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Richard Wilkinson and Kate Pickett’s “The Inner Level: How More Equal Societies Reduce Stress, Restore Sanity and Improve Everyone’s Well-Being” is a follow-up to their 2009 best-seller “The Spirit Level.” That book reviewed several hundred studies expounding on their book’s subtitle: “Why Greater Equality Makes Societies Stronger,” and came to the breakthrough finding that “inequality affects the vast majority of the population, not only the poor minority.”