Most days present us with opportunities to commit minor altruistic acts, small choices to forgo an advantage for the good of someone else—waking early to make the coffee, standing to let an elderly person sit on a crowded train, slowing down to allow another car to merge onto the highway. These moments of mild self-sacrifice seem like the right thing to do, but often defy logical analysis on reflection. After my good deed, I arrive at work a bit drowsy, a little footsore, or a few seconds late, to what end? In a personal cost-benefit calculation, each act exacts a price but rarely seems to yield a direct, tangible dividend.

If these acts are “right,” why are they right? This question becomes more profound when the sacrifices increase, when choosing selfless actions might impact o...