The other day I was having a debate with a supporter of high taxes. I decided to argue against this by explaining the concept of the deadweight loss of taxation. In order to do so, I started with the following example, to explain the deadweight gain of trade:
Bob and Sam live next door. Bob can mow his lawn in 4 hours, and clean his gutters in 2 hours. Sam is the other way around: He can mow his lawn in 2 hours and clean his gutters in 4. If Bob cleans Sams gutters in exchange for Sam mowing his lawn, they both save 2 hours.
My friend replied: "I understand the mathematics, and don't dispute that in your example both people would be better off. But is there any evidence that this works in the real world? Perhaps there is some unforeseen factor that causes voluntary exchange to not be mutually beneficial."
This question left me momentarily stumped, because I had always found the concept self evident and did not have any obvious examples at hand.
Improvising, I tried using the example of China and the USA both benefiting from trading with each other (my thinking was that this was a more contemporary form of Ricardo's original wheat example). However this caused my friend to object on grounds of geopolitical issues. So I am hoping to find a better example, that does not raise other issues.
Therefore I wanted to ask: Can anyone suggest some unambiguous, real world (ie non-hypothetical) evidence of the mutual benefits of voluntary exchange? If this could relate to exchange between individuals rather than countries I believe this would make the case more clearly.