If you are a territory sales manager you may occasionally give away a little gift as a kind of thank-you to your customers. Nothing too extravagant, just a nice little gesture, neatly wrapped. However, when some people get a gift out of the blue, alarm bells start ringing in their heads. Not literally, of course, that would be terrible. But metaphorically, they become suspicious of this unexplained act of kindness.

For instance, when presented with a lovely bouquet of flowers, my wife always asks me what I've done. Or where am I about to jet off to. Or if the car is still in one piece.

New research conducted by Tim Wilson of the University of Virginia supports the theory that it's sometimes better to be kept in the dark. And he's never even met my wife.

To prove his theory that mystery increased pleasure, Wilson had people distribute two types of greeting card, both with a £1 coin attached. The design was identical with a big smiley face at the top and the words: 'This is for you!'. Underneath this was written: 'The Smile Society, A Student/Community Secular Alliance. We like to promote Random Acts of Kindness! Have a nice day!' The only difference between the two sets of cards was that half of them also carried the words: 'Who are we?' and 'Why do we do this?'

People who received the cards with the added questions could see a logical explanation of the unexpected gift, whereas without the questions the gift was more mysterious.

Then, under the guise of carrying out a different study a short time later, the researchers got a measure of their mood. What they found was that those who didn't know why they'd been given a gift remained happier than those who were more certain. This proves that mystery prolongs the pleasure.

Why is this? Well, we normally associate uncertainty with worry and anxiety. We don't like surprises. When people are exposed to traumatic events, the sooner they 'make sense' of what has happened, the sooner the negative emotion is reduced and they recover. The same process seems to operate for positive emotions. We try to reduce our uncertainty by explaining positive events and thereby reduce the amount of positive emotion we feel.

So if you are a territory sales manager, consider giving a little gift. The gift of far exceeding your customer's expectations. No explanation.

People don't like surprises - unless they are nice ones.

Professor Philip Hesketh is a professional speaker on the psychology of persuasion and author of LIfe's A Game So Fix the Odds. www.heskethtalking.com