Red-faced scientists have admitted that, despite years of research, no one can explain why people go red in the face when they are embarrassed, ashamed or shy.
A leading scientist admitted that lack of understanding surrounding the reasons for blushing was the biggest gap in Charles Darwin's theory of evolution, reports the Daily Express.
Writing in the New Scientist, Frans de Waal said: "We're the only primate that (blushes) in response to embarrassing situations (shame) or when caught in a lie (guilt), and one wonders why we needed such an obvious signal to communicate these self-conscious feelings.
"Blushing interferes with the unscrupulous manipulation of others."
de Waal, professor of primate behaviour at Emory University in Atlanta, Georgia, was one of several experts asked by the magazine to identify the biggest gap in their field when it came to Darwin's theory, to mark the 200th anniversary of the scientist's birth.
Blushing is caused when adrenalin generated by a situation makes veins expand, allowing more blood to flow and leaving the sufferer red-faced.
As for why people blush, Professor de Waal speculates it could be a way of keeping humans honest.