Peter Brown of Princeton University is one of Mark Koyama’s favorite historians of late antiquity, but Koyama doesn’t agree with Brown’s explanation for why the Roman economy declined:
In The Rise of Western Christendom, Brown summarizes the new wisdom on the transition from late antiquity to the early middle ages. He accepts that this transition brought about an economic decline — a decline evident in the radical simplification in economic life that took place. Long distance trade contracted. Cities shrank and emptied out. The division of labor became less complex. Many professions common in the Roman world disappeared.
All of this is relatively uncontroversial. At issue is what caused this decline? Traditional accounts emphasized the destruction brought about by barbarian invasions and civil wars as the frontiers of the Western Empire collapsed. These accounts emphasized a collapse in trade and increased economic insecurity. Brown, however, argues that the bulk of modern research rejects this old fashioned view.
The barbarian invasions, of course, play a role in this story because they put pressure on the Roman state. But their role is peripheral. Rather, Brown contends that the Roman state was the engine of economic growth of late antiquity. Turning on its head the old view associated with Michael Rostovtzeff that attributed the decline of the Roman economy to high taxes imposed by the Emperor Diocletian and his successors, Brown argues that these high taxes were in fact the source of economic dynamism.
The collapse of the Roman state was catastrophic, not because the Roman state was an engine of economic growth, as Brown contends, but because it provided, albeit imperfectly, the public good of defense. In the absence of this, transactions costs greatly increased, long-distance trade declined, markets contracted, and urbanization declined.
The notion of a Malthusian Trap helps explain how high taxes — especially efficient land taxes — might help, rather than hurt, per capita income.