Left-right asymmetries in behavior associated with asymmetries in the brain are widespread in the animal kingdom [1], and the hypothesis has been put forward that they may be linked to animals’ social behavior [2, 3]. Dogs show asymmetric tail-wagging responses to different emotive stimuli [4]—the outcome of different activation of left and right brain structures controlling tail movements to the right and left side of the body. A crucial question, however, is whether or not dogs detect this asymmetry. Here we report that dogs looking at moving video images of conspecifics exhibiting prevalent left- or right-asymmetric tail wagging showed higher cardiac activity and higher scores of anxious behavior when observing left- rather than right-biased tail wagging. The finding that dogs are sensitive to the asymmetric tail expressions of other dogs supports the hypothesis of a link between brain asymmetry and social behavior and may prove useful to canine animal welfare theory and practice.