Why do we insist on "just one more level"? Nir Eyal explains in his book, Hooked: How to Build Habit-Forming Products.

In the 1940s, two researchers named James Olds and Peter Milner accidentally uncovered some peculiar properties of a special area of the brain. The researchers implanted electrodes in the brains of lab mice that enabled the mice to give themselves tiny electric shocks to a small area called the nucleus accumbens. The mice quickly became hooked on the sensation.

Olds and Milner demonstrated that the lab mice would forgo food, water, and even run across a painful electrified grid for the opportunity to continue pressing the lever that administered the shocks. A few years later, other researchers tested human response to self-administered stimulus in the same area of the brain. The results were just as dramatic as in the mouse trial — subjects wanted to do nothing but press the brain-stimulating button. Even when the machine was turned off, people continued pressing the button. Researchers even had to forcibly take the devices from subjects who refused to relinquish them.

Given the powerful response demonstrated in these experiments on lab animals and later people, it's no wonder companies stimulate consumers' nucleus accumbens all the time. For example, the reason we find ourselves incessantly checking our smart phones is deeply rooted in the psychology of reward.

The Stress of Desire

Olds and Milner concluded that they had discovered the brain's pleasure center. In fact, we now know that other things that feel good also activate the same neural region. Sex, delicious food, a bargain, and of course, our digital devices, all tap into this deep recess of the brain, providing the impetus for many of our behaviors.

However, more recent research has shown that Olds and Milner's experiments were not stimulating pleasure per se. Stanford Professor Brian Knutson, conducted a study exploring blood flow in the brains of people wagering while inside of an fMRI machine.

The test subjects played a gambling game while Knutson and his team looked at which areas of their brains became more active. The startling results showed that the nucleus accumbens was not activating when the reward (in this case a monetary payout) was received, but rather, in anticipation of it.

The study revealed that what draws us to act is not the sensation we receive from the reward itself, but the need to alleviate the craving for that reward. The stress of desire in the brain appears to compel us, just as it did in Olds' and Milner's lab mouse experiments.

We Love the Unexpected

If you've never watched a YouTube video of a baby's first encounter with a dog, it's worth doing. Not only are these videos incredibly cute, but they help demonstrate something important about our mental wiring.

At first, the expression on the baby's face seems to ask, "What is this hairy monster in my house? Will it hurt me? What will it do next?" The child is filled with curiosity, uncertain if this creature might cause harm. But soon the child figures out the dog is not a threat. What follows is an explosion of infectious giggles. Researchers believe laughter may in fact be a release valve when we experience the discomfort and excitement of uncertainty, but without fear of harm.

Our brains have evolved over millennia to help us figure out how things work. Once we understand causal relationships, we retain that information in memory. Our habits are simply the brain's ability to quickly retrieve the appropriate behavioral response to a routine or process we have already learned. Habits help us conserve our attention for other things while we go about the tasks we can perform with little or no conscious thought.

However, when something breaks the cause-and-effect pattern we've come to expect — when we encounter something outside the norm — we suddenly become aware of it again. Novelty sparks our interest, makes us pay attention, and — like babies encountering friendly dogs for the first time — we seem to love it.

At the heart of this response is a powerful cognitive quirk described by B.F. Skinner in the 1950s, called a intermittent reinforcement. Skinner observed that lab mice responded most voraciously to random rewards. The mice would press a lever and sometimes they'd get a small treat, other times a large treat, and other times nothing at all. Unlike the mice that received the same treat every time, the mice that received variable rewards seemed to press the lever compulsively.

If you've ever asked someone a question while he or she was engrossed in a video game, only to receive a mumbled "sure, ok, whatever," you've seen this mental state. Players will agree to almost anything to get rid of distraction and keep playing.

As B.F. Skinner discovered over 50 years ago, variable rewards are a powerful inducement to creating compulsions. Today, technology companies are creating new habits by running users through a series of what I call "Hooks" — and variable rewards fuel the chain reaction.
Nir Eyal is the author of Hooked: How to Build Habit-Forming Products and blogs about the psychology of products at NirAndFar.com.