Does civilization progress in a straight line or in cycles?

The idea that civilizations rise and fall is much older than genetics. In the second century BCE, Polybius described political constitutions deteriorating into corrupt forms and being replaced. In the fourteenth century, Ibn Khaldun explained dynastic expansion through asabiyyah, the solidarity that enables a group to cooperate and defend itself. His account traced how concentrated power, luxury and dependence could erode that solidarity across generations.

One recurring idea is a feedback loop: the conditions that help a society expand can be changed by its own success. Security and prosperity alter incentives, weaken older restraints or undermine cooperation, allowing a new ruling group to emerge. These traditions offer different accounts of political and dynastic turnover; they do not imply that every society follows the same timetable.

Edward Dutton and Michael Woodley of Menie developed a genetic account of civilization cycles in At Our Wits’ End. They propose that selection favouring intelligence contributes to a civilization’s rise, while greater prosperity relaxes survival pressures and changing fertility patterns eventually reverse the direction of selection. In this account, the achievements of a civilization can weaken the conditions that helped produce them. Over generations, declining cognitive capacity would make innovation and the maintenance of technological achievements harder. Their discussion of Rome and other civilizations applies this rise-and-decline argument to the ancient world as well as the modern West. A renewed rise after decline would require conditions favouring those traits to return. These are proposed mechanisms to test against historical evidence. Figure 1 illustrates this proposed feedback and the conditions a renewed rise would require.

Figure 1. The proposed civilization cycle

Modern Icelandic research by Kong and colleagues found that people with higher education-associated genetic scores tended to have children later and have fewer children overall. That observation makes reproductive differences a plausible mechanism for changing score averages. Whether a comparable process operated during ancient expansion, imperial decline and recovery requires historical evidence.

Ancient DNA gives us a way to ask whether anything resembling this sequence actually happened. In Rome Was Ahead. Then the Barbarians Caught Up, I followed education-associated genetic scores through the Roman transition in Italy and northern Europe. That raised a larger question: was the Roman-era pattern one stretch of a much longer European rise, decline and recovery?

The Roman comparison also raises a regional question: how much does the European pattern differ from place to place? Different political and demographic histories could leave different genetic trajectories. I test that directly by asking whether allowing regional differences improves on one European trend.

The larger question calls for a European view, extending back into prehistory. I compare two polygenic scores (PGS): one associated with educational attainment (EA4), and another associated with fluid intelligence (FI), or reasoning ability.

From the Holocene to the Black Death

The historical hypothesis has three landmarks. The first is the beginning of the Holocene, around 9750 BCE, or 11,700 years before present. The second is 27 BCE, the beginning of the Roman imperial period. The third is 1350 CE, around the Black Death. These dates give the theory a concrete sequence to confront: a long rise, a decline across the imperial and medieval centuries, then a renewed rise.

The 1350 date comes from the question I explored in Did the Black Death Select for Intelligence?. Here the plague hypothesis becomes part of a longer story. Instead of asking only whether scores rose after the pandemic, I ask whether that rise followed an earlier reversal.

At each date, the fitted line can change direction or speed without making a sudden jump. This is what a statistical “hinge” does. The dates are historical reference points chosen for the hypothesis; the DNA still has to tell us whether the proposed rise and fall are present.

Figure 2 puts the two scores side by side.