“The flow of traffic through a city is complex and dynamic — sometimes jamming up inexplicably and other times flowing smoothly, even at rush hour. The stock market and larger economy follow similar patterns as does the weather and even pop culture. Mathematically, these types of complex dynamic systems share many similarities, including the way the whole system — whether a traffic jam, a tornado, a recession or recovery, a viral tweet, or the next fad — can get caught in a runaway pattern: either an upward or downward spiral.
“So why do tornadoes happen in Oklahoma but not in New York? Because conditions are just right — the flatness of the land, temperature changes, humidity, and wind direction and speed. But there’s nothing wrong with Oklahoma.
“The same is true of your brain. In depression, there’s nothing fundamentally wrong with the brain. It’s simply that the particular tuning of neural circuits creates the tendency toward a pattern of depression. It has to do with the way the brain deals with stress, planning, habits, decision making, and a dozen other things — the dynamic interaction of those circuits. And once the pattern starts to form, it causes dozens of tiny changes throughout the brain that create a downward spiral.”