The iPhone is less than two decades old. Yet the psychology behind why it is so hard to put your phone down has been understood for almost a century.

In 1898, the great psychologist Edward Thorndike proposed one of the first rules of behavior: The Law of Effect. Stated simply, rewarded actions tend to increase.

Just as a rat that gets rewarded with food when it pushes a lever tends to push the lever more often, every time we check our phones and get a viscerally rewarding stimulus, it strengthens our habit of phone-checking.

Given this relationship, a reasonable assumption would be that the more consistently an action is rewarded, the more durable the behavior will be. We might expect that a rat that gets a pellet every time it pushes a lever would push the lever consistently. Likewise, we might assume that a burst of entertainment every time we open our phones ought to make us thoroughly obsessed with them.

Except, this isn’t what early behaviorist psychologists discovered. Instead, unpredictable rewards tend to result in far more durable behavior. The rat that only sometimes gets a food pellet will persist in pushing the lever long after the machine stops dispensing pellets. The human being who only sometimes sees an interesting bit of news will keep refreshing her feed long after there ceases to be anything new or interesting.

Explaining Compulsive Phone-Checking

Why should unpredictable rewards be more robust than consistent ones? One explanation is adaptive: if I pull the lever and it reliably gives a treat, not getting a treat might indicate the resource has run out. In contrast, if treats only arrive some of the time, a steady period without rewards might just be a streak of bad luck.

This psychological quirk helps explain why slot machines are so addictive — and why social media algorithms compel us to check our phones, even though most of their content isn’t particularly interesting. The last twenty times may have been a dud, but maybe this next time …

Unfortunately, this law of behavior makes kicking our wasteful social media habits much harder. Just as a compulsive gambler keeps pulling the arm on a slot machine long…