you can have money automatically deducted from your paycheck. An extreme example in literature, Professor Pychyl said, was Odysseus asking his men to tie him to the mast to avoid his being lured by the Sirens’ singing.

But we don’t always have shipmates around to shield us from temptation, so we need to learn to do it on our own. Some new research has shown that self-control may be increased if it is seen as fun, not work.