This is an excellent short, punchy and important book. It contains some very useful ideas, both for personal use, and which will help us in business and political settings.
Its basic point is two fold. Firstly that birds of a feather flock together. Secondly, as they do this they tend to narrow their field of options, and magnify each other's prejudices and misconceptions.
This phenomenon which affects all of us up to a point, becomes dangerous quickly, particularly when we do not accept the discipline of wide reading or other exposure to many different people and ideas. One of the privileges of working as a doctor is that by default I meet people from most walks of life, and learn a lot about them, and about how to adapt my style to meet the needs of different patients. The medicine is the same- but my presentation of it alters according to who I am treating. My medical experience has led to me becoming more moderate over time, and to recognition that there are often many options to approach any one particular problem.
The opposite of meeting, learning and debating with many others is the in group, the phenomenon of looking for reinforcement of previous prejudices, rather than for new knowledge, or counter examples. The extreme of this in group thinking, and ignoring, or misinterpreting the rest of the world is seen in terrorism, and other single issue fanaticisms.
Sunstein has done us a great favour by summarising the cognitive work needed to be done to become a dogmatic fanatic or terrorist, and by showing us what we need to do to avoid this.
Some degree of associating with birds of a feather is useful (e.g. a learned society, a local football club) in terms of sharing experience and developing focused expertise. But we lose so much if we go too far down this specialisation process and develop a blind spot for the rest of the world.
Great minds think alike? Fools seldom differ? This book navigates the balance between these two opposites beautifully.