But it's a problem at the heart of education. One common reason is the incompatibility between short-term and long-term goals. For instance, a short-term goal of watching a lot of telly is probably incompatible with the longer-term goal of passing your exams in a month's time.
Psychological experiments have found that we have a preference for valuable outcomes sooner rather than later. This is known as "positive time preferences" or an "inability to delay gratification". Most adults show a puzzling failure to delay gratification. For example, in one experiment, when offered a choice between pound;5 now and pound;10 in two months' time, the majority chose the pound;5. Unless the death rate among those taking part in the experiment is way above average, or interest rates are set at 5,000 per cent, this is essentially an irrational choice.
One theory about why we tend to spend now rather than save for later, even though this is often the better long-term decision, comes from evolutionary psychology. Wired into our genes is behaviour designed for the environment we evolved into several hundred thousand years ago when survival was a minute-by-minute affair. The concept of next week - let alone tomorrow - was not one we were equipped to grasp. In such a situation, it makes sense to go for definite gains or pleasures now rather than some hypothetical benefit in an uncertain future.
This theory also helps to explain why so many of us find it difficult to diet or exercise. Our Stone Age genes still figure it is better to eat that chocolate biscuit now and gain the pleasure, than to turn it down and perhaps put up with the pain of hunger. And all that pain and chocolate biscuit deprivation for the uncertain benefit in a few months' time of looking attractive enough to pull George Clooney or Claudia Schiffer (left).
These two could actually help you exercise and diet. Put up a poster of Schiffer or Clooney in front of your rowing machine to motivate you; your possible future reward is being made more concrete. You should also put a picture on your fridge. But adopt these techniques with caution; in my experience, partners seem not to understand the finer details of evolutionary psychology when they suddenly find images of Ms Schiffer everywhere. (Though you could protest that this is what your psychiatrist advised.)
Making distant rewards seem closer or more concrete, and making people more aware at crucial times of the distant implications of their short-term choice, is the essence of how you motivate them to delay gratification.
It could be that a lot of children brought up in tough circumstances find the future so uncertain it makes more sense to go for positive gains now and not, for instance, to study for their longer-term future. In some cases, however, we delay gratification without thinking about it; often we save the best chocolate in the box for last. This seems to add to the piquancy and pleasure. Psychologists refer to this as savouring; the process of looking forward to or anticipating a future pleasure. But we can do this only if we know what is in that last chocolate - we usually have to have had it before and know we will like it. In a sense, then, you can only savour certainty.
This psychological research and theory suggests teachers need to help children see more clearly the consequences of decisions taken today if they are to motivate them.
And I really must see someone about this Claudia Schiffer fixationI Dr Raj Persaud is a consultant psychiatrist at the Maudsley hospital and senior lecturer at the Institute of Psychiatry in London. He is a fellow of University College London, and author of 'From the Edge of the Couch' published by Bantam Press, pound;12.99. Email: email@example.com