Tainter's project here is to articulate his grand unifying theory to explain the strange and disturbing fact that every complex civilisation the world has ever seen has collapsed.
Tainter first elegantly disposes of the usual theories of social decline (disappearance of natural resources, invasions of barbarians, etc). He then lays out his theory of decline: as societies become more complex, the costs of meeting new challenges increase, until there comes a point where extra resources devoted to meeting new challenges produce diminihsing and then negative returns. At this point, societies become less complex (they collapse into smaller societies).
Complexity, writes Tainter, describes a variety of characteristics in a number of societies-- many differentiated social roles, a large class of administrators not involved in the production of primary resources, energy devoted to different kinds of communication, centralised government, etc. Societies become more complex in order to solve problems. Consider this example:
A simple hunter-gatherer society with limited agriculture (i.e. garden plots) is faced with a problem, such as a seasonal drop in food production (or an invasion from its neighbours who have the same problem and are coming over for food). The bottom line is, this society faces an energy shortage. This society could respond to the food crisis by either declining in numbers (die-off) or by increasing production. Most societies choose the latter. In order to increase production, this society will need to either expand territorially or increase agricultural production . In either case, this investment can pay off substantially in either increased access to food or increased food production.
Herein, however, lies the rub. Since, as Tainter writes, the "number of challenges with which the Universe can confront a society is, for practical purposes, infinite," complex societies need to keep on increasing their level of complexity in order to survive new challenges. Tainter's thesis is that these "investments in aditional complexity" produce fewer and fewer returns with time.
The hunter-gatheres of the above example incur costs as they try to solve their food-shortage problem. If they conquer their neighbours, they have to garrison those territories, thus raising the cost of government. If they start agriculture on a larger or more intense scale in their own territories, they have to create a new class of citizens to man the farms, distribute and store the grain, and guard it from animals and invaders. In either case, the increases in access to energy (food) are offset somewhat by the increased cost of social complexity.
But, as the society gets MORE complex to confront newer challenges, the returns on these increases in complexity diminish. Eventually, the costs of maintaining garrisons (as the Romans found) is so high that both home and occupied populations revolt, and welcome the invaders with their simpler way of life and their lower taxes. Or, agricultural challenges (a massive drought, or degradation of soils) are so great that the society cannot muster the energy reserves to deal with them.
Tainter's book examines the Maya, Chacoan and Roman collapses in terms of his theory of diminishing marginal returns on investments in complexity. This is the fascinating part of the book; the disturbing sections are Chapter Four and the final chapter. In Chapter 4, Tainter musters a massive array of statistics that show that modern society has been facing diminishing returns on investments in complexity. There is a very simple reason for this: we solve the easiest problems first. Take oil, for example. In 1950, spending the energy equivalent of one barrel of oil in searching for more oil yielded 100 barrels in discovered oil. Now, the same investment yields 5. The per-dollar return on R&D investment has dropped for fifty years. In education, additional investments in programs, technology etc. no longer produce increases in outcomes. In short, industrial society is looking at steadily fewer returns on its investments in both non-human and human capital. When a new challenge comes, Tainter argues, society will eventually be unable to muster the necessary resources to deal with the crisis, and will revert-- in a painful and unhappy way-- to a much simpler way of life.
In his final chapter, Tainter describes the modern world's "arms race of complexity" and makes some uncomfortable suggestions about our own future. (...). In an age where, for example, the U.S. invasion of Iraq has yielded net negative returns on investment even for the invaders (where's that cheap oil?), and where additional investments in education and health care in industrialised countries make no significant increases in outcomes, the historical focus of Tainter's work starts to become eerily prescient.
The scary thing about this deeply thoughtful and thoroughly researched book is its contention that the future, for all our knowledge and technology, might be an awful lot like the past.