When you think of gambling, your mind probably goes to high-stakes bouts of chance where fortunes can be won or lost in a split second. But, as you may have guessed from watching Chevy Chase in National Lampoon’s Vegas Vacation, not much in a casino is more carefully engineered than the slot machines. In fact, Schull reports that they account for more than half of the industry’s profits and are responsible for the majority of the public’s perception that casinos are seductive playgrounds where people go to win big money.

One theory that explains the popularity of slot machines is that near misses act as conditional reinforcers by approximating the feedback of a loss. This is said to increase the frequency of a person’s gambling response, but experimental tests have not confirmed this idea.

The most recent study to examine this hypothesis was by Kassinove and Schare, who had participants play a four-reel slot machine simulation for money. Then, they manipulated the frequency of near-miss presentations, giving some groups 15% and others 30% or 45% of chances for a near miss. After a set number of required trials, the machines stopped paying out wins. The 30% group showed the most persistence and a self-report questionnaire found that they were more likely to be willing to return and play in the future.

So, next time you see a slot machine, take a closer look at the layout to understand how it works. But don’t be like Clark from the movie, playing slots for hours on end. Instead, learn to appreciate the odds and use probability to make smart decisions.