A narrow notch, groove or opening, as in the head of a key, or the slit for a coin in a machine.

Schull spent 15 years in Las Vegas tracking how slot machines became so addictive, and she found that casinos use a dizzying array of strategies to keep people gambling. Many of the players she talked to told her they weren’t hooked on the big jackpot, but on being in the zone and losing themselves in the machine. They liked the “gradual drip feed,” and they didn’t like it when winning interrupted their game.

Several studies have shown that near-misses increase gambling persistence. For example, Kassinove and Schare manipulated the frequency of near-misses in a four-reel slot machine simulation that participants played for money. They gave some groups of players a chance to win every other spin, while others were given a 75% chance of hitting a win on each spin. They found that those who had the higher density of near-misses were more persistent gamblers.

But this finding raises some concerns. One is that the experiment relied on a putative conditional reinforcer, which does not exist in typical slot machines. Another is that a self-report measure was used to assess gambling persistence after extinction, which is prone to biases. And finally, Ghezzi and colleagues replicated the results of Kassinove and Schare, but only in one out of three experiments. (The other two experiments failed to show an effect of near-miss density.)