Pretty expensive study -- unless I'm misunderstanding, it seems like they gave away hundreds of thousands of dollars in these wallets.
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Also, the wallets have a cost too, so it is not only the equivalent of $13 being lost in some cases.
They did in some cases (seven cities in the Czech Republic and Switzerland) then drop by in person to try to retrieve the wallets, but (unless I misunderstood) that's only about 1% of the wallets.