I’ve just found this wonderful study that investigated why people are reluctant to exchange lottery tickets before the draw – when each is equally as likely to win the jackpot.
It seems that swapping the ticket sparks images of it winning the lottery. This tends to make us think it’s more likely to occur because the possibility becomes more vivid and hence holds more weight in our minds when we’re trying to judge likelihood – a cognitive bias known as the availability heuristic.
I found the paper on psychologist Jane Risen’s website, whose work on ‘one shot illusory correlations’ and minority stereotyping we featured the other day.
Another look at why people are reluctant to exchange lottery tickets.
J Pers Soc Psychol. 2007 Jul;93(1):12-22.
Risen JL, Gilovich T.
People are reluctant to exchange lottery tickets, a result that previous investigators have attributed to anticipated regret. The authors suggest that people’s subjective likelihood judgments also make them disinclined to switch. Four studies examined likelihood judgments with respect to exchanged and retained lottery tickets and found that (a) exchanged tickets are judged more likely to win a lottery than are retained tickets and (b) exchanged tickets are judged more likely to win the more aversive it would be if the ticket did win. The authors provide evidence that this effect occurs because the act of imagining an exchanged ticket winning the lottery increases the belief that such an event is likely to occur.
I love studies on the quirks of human psychology. While they often have wider implications and help us understand more general principals of our thought and behaviour, in this case – the role of imagination in fuelling cognitive biases, they are also wonderful windows into the curiosities everyday reasoning.
By the way, psychologist Thomas Gilovich is a co-author on both of these studies. He’s also the author of one of the best books on cognitive biases, called How We Know What Isn’t So: The Fallibility of Human Reason in Everyday Life (ISBN 0029117062) which I highly recommend.
Mind Hacks 
The obvious reason for me, a total layman interested in this kind of stuff, is that you know you would kick yourself more after a win on your own ticket which you’d given up, than a win on another ticket that you turned down. There is a sense of ownership of the numbers you chose (or asked a lottery machine to choose randomly – you still asked.)
Ellen Langer looked into this phenomenon in the 70’s. Like Risen and Gilovich, she found that salience of the ticket produced the effect. She also found other factors, like stimulus familiarity, that produced the effect, and chalked up the general cause to the notion that participants had the illusion of control – they believed they could affect the outcome.