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In his work on the pre-800 AD Classic Maya economy, the historian Philip Curtin recounts a striking finding: Archaeologists measured the ratio of cutting length to weight in obsidian blades and discovered that the ratio varied inversely with distance from obsidian sources. 

The economist Deirdre McCloskey cites this as evidence that the human impulse toward exchange and profit-seeking gain has always existed.

“If Mayans lived in a gainless, profitless, nonmarket economy, it would not matter to them how expensive obsidian was. But…the ratio varied inversely with the distance from the…

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