From the "How to Change Your Mind" sequence: Lonely dissent doesn't feel like going to school dressed in black. It feels like going to school wearing a clown suit. (In other words, "leaving the pack" vs. "joining the rebellion")

What is this "social acceptance" you speak of?

The first rule of human club is you don't explicitly discuss the rules of human club.

Is it defined by some authority?

There is no authority with the power to define, despite the best efforts of Dear Abby and others.

How is it measured?

With great difficulty and much controversy.