ANN ARBOR—Humans are altruistic by nature, according to a new theory published in the current issue of Psychological Inquiry.

The theory focuses on explaining the kind of altruistic behavior that involves costly long-term investment in others, such as parenting, caring for the sick or injured, and protecting family and comrades in times of conflict or war. This behavior typically entails considerable sacrifice” of time, effort, health and even life itself.

“Considering the self-centered motives that are evolutionarily ancient and that continue to drive human behavior today, it’s worth considering why people make these kinds of sacrifices,”