Charles Darwin corresponded with missionaries and doctors worldwide to learn whether all peoples blush, and called it the most peculiar and most human of all expressions. This episode separates blushing, which needs an emotional trigger, from flushing caused by heat, exercise, fever, or alcohol. Blushing occurs in the blush region of the cheeks, forehead, ears, neck, and upper chest, where research by Wilkin found denser, wider, shallower capillaries. A 1982 study by Mellander and colleagues showed that facial veins hold a constant basal tone and carry beta adrenoceptors that release that tension and let blood rush in.

In 1997 Drummond fitted undergraduates with laser Doppler flow meters on their foreheads. Blocking alpha receptors had no effect on blushing, and beta blockers delivered through the skin reduced it but did not stop it, pointing to backup mechanisms. Psychologist Ray Crozier calls the blush an involuntary apology that cannot be faked, while Salzen proposed in 2010 that it is thwarted fight or flight energy. Imaging links blushing to the cerebellum and the left paracentral lobe.

• Flushing is about physical heat, while blushing responds to social situations
• Facial veins act like coiled springs, held tight until a social trigger releases them
• For people with social anxiety, fear of blushing can itself trigger more blushing
• Idiopathic craniofacial erythema causes intense blushing with almost no provocation
• Endoscopic thoracic sympathectomy cuts or clamps sympathetic nerves in the chest to block the signal