There are plenty of macro examples of this behavior: we form complex social hierarchies, our economy is highly reliant on the trust of people we’ve never met, and we punish unprovoked violence and harm. This is also true when it comes to interactions between individuals. In the famous Ultimatum experiment (pdf) in which people are given $20 and the choice to either take all of it, split it $18/$2, or split it $10/$10, most split it evenly. In another experiment, researchers found that deliberately dropped envelopes were stamped and mailed one fifth of the time by complete strangers. Moreover, psychologist Jenifer Kunz found that (pdf) when people receive a Christmas card from a family they do not know, they usually send one back in return.
Notes: I interviewed Yale Psychologist Paul Bloom for this article.