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What makes some people give blood and bake casseroles for their neighbors, while others mutter about taxes from behind closed blinds? A new paper published in Psychological Science, a journal of the Association for Psychological Science finds that part of the answer—but not all—may be in their genes.

The hormones oxytocin and vasopressin are thought to affect how people behave toward each other. For example, lab tests have found that people play nicer in economic games after having oxytocin squirted up their nose. “This is an attempt to take this into the real...