Why do certain ideas become popular? The naive view is that it's because they're true, or at least justified. This fascinating book, influenced by evolutionary biology and epidemiology, is the first full-scale examination of some of the other reasons. Consider Aaron Lynch's example of optimism--it may not be true or warranted, but it tends to prevail because optimists tend to have more children to pass along their outlook to. Sometimes, Lynch points out, there is a paradoxical but predictable expansion-contraction pattern to the social spread of ideas. If nothing else, lobbyists need to look into this stuff to see which side their bread is really buttered on. Warning: this book is densely written. But it's worth the wade.