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For a 2010 study, social network gurus James Fowler and Nicholas Christakis had people play a series of “public goods” games with different groups of strangers.

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In these games, subjects staked with a modest amount of money can give whatever they want to a communal pot of money that is then tripled in value and split evenly.

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After each round, everybody learns who gave what.

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Usually in these games, people start off trusting each other to some degree.

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Then some players inevitably try to get something for nothing, or next to nothing, and their free riding eventually kills trust and drives donations into the ground.

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In Fowler and Christakis’s study, however, subjects dealt with a new group of players every time.

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Each round was a fresh start and a one–shot deal, so neither reputation nor reciprocity had any pull.

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Nevertheless, the players’ contributions in any given round rose and fell according to how generous their fellow players had been in earlier rounds.

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The more those previous partners had given — indeed, the more that previous partners of those previous partners had given — the more trusting and generous subjects would be with their new group.

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Even without concern over developing a reputation, or an opportunity to reciprocate, trust ripples out.

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Trust can evaporate just as easily.

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Ironically, punishments for not cooperating can backfire precisely because they imply low expectations for trustworthy behavior.

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If people could be trusted, after all, then no sanctions would be necessary.

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In one recent experiment, people played two rounds of a trust game.

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Some initially played with punishments for violating trust and then played a second round without these sanctions.

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Everyone rated how much they trusted their fellow group members on a sliding scale.

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People who played under the threat of sanctions cooperated more, but trusted each other less than those who played without sanctions.

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When the sanctions were removed, the value of trust dropped even further and cooperation among these players took a nosedive.