University of Texas atSan
Abstract
Why do human populations remain vulnerable to collapse, even when they are
large? Classical demographic theory predicts that volatility in growth should
decline rapidly with size due to the averaging effects of the law of large
numbers. As such, while small-scale societies may be demographically fragile,
large-scale societies should be much more stable. Using a large census dataset
of 228 indigenous societies from Brazil, we show that this prediction does not
hold. Instead of volatility declining as the square root of population size, it
falls much more slowly. This means that individuals within communities do not
behave as independent demographic units as their lives are correlated through
cooperation, shared subsistence practices, overlapping land use, and exposure
to common shocks such as disease outbreaks or failed harvests. These
correlations build demographic synchrony, drastically reducing the effective
demographic degrees of freedom in a population, keeping volatility higher than
expected at all scales. As a result, large-scale populations fluctuate as if
they were much smaller, increasing their vulnerability to collapse. This helps
explain why human societies of all sizes seem vulnerable to collapse, and why
the archaeological and historical record is filled with examples of large,
complex societies collapsing despite their size. We suggest demographic
synchrony provides a general mechanism for understanding why human populations
remain vulnerable across all scales: Scale still stabilizes synchronous
populations via density increases, but synchrony ensures that stability grows
only slowly with size, leaving large populations more volatile, and more
vulnerable, than classical demographic theory predicts.