In that case, you might just be getting confused by regression to the mean. Over a long enough timescale, it's inevitable that outlier societies will regress ba

In that case, you might just be getting confused by regression to the mean. Over a long enough timescale, it's inevitable that outlier societies will regress back to being average. That would look like especially prosperous societies and especially poor societies becoming average, which you could mistakenly interpret as "they became average because they were weak from decadence/strong from adversity".