Emotional Flushing (Blushing)

          Humans express a "blushing" response to situations of personal embarrassment that is very visible in fair-skinned individuals, but susceptibility to this response is highly variable. In Chapter 13 of Darwin's "The Expression of the Emotions in Man and Animals", he described blushing as "... the most peculiar and most human of all expressions." Blushing is a uniquely human characteristic that does not appear in the great apes or other primates. In response to psychologic arousal, involuntary dilation in facial capillary beds produces a pronounced reddening in the cheeks that often presents during embarrassment. Violation of social taboos, recognition of scrutiny, praise, and accusation of blushing may all trigger a blushing response. Surprisingly, children do not blush until 2-3 years of age. Blushing, it would seem, requires an awareness of being observed and a knowledge of social norms that begin to develop at these ages (Doherty-Sneddon 2003, Crozier 2010). Blushing may also serve as an involuntary form of non-verbal apology, demonstrating embarrassment in order to acknowledge a social mistake. Physical acknowledgement displays may work in conjunction with verbal apologies and other appeasement behavior to elicit sympathy that prevents conflict escalation. Since blushing is involuntary, the sincerity of the apology is also less likely to be questioned (Crozier 2010, Leary 2006, Edelmann 2001). Although all humans appear to share the capacity to blush, the ability to detect this response varies with skin complexion. The blushing response may have been less evident in ancestral humans, arguing against selection for this signaling function.

           Studies using thermal imaging suggest that blushing functions as an involuntary signal. These studies demonstrate that blushing is more prounounced in observed skin areas and occurs independent of conscious emotions. In the first study, subjects were asked to sing aloud while a researcher stared closely at one of the subject's cheeks. Increases in cutaneous blood flow, measured by changes in facial temperature, were larger in the observed cheek (Drummond 2004). A second study measured changes in facial temperature in response to physical contact. Tactile contact (under the pretense of making measurements) with the face or chest increased facial temperature even though subjects did not report a strong emotional response to the exam (Hahn 2012). Physiologic changes that precede conscious responses, vary according to external observation, and produce detectable differences are strong candidates for signaling mechanisms.

           On a physiologic level, a few studies address the distribution and control of the blushing response. In 1988, Wilkin describes “flushing” as a vasodilatory effect in capillary beds of the face and forearms. These capillary beds lie close to the surface of the skin, allowing greater visibility during dilation. Flushing may be associated with a number of conditions, including menopause, migraine, and pharmacologic therapies. The condition can also be triggered by spicy food or alcohol consumption. Wilkin distinguishes flushing, with its various physiologic triggers, from blushing, which is triggered by an emotional response (Wilkin 1988, Wilkin 1993). This emotional response appears to trigger blushing through the sympathetic nervous system. A 1982 study by Mellander demonstrates B-adrenergic dilation of human buccal (cheek) veins. Adrenergic innervation is uncommon in veins, and the presence of adrenergic receptors in buccal veins may be the mechanism that ties blushing to emotional distress (Mellander 1982). Treatment with B-adrenergic antagonists can reduce blushing. Excessive blushing can also be treated surgically: thoracic sympathectomy prevents blushing by severing sympathetic innervation to the face (Crozier 2010). Surprisingly, adrenergic innervation is also present in the buccal veins in rabbits (Mellander 1982).

Although changes in blood flow may precede conscious emotions, blushing in humans strongly correlates with emotional arousal. Other animals share vascular blushing machinery, but humans are unique in flushing from emotional rather than physiologic stimuli.