Is it possible that the coffee drinkers have more social interaction with the barista and others? It's unclear from the paper if they eliminated the confounding factors around coffee drinking.
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I think most baristas who do it for more than a year or two learn to not primarily be a coffee factory but first to make a positive impact on the people they see. The coffee is something that can be made consistent (and in a way, boring) through practice, but personal connection, especially when it is genuine, has a real draw.