This book employs the comparative method to understand societal collapses to which environmental problems contribute. My previous book (Guns, Germs, and Steel: The Fates of Human Societies), had applied the comparative method to the opposite problem: the differing rates of buildup of human societies on different continents over the last 13,000 years. In the present book focusing on collapses rather than buildups, I compare many past and present societies that differed with respect to environmental fragility, relations with neighbors, political institutions, and other "input" variables postulated to influence a society's stability. The "output" variables that I examine are collapse or survival, and form of the collapse if collapse does occur. By relating output variables to input variables, I aim to tease out the influence of possible input variables on collapses.
