Smartphones provide people with a variety of benefits, but they may also impose subtle social costs. We propose that being constantly connected undercuts the emotional benefits of face-to-face social interactions in two ways. First, smartphone use may diminish the emotional benefits of ongoing social interactions by preventing us from giving our full attention to friends and family in our immediate social environment. Second, smartphones may lead people to miss out on the emotional benefits of casual social interactions by supplanting such interactions altogether. Across field experiments and experience-sampling studies, we find that smartphones consistently interfere with the emotional benefits people could otherwise reap from their broader social environment. We also find that the costs of smartphone use are fairly subtle, contrary to proclamations in the popular press that smartphones are ruining our social lives. By highlighting how smartphones affect the benefits we derive from our broader social environment, this work provides a foundation for building theory and research on the consequences of mobile technology for human well-being.