In particular, this influence was most robust for the negative video clip, and less so for the neutral video clip - which itself elicited mild positive emotions. The facial feedback hypothesis, "that skeletal muscle feedback from facial expressions plays a causal role in regulating emotional experience and behaviour", [3] developed almost a century after Darwin. Hair Drug Testing Facts. In essence, the same point that Charles Darwin stressed on when he suggested that physiological changes were not just consequences of an emotion, but also affected that particular emotion.