“The plastic paradox is that the same neuroplastic properties that allow us to change our brains and produce more flexible behaviours can also allow us to produce more rigid ones. All people start out with plastic potential. Some of us develop into increasingly flexible children and stay that way through our adult lives. For others of us, the spontaneity, creativity, and unpredictability of childhood gives way to a routinised existence that repeats the same behaviour and turns us into rigid caricatures of ourselves. Anything that involves unvaried repetition - our careers, cultural activities, skills, and neuroses - can lead to rigidity. Indeed, it is because we have a neuroplastic brain that we can develop these rigid behaviours in the first place. As Pascual-Leone’s metaphor illustrates, neuroplasticity is like pliable snow on a hill. When we go down the hill on a sled, we can be flexible because we have the option of taking different paths through the soft snow each time. But should we choose the same path a second or third time, tracks will start to develop, and soon we will tend to get stuck in a rut - our route will now be quite rigid, as neural circuits, once established, tend to become self-sustaining. Because our neuroplasticity can give rise to both mental flexibility and mental rigidity, we tend to underestimate our own potential for flexibility, which most of us experience only in flashes.”