Empathy is often thought to occur automatically. Yet, empathy frequently breaks down when it is difficult or distressing to relate to people in need, suggesting that empathy is often not felt reflexively. Indeed, the United States as a whole is said to be displaying an empathy deficit. When and why does empathy break down, and what predicts whether people will exert effort to experience empathy in challenging contexts? Across 7 studies, we found that people who held a malleable mindset about empathy (believing empathy can be developed) expended greater empathic effort in challenging contexts than did people who held a fixed theory (believing empathy cannot be developed). Specifically, a malleable theory of empathy--whether measured or experimentally induced--promoted (a) more self-reported effort to feel empathy when it is challenging (Study 1); (b) more empathically effortful responses to a person with conflicting views on personally important sociopolitical issues (Studies 2-4); (c) more time spent listening to the emotional personal story of a racial outgroup member (Study 5); and (d) greater willingness to help cancer patients in effortful, face-to-face ways (Study 6). Study 7 revealed a possible reason for this greater empathic effort in challenging contexts: a stronger interest in improving one's empathy. Together, these data suggest that people's mindsets powerfully affect whether they exert effort to empathize when it is needed most, and these data may represent a point of leverage in increasing empathic behaviors on a broad scale.