Toward the center of the skull is a golf ball–sized lump of tissue that is similar to what you might find inside the head of a fish, reptile, or mammal.1.12 This is the basal ganglia, an oval of cells that, for years, scientists didn’t understand very well, except for suspicions that it played a role in diseases such as Parkinson’s.

But conserving mental effort is tricky, because if our brains power down at the wrong moment, we might fail to notice something important, such as a predator hiding in the bushes or a speeding car as we pull onto the street. So our basal ganglia have devised a clever system to determine when to let habits take over. It’s something that happens when

First, there is a cue, a trigger that tells your brain to go into automatic mode and which habit to use. Then there is the routine, which can be physical or mental or emotional. Finally, there is a reward, which helps your brain figure out if this particular loop is worth remembering for the future:

And in almost every experiment, researchers have seen echoes of Squire’s discoveries with Eugene: Habits are powerful, but delicate. They can emerge outside our consciousness, or can be deliberately designed. They often occur without our permission, but can be reshaped by fiddling with their parts. They shape our lives far more than we realize—they

If you picture the human brain as an onion, composed of layer upon layer of cells, then the outside layers—those closest to the scalp—are generally the most recent additions from an evolutionary perspective. When you dream up a new invention or laugh at a friend’s joke, it’s the outside parts of your brain at work. That’s where the most complex thi

This explains why it’s so hard to create exercise habits, for instance, or change what we eat. Once we develop a routine of sitting on the couch, rather than running, or snacking whenever we pass a doughnut box, those patterns always remain inside our heads. By the same rule, though, if we learn to create new neurological routines that overpower th

incredibly complex or fantastically simple (some habits, such as those related to emotions, are measured in milliseconds).

the brain spends a lot of effort at the beginning of a habit looking for something—a cue—that offers a hint as to which pattern to use.

Left to its own devices, the brain will try to make almost any routine into a habit, because habits allow our minds to ramp down more often. This effort-saving instinct is a huge advantage. An efficient brain requires less room, which makes for a smaller head, which makes childbirth easier and therefore causes fewer infant and mother deaths.