Why do our jaws drop?
September 23, 2011 11:21 AM   Subscribe

Why do our jaws drop when we are dismayed or surprised?

For instance, I just read about a scene in The Cove involving a baby dolphin and a very disturbing death. Cue the jaw dropping.

posted by Fister Roboto to Health & Fitness (17 answers total) 2 users marked this as a favorite
It mimics a scream or shout without the sound. It's also mimcry of others we've seen doing that.
posted by michaelh at 11:22 AM on September 23, 2011

I think it's a reflex to ensure we get adequate oxygen to make it through whatever the situation is. That is, I think it's more about the intake of air than the dropping of the jaw.
posted by cocoagirl at 11:25 AM on September 23, 2011 [1 favorite]

Because we open our mouths to express dismay or shock, and then we find ourselves speechless.
posted by Decani at 11:37 AM on September 23, 2011 [3 favorites]

"slack-jawed" is probably a more accurate way to describe that, but when you're looking for an adverb nothing works like "jaw-droppingly."

I think it reflects how your facial muscles kind of go slack when you're shocked at something. Your mouth only opens a tiny bit extra, or maybe your lips part where they were closed before, but you know writers, with the hyperbole and all!
posted by stupidsexyFlanders at 11:50 AM on September 23, 2011

Defensive physiology

When the body executes the “fight-or-flight" reaction or stress response, the nervous system initiates, coordinates and directs specific changes in how the body is functioning (physiology), preparing the body to deal with the threat.
posted by Cool Papa Bell at 11:57 AM on September 23, 2011 [1 favorite]

I'm not sure there's a reason. Great question. Facial responses to emotions are instinctive and universal (see Paul Ekman's "Emotions Revealed" and others) but as for why your jaw drops with surprise or dismay, or why your eyebrows go down with sadness, or all the other classic identifiers for happiness, anger, disgust, contempt, etc,... I hadn't thought.

I suppose... yeah, it must be to transmit the information, so others can empathically feel what you're feeling and you have that group identity thing. How does any language take shape?

If you pull the face you feel the emotion, that's how hardwired it is.
posted by fraac at 12:17 PM on September 23, 2011 [2 favorites]

It's not for oxygen - it's from loss of attention.

Our brain is a greedy beast - if it experiences sensory overload, it will release its previous focus in favor of a more tantalizing trip...and so your jaw drops (your jaw naturally slacks at time of rest).

But steely men (...and women) do not suffer from jaw drop - they witness the scene but do not flinch.
posted by Kruger5 at 12:47 PM on September 23, 2011

I too think it's communicative. Any kind of mouth opening in primates is a display (of some kind) and I think the distinction here is that the lips are covering the teeth, so it's a show of submission and of being overwhelmed.
posted by bonobothegreat at 1:40 PM on September 23, 2011

I would agree with the loss of attention/muscle slackness explanations, because when I experience this, it certainly isn't because I'm halfway to a scream or shout, or intending to express something and simply at a loss for words. I sense no intention to vocalize at all.
posted by Durn Bronzefist at 1:46 PM on September 23, 2011

I believe this supports Kruger5's attention loss hypothesis.

It could possibly be a by-product of the eyes bulging out in surprise- in that the movement of the top of the head leaves the bottom of the head behind.

There could also well be a role for the collection of more oxygen to help prepare the body to deal with the surprising stimuli, in that sense it is a less intense form of a gasp of shock.
posted by leibniz at 1:53 PM on September 23, 2011

Yeah, I also think it's that you are so shocked the seemingly involuntary expression of being not slack-jawed is no longer maintained.
posted by abirdinthehand at 1:55 PM on September 23, 2011

Darwin's opinion, from his The Expression of the Emotions in Man and Animals:
Now when we start at any sudden sound or sight, almost all the muscles of the body are involuntarily and momentarily thrown into strong action, for the sake of guarding ourselves against or jumping away from the danger, which we habitually associate with anything unexpected. But we always unconsciously prepare ourselves for any great exertion, as formerly explained, by first taking a deep and full inspiration, and we consequently open our mouths. If no exertion follows, and we still remain astonished, we cease for a time to breathe, or breathe as quietly as possible, in order that every sound may be distinctly heard. Or again, if our attention continues long and earnestly absorbed, all our muscles become relaxed, and the jaw, which was at first suddenly opened, remains dropped. Thus several causes concur towards this same movement, whenever surprise, astonishment, or amazement is felt.
(p.284, summarizing the discussion that started on p.282; he goes on to discuss various sounds and gestures made when astonished)
But I imagine there is more recent research.
posted by stebulus at 3:32 PM on September 23, 2011

Great question.

When an explosion is known to be in the offing, people are advised to keep their mouths open to reduce the possibility of damage to their ears and lungs.

At first, I didn't see any relevance this could have to an apparently universal response to surprise, but perhaps surprising lightning flashes and the often delayed enough to open your mouth-- whether by distance or from a subsequent bolt-- shock wave of thunder could be a part of the selective pressure which has made this such a widespread trait.
posted by jamjam at 5:28 PM on September 23, 2011

I'm sure if a study could be done.. where some people were brought up in an environement where you dropped your jaw when shocked... and another environment where you didn't do this, you'd find a high correlation between jaw dropping and people brought up to be jaw droppers.
posted by herox at 8:29 PM on September 23, 2011

It's universal, not cultural.
posted by fraac at 1:22 AM on September 24, 2011

i don't think it's universal. just because a book says it's universal doesn't necessarily make it so, for instance some people believe that learning language is an innate human skill while others believe it's an acquired skill. personally i've never witnessed anybody involuntarily dropping their jaw in shock or astonishment other than on TV and the movies. i've only seen it used in an exaggerated fashion with a sharp intake of breath making a sound to convey shock or astonishment and that behaviour is learnt.
posted by canned polar bear at 12:32 PM on September 24, 2011

Thousands of hours of scientific research went into Paul Ekman's work. At this stage it isn't controversial. Jaw dropping is a universal reaction to surprise. You could probably find apes that do it. It would have to be a lot of surprise for the full effect. Perhaps jaw dropping for 'dismay' is cultural, I don't know.
posted by fraac at 1:18 PM on September 24, 2011

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