This book kept appearing on Waterstone's tables and in the end I succumbed.
My first reaction was one of surprise: Introversion / Extroversion? Is that still around? I thought it was something we did in grammar school fifty years ago, administering Eysenck personality tests to ourselves. But, yes, it is still around and - at least in America - seems to be big business. Cain acknowledges the help of over 200 individuals.
Cain writes a reasonably interesting book axed around the Introversion / Extroversion dichotomy and taking the side of the Introverts. She combines interview material with summaries of experimental (behavioural) research, and laces the text with folksy self-help advice. The book will do no harm.
The thing is this: you could write an indefinite number of books around popular dichotomies: feminine / masculine; neurotic / psychotic; narcissism / whatever is the opposite of that. Each dichotomy will have its enthusiasts as well as its critics. Each dichotomy can bolster itself by drawing on the vast reserve of experimental psychological research which has now accumulated in American journals - much of it banal and tautologous ("extroverts tend to interact with more people than introverts"), some of it faked, most of it routinised.
I have a huge distrust of such material. It is the product, on the one hand, of a desire to avoid the challenges of psychoanalysis and, on the other, of a desire to build a subject (a discipline) which can hold its own in any university as a source of research income. And yet to me, I can't really believe that it is real science.