Why do people blush?

Actress Sophie Marceau begins to blush from embarrassment after a wardrobe mishap at the Cannes Film Festival in 2005. See more emotion pictures.
Pascal Le Segretain/Getty Images

You can't believe the words that have just come out of your mouth. You certainly didn't mean to insult the woman in line ahead of you, but it's too late now. What business is it of yours how many chicken nuggets she orders? She's staring at you angrily. You feel a sudden jolt as your heart rate increases. You have a coppery taste in your mouth, and your cheeks begin to feel warm. You, my friend, are embarrassed, and you're blushing because of it.

Blushing and embarrassment go hand in hand. Feeling flushed is such a natural response to sudden self-consciousness that if it weren't part of an emotionally crippling experience, it could almost be overlooked. But blushing is unique, which is why scientists want to know more about blushing. While the psychology of blushing remains elusive, we do understand the physical process involved. Here's how it works.

Blushing from embarrassment is governed by the same system that activates your fight-or-flight response: the sympathetic nervous system. This system is involuntary, meaning you don't actually have to think to carry out the processes. In contrast, moving your arm is a voluntary action; You have to think about it, no matter how fleeting the thought is. This is good, because if moving your arm was involuntary, people would end up buying a lot of stuff they don't want at auctions.

When you're embarrassed, your body releases adrenaline. This hormone acts as a natural stimulant and has an array of effects on your body that are all part of the fight-or-flight response. Adrenaline speeds up your breathing and heart rate to prepare you to run from danger. It causes your pupils to grow bigger to allow you to take in as much visual information as possible. It slows down your digestive process so that the energy can be redirected to your muscles. All of these effects account for the jolt you feel when you find yourself embarrassed.

Adrenaline also causes your blood vessels to dilate (called vasodilation), in order to improve blood flow and oxygen delivery. This is the case with blushing. The veins in your face respond to a signal from the chemical transmitter adenylyl cyclase, which tells the veins to allow the adrenaline to do its magic. As a result, the veins in your face dilate, allowing more blood to flow through them than usual, creating the reddened appearance that tells others you're embarrassed. In other words, adrenaline causes more local blood flow in your cheeks.

­­This sounds reasonable enough, but it's interesting to note that this is an unusual response from your veins. Other types of blood vessels are responsive to adrenaline, but veins generally aren't. In other regions of your body, veins don't do much when adrenaline is released; the hormone has little or no effect on them.

Blushing from embarrassment is a unique phenomenon. There are other means by which our cheeks become flushed: Drinking alcohol or becoming sexually aroused can cause us to blush, but only being embarrassed causes the type of blushing that is triggered by adrenaline.

Some people opt to undergo surgery to limit their blushing response. Erythrophobia is the fear of blushing and it can be enough that it could lead to a person choosing to have the tiny nerves at his or her spine, which control blushing, snipped. This surgery -- called endothoracic sympathectomy -- has been shown to limit blushing.

­Blushing is part of a powerful experience, but why have we developed this response to being embarrassed? Science hasn't been able to answer that question definitively, but there are some interesting theories about the reasons for blushing. Read about those on the next page.