All of the little signs we instinctively register during a conversation—the muscle shifts and tensions in the other person’s face, eye movements and pupil dilation, pitch and speed of the voice—as well as the fluctuations in our own inner landscape—salivation, swallowing, breathing, and heart rate—are linked by a single regulatory system.5 All are a product of the synchrony between the two branches of the autonomic nervous system (ANS): the sympathetic, which acts as the body’s accelerator, and the parasympathetic, which serves as its brake.6 These are the “reciprocals” Darwin spoke of, and
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