Decisions to be altruistic (or not) are typically thought to result from explicit, salient internal conflicts between self-serving and other-regarding motives. We show, however, that, when faced with a choice between a selfish or altruistic outcome, people’s decisions are biased by a fundamental, yet implicit, asymmetry in the attention (or weight) given to the opportunity costs associated with each alternative. We provide converging evidence for the existence of this subtle, but consequential, default asymmetry in how people attend to their own vs. others’ outcomes, and we highlight a novel semantic “nudge” that increases generosity by countering this tendency. We also elucidate the underlying cognitive processes that drive the initial asymmetry and the nudge to counter it.