A slot (plural slots) is a narrow opening for receiving something, especially money or a card. A slot can also refer to a position, as in “a slot in the schedule” or “a slot in the wall.” The word comes from the Dutch word sleutel, which is related to the Dutch noun slotte (“lock”). The term is sometimes used to denote an area of a field on an ice hockey rink that is marked off for face-offs.

A modern casino is a huge, noisy place filled with beeps and bells and tall machines decorated with images of pigs and pirates and martini glasses. Inside the machines are well-dressed men and women pressing buttons and pulling levers, watching cherries and sevens spin on liquid crystal displays. These men and women aren’t playing for fun—they’re here to do business. The business is gambling, and the men and women in these suits are called “slot operators.”

Slots are a vital source of income for casinos. In fact, they contribute more than 40 percent of total revenues and profits. Therefore, research that can improve slot performance would be of tremendous value to casino management.

While no one has uncovered the Platonic ideal of a slot machine, there are certain principles that undergird most games. First, there’s a vague aesthetic uniformity: colors tend toward primary or pastel and franchise tie-ins are a must. Second, the game must be accessible to people who aren’t familiar with the rules. Finally, it must be able to keep players engaged and betting.

Modern slot machines do all of this through a process called random number generation, or RNG. The computer inside the machine chooses a combination of stops on each reel at random, and if that combination appears in the paytable it pays out. The reels spin sort of as a courtesy to the player—they’re there only to show what the computer has already selected.

The number of different combinations that a machine can make is huge, but the odds of hitting a particular combination are not. Each symbol has a specific probability of appearing, but the probabilities of hitting each higher-paying symbol increase with each additional reel. This means that players will often believe they are close to winning on a particular pull when, in reality, they’re not.

Another factor in the popularity of slot machines is their ubiquity. They can be found in many public places, from airports to malls to gas stations. Some people even play them in their own homes. It’s no surprise that many people struggle with slot addiction. In fact, many researchers have compared it to other forms of addiction, such as drug or alcohol addiction. In a recent interview, I asked Nir Eyal to explain what sets tech addiction apart from gambling addiction. His answers ranged from the comprehensive—that mobile games and dating apps don’t reach the level of severity that is associated with slot addiction—to the somewhat defensive, asserting that people prone to addiction will be addicted no matter what technology they use.