Schwartz's research suggests something important: we can stretch our personalities, but only up to a point. Our inborn temperaments influence us, regardless of the lives we lead. A sizeable part of who we are is ordained by our genes, by our brains, by our nervous systems. And yet the elasticity that Schwartz found in some of the high-reactive teens also suggests the converse: we have free will and can use it to shape our personalities.

Shyness is inherently uncomfortable; introversion is not. The traits do overlap, though psychologists debate to what degree.

The other thing Aron found about sensitive people is that sometimes they're highly empathic. It's as if they have thinner boundaries separating them from other people's emotions and from the tragedies and cruelties of the world. They tend to have unusually strong consciences. ... they're acutely aware of the consequences of a lapse in their own behavior.

(Finland is a famously introverted nation. Finnish joke: How can you tell if a Finn likes you? He's staring at your shoes instead of his own.)

I've never given a speech without being terrified first.

I use a lot of old-fashioned expressions.

Love is essential; gregariousness is optional. Cherish your nearest and dearest. Work with colleagues you like and respect. Scan new acquaintances for those who might fall into the former categories or whose company you enjoy for its own sake. And don't worry about socializing with everyone else. Relationships make everyone happier, introverts included, but think quality over quantity.