edit: here's an example:

If you're risk-neutral, you still can't just do whatever has the highest chance of being right; you must also consider the cost of being wrong. You will probably win a bet that says a fair six-sided die will come up on a number greater than 2. But you shouldn't buy this bet for a dollar if the payoff is only $1.10, even though that purchase can be summarized as "you will probably gain ten cents". That bet is better than a similarly-priced, similarly-paid bet on the opposite outcome; but it's not good.

You have a 1/3 chance of losing a dollar and a 2/3 chance of gaining ten cents. On average, you will lose 13 (edit for, um, the fifth time: 26) cents per dollar. Unless you need that dime to buy a ticket for the last plane leaving your doomed volcanic island home... it's a bad bet.

Also see: applause lights