Taken together, the existing evidence suggests that happier people do indeed help more in a variety of contexts. Studies using random assignment to experimentally induce positive mood have provided important evidence that happiness causes increased helping behavior. Supporting the external validity of these findings, naturally occurring positive moods have also been shown to facilitate prosocial behavior. While we have focused on the impact of positive mood on giving, however, another well-documented area of inquiry has documented the impact of negative mood on helping as well, a seeming contradiction (see Batson, 1987; 1991). For example, Cialdini et al. (1987) showed that watching another person suffer a mild electric shock motivates helping in an observer through a sense of heightened empathy and increased personal sadness.