This paper explores whether there has been a recent decline in helping behavior in the United States. In a lost letter experiment, 7,466 letters were “lost” in 63 urban areas in the United States and Canada in 2001 and 2011. There has been a 10 percent decline in helping behavior in the United States, but not in Canada. Two arguments anticipate change in the level of help provided to strangers: the rise of new technologies, and neighborhood racial and ethnic diversity. Findings exclude increased privatism as a source for the decline in helping. In 2001 there was no variation in altruistic behavior based on neighborhood diversity. However, areas of the United States where the proportion of noncitizens increased since 2001 experienced reduced helping; the opposite was found in Canada. Possible explanations include changing attitudes toward noncitizens, and differences in public policy related to economic inequality, social inclusion, and the acceptance of multiculturalism.