The book main point is simple: wealth and income inequality have corrosive effect on societies, and lie at the root of the most pressing problems, such as declining health, despair death or the lack of action in the face of climate change.

The authors set forth to document how outcomes in various fields, from suicide rates to education, subjective well-being, environmental compliance, cultural or political participation are all better in more equal societies. The core insight behind this regularity is that the human psyche is geared to act on either a cooperate/reciprocate set of values, or a compete/aggressive one. When income inequalities get larger, many more of our opportunities to cooperate become limited to people around our own level - hence a stronger desire to increase, status and defend against people coming up from below. This results in increased anxiety, wasteful conspicuous consumption, drugs and alcohol abuse to cope with anxiety, and a dereliction of public trust and institutions.

It is a well-known fact that cross-country comparisons are always tricky beasts. Every country-wide metric has blind spots and very different distributions of a phenomenon (income, mental health) can return similar levels of a given metric. In the literature I happen to be familiar with - subjective well-being - results are much less clear-cut than what the authors make. Subjective well-being is indeed very high in the poster countries for income equality (Scandinavian countries), but also in countries with much higher levels of inequality, especially among emerging countries. Among the latter, economic inequality is seen by people lower in the income distribution not as a reflection on their personal worth, but as a sign of the opportunities open to them to improve their lives. Even restricting to developed countries, happiness inequality actually declined during the last decade, along a large increase of income inequality (see https://ourworldindata.org/happiness-and-life-satisfaction#happiness-inequality). I strongly suspect the same holds in the other areas the authors touch.

Even if the evidence on each point is less consistent than the authors would have, the book still piles up an impressive amount of it. A hint of a negative relation between inequality and a crucial social outcome is interesting. When the same hint crops up everywhere in social life, if becomes close to a stylized fact, and an important one. All the more when it chimes with other works such as Thomas Piketty's Capital or Julia Cagé's The Price of Democracy, among others. I would thus advise to pay strong attention to this book, which also happens to be extremely accessible for laypeople. ( )