If you’ve ever used online pornography, you know that it has a powerful pull. There are several obvious reasons for this, and a few not so obvious reasons. When asked about the allure of Porn, most porn users will answer with some version of one or more of the following statements.

• It’s always available. Any time I’m horny or lonely, it’s there.
• It’s easy to hide. Nobody else ever has to know about it.
• It’s one of the few places in my life where I’m in complete control of my sexuality, my perceived desirability.
• It never says no. I don’t have to be vulnerable or take any emotional risks.
• I can find and explore whatever I want with no fear of judgment.

And it’s true that pornography provides all of those “positives” and more. But those reasons are not what pushes porn over the top in terms of its allure. The real reasons porn becomes what we refer to as a super-stimulant are:

• Porn offers endless variety.
• Porn sites provide intermittent reward.

Variety is easy to understand. Humans are wired to desire Variety. For example, our brain knows that our body needs a variety of foods to get all the nutrients that support health. This is why we don’t want the same thing every meal, even if that thing is our favorite food.

The desire for variety carries over into our sex lives. This is best evidenced by the fact that when a porn user sees a video that fits solidly within their sexual arousal template, it’s super-hot. Then, when they see it again, it’s kind of hot but not super-hot. By the third or fourth viewing, it’s still objectively hot, but they find themselves clicking past it and looking for something new. After a few viewings, the neurochemical rush provided by variety is no longer there.

The appeal of Intermittent Reward is less obvious but equally powerful. Intermittent reward is, stated simply, the power of maybe. For example, “The next video might be the super-hot video I’ve been searching for, so I’ll click to see.” The possibility can keep porn users clicking on new videos for hours on end, constantly telling themselves, “Just one more. Just one more. Just one more.”

This phenomenon is easily seen in experiments with lab rats. When we put a rat in a cage with lots to do—cedar shavings, hamster wheels, toys, other rats to play and fight with—the rat will first and foremost check the food dispenser. If they get a nugget of food every time they push the lever, they figure that out, eat three or four nuggets, and go do something else. If they get a nugget every other time they push the lever, they figure that out as well, eat three or four bites, and go do something else. But if a randomization algorithm is programmed into the food dispenser so the rats don’t know if/when food is coming, they will push the lever and eat until they vomit. They simply can’t stop.

Slot machines have a similar algorithm, as do video games, and from a neurochemical-attraction standpoint, those are the closest relatives we have to pornography, as they offer both intermittent reward and a modicum of variety. The finely honed algorithms in gambling and gaming programs mix up both the frequency and amount of reward just enough to hook users Think about the retired slot jockey who feeds her entire Social Security check into the slot machine, unable to stop pressing the button because if she does, the next person might come along and win big on the very next play.

That said, there are some important differences between slot machines, video games, and porn. For starters, porn doesn’t need a built-in algorithm. By its very nature, usually via “tube sites,” it provides intermittent reward all on its own. Even if there are screenshots indicating possible content, users never really know what a video provides until they open it up. It might be awesome; it might also be not very interesting at all.

The truest and deepest allure of pornography is related not to its constant availability or the fact that it never says no but to the perfect storm of three things humans are wired to want: sexual arousal, variety, and intermittent reward. Admittedly, there are differences in what people feel is arousing (especially between men and women), but variety and intermittent reward will up the ante for everyone.