Sun-induced frowning fosters aggressive feelings

We tested whether aggressiveness can be triggered by the involuntary frowning that occurs when people face the sun, due to the fact that sun-induced frowning involves the same pattern of facial muscle activation as in the expression of anger (interestingly, Charles Darwin remarked on the sunshade-like nature of frowning). In line with data showing that experimentally and unobtrusively induced facial and body displays facilitate congruent feelings, we found that participants walking against the sun without sunglasses scored higher in a self-report measure of anger and aggression compared to those walking with the sun behind and/or wearing sunglasses. We also suggest that frowning at the sun affects mood very quickly, because we did not find any effect of walking time on self-reported aggressiveness. Our results provide the first evidence of the ecological validity of the facial feedback hypothesis.