Human societies are problem solving organizations, which supply the necessities along with entertainment, as well as training to ensure continuation. The over-arching Sociopolitical systems require energy for their maintenance. If left without proper tending, they fall apart like a garden without attention. Increasing complexity carries with it increased cost per capita. Simple monarchies need much less maintenance than democracies, as voices are heard and justice grinds slowly. Investment in sociopolitical complexity often reaches a point of declining marginal returns, as time passes, interest wanes, justice disappoints. When debt saturation occurs, like today, the cost of maintenance and decline can be overwhelming. The complex system becomes vulnerable to collapse after great stresses and what Tainter calls perturbations, better thought of as disruptions or disturbances.
Stress and perturbation are a constant feature of any complex society, always occurring somewhere in its territory. The developed operating regulatory machinery is designed to deal with unplanned changes. Imagine spotty agricultural failures, border conflicts, large scale accidents, social eruptions, and disease outbreaks. Such localized stress tends to recur with regularity, and thus can be anticipated with preparations. To meet major unexpected stress surges, the society must have some form of net reserve. It can take the form of excess productive capacities in farm output, stored energy, or stockpiled minerals, or hoarded surpluses from past production, even reserve savings. Stress surges of great magnitude cannot be accommodated without such a reserve. A society beset by regular self-inflicted crises will be forced to draw upon its stored reserves too quickly. As declining marginal returns are experienced, the society must contend with rising systemic risk. The costs outweigh the yield put in reserve. Excess productive capacity will at some point be exhausted, and accumulated surpluses allocated to current operating needs will be used up. Even when the adversities are met and the surges are dealt with, a detriment to the system as a whole occurs. The society is weakened in the process, and made more vulnerable to the next crisis. Once a complex society develops the vulnerabilities of declining marginal returns, collapse might require the sufficient passage of time to render probable the occurrence of an insurmountable calamity.
More importantly, declining marginal returns make complexity a less attractive goal to maintain. The problem solving strategy takes a secondary role to instituting simpler systems. The society tends to accept the option for less costly and less sophisticated social forms. The alternative embraced eventually is an endorsed level of disintegration.