An excellent question. The way Pickett and Wilkenson did the research, the "proper" amount of equity remains unanswered. Instead, they just took all the info they could on the various nations and states and compared that info to the GINI coeffficient, an international measure of financial inequality.

It's amazing how well some phenomenon track inequality. Please, head over to the evidence link I gave and see for yourself.

Before I read this book, I was pretty Meh about equity talk. I didn't give a rip how much someone made. (Voted Reagan, after all.) After that book, I'm not so sure. The gap between rich and poor has ripple effects throughout society. Narrowing that gap proves as good as throwing money at a societal problem, if not more effective. In fact, after reading the book, I would say cut back on the social programs and attack the gap instead. It seems the gap is if not the cause, at least an exasperating factor.