The book is a history of wealth and income gaps in the US in the 20th century. The book documents that the gap between rich and poor declined greatly in mid-century—he refers to this as the "Great Compression"—then widened again, starting in the 1980s, to levels higher than those in the 1920s. Most economists—including Krugman himself—have regarded the late 20th century divergence as resulting largely from changes in technology and trade, but now Krugman writes—particularly in Chapters 1, 3, and 4—that government policies—particularly the establishment of, and subsequent attacks on, the social safety net or "welfare state"—has played a much greater role both in reducing the gap in the 1930s through 1970s, and in widening it in the 1980s through the present.