Blushing

Why would a body build a signal that broadcasts your embarrassment exactly when you least want it seen?

You say the wrong thing at a meeting, and your face announces it to the room. You cannot summon this on demand, you cannot call it off, and it arrives at precisely the moment concealment would serve you best. Design a body from first principles and you would never install it.

So there are two possibilities. Either it is a leak — an accident of plumbing with no function — or the very thing that makes it feel like a betrayal, the fact that your will is nowhere near the controls, is the point of it. Which would we expect to find?

Take the plumbing first, since it is the part we know best. A blush is vasodilation: blood vessels in the face, neck and upper chest widen, and more blood arrives close to the surface. The distribution is peculiar and not accidental — the facial skin carries an unusually dense superficial venous supply, so a change in flow there becomes visible in a way the same change on your thigh would not. The route is sympathetic, which is why interrupting the upper sympathetic chain surgically abolishes blushing, and beta-adrenergic mechanisms are implicated, since beta-blockers reduce it in some people. Whether emotional blushing is best described as active dilation or as the release of an existing constriction is still argued, and I will not pretend otherwise.

Now notice what is missing from that description: any voluntary route in. Actors cannot blush on cue. Nor is there a route out — and this is the interesting half. Attempting to suppress a blush reliably makes it worse, because the attempt requires attending to your own face, and self-focused attention is one of the things that drives blushing hardest. The system is closed at both ends.

Hold that thought and ask a different question: what makes any signal worth believing? Consider the alternative available to you after the gaffe. You can say you are sorry. That costs nothing, requires no particular inner state, and is exactly what someone who did not care would also say. A listener has no reason to weight it heavily. Anything a sender can produce at will is, for that reason, weak evidence about the sender.

Now the blush looks different. It cannot be produced by someone who feels nothing, and it cannot be withheld by someone who does. Its presence is therefore genuine evidence — not that you are a good person, but of something narrower and quite useful: that you registered the norm you just broke and that this room's judgement matters to you. Which is precisely the information an onlooker deciding whether to write you off would want.

That is a claim about receivers, and receivers can be tested. If the account is right, an identical transgression should be judged differently depending on whether the transgressor blushes. It is: presented with the same mishap, observers rate the person who blushes as more trustworthy and judge the offence less harshly than when no blush is shown. That is the load-bearing evidence for the whole story. Had the blush made no difference to observers — or had it hardened them — the signalling account would have nothing left to stand on.

There is a corollary worth noticing, because it constrains the account. The blush helps after a transgression. A blush with no visible cause does not read as remorse at all, and can invite suspicion that something is being concealed, which is roughly why fear of blushing is so miserable a condition: the signal fires with nothing to explain it.

Here is the refinement that ties the reasoning together, and it corrects the usual assumption that honest signals are honest because they are expensive. In the classic cases — an aposematic beetle's warning colour, an ornament only a healthy animal can carry — the guarantee is cost: a poor-quality sender cannot afford the display. A blush costs essentially nothing. Widening some facial vessels is metabolically trivial.

Its guarantee comes from somewhere else entirely. A blush is an index: it is caused by the state it advertises, through a pathway the sender has no access to. Cost is not doing the work; the absence of a control lever is. That is a genuinely different way for a signal to be unfakeable, and blushing is one of the cleanest examples of it in human behaviour. Darwin's description of it as the most peculiar and most human of all expressions has held up rather well.

Two honest limits. We have good evidence that receivers benefit from reading blushes and respond to them, but that is not the same as showing that blushing was selected for signalling rather than being a byproduct that observers learned to exploit — the selection history is not established. And there is an awkward fact for any purely visual account: a blush is far less visible on darker skin, which suggests that the accompanying gaze aversion, lowered head and the felt heat may carry more of the message than the colour alone.

• Why shame, embarrassment and guilt produce different displays, and which of them blushing actually belongs to.
• How other involuntary indices — pupil dilation, voice tremor — compare, and why so few of them are as legible as this one.

Every term the collection defines is gathered in the glossary.