There are actually two questions hidden in this one. For a clear answer we can best treat them separately:

(1) Why do we cry when we are in sudden pain?

(2) Why do we say ‘ouch’ and not something else?

With regard to the first question, let’s start by noting that we’re not alone: a lot of other animals also cry when in pain. Why? Darwin, who wrote a book in 1872 about emotions in humans and animals, thought it had to do with the fact that most animals experience strong muscle contractions when they’re in pain — a ritualized version of quickly escaping a painful stimulus. Still, why should this be accompanied with an actual cry? Research has since shown that pain cries also have communicative functions in the animal kingdom: for example to alarm others that there is danger, to call for help, or to prompt caring behavior. That last function already starts in the first seconds of our life, when we cry and our mothers caringly take us in their arms. Human babies, and in fact the young of many mammals, are born with a small repertoire of instinctive cries. The pain cry in this repertoire is clearly recognizable: it has a sudden start, a high intensity and a relatively short duration. Here we already see the contours of our ‘ouch’. Which brings us to the second part of our question.

Why do we say ‘ouch’ and not something else? Let us first take a more critical look at the question. Do we never shout anything different? Do we utter a neat ‘ouch’ when we hit our thumb with a hammer or could it also be ‘aaaah!’? In reality there is a lot of variation. Still, the variation is not endless: no one shouts out ‘bibibibi’ or ‘vuuuuu’ when in pain. Put differently, pain cries are variations on a theme — a theme that starts with an ‘aa’ sound because of the shape of our trachea when our mouth is wide open, and then sounds like ‘ow’ or ‘ouch’ when our mouth quickly returns to being closed. The word ‘ouch’ is a perfectly fine summary of this theme — and here we touch on an important function of language. Language helps us share experiences which are never exactly the same and yet can be categorised as similar. That is useful, because if we want to talk about “someone crying ouch” we don’t always need or want to imitate the cry. In that sense ‘ouch’ is more than just a pain cry: it is a proper word.

Does this all imply that ‘ouch’ may be the same in every language? Almost, but not quite, since each language will use its own inventory of sounds to describe a cry of pain. In German and Dutch it is ‘au’, in Israeli it is ‘oi’, and in Spanish it’s ‘ay’ — at least that is how Byington described it in 1942 in one of the first comparisons of these words.

Each of us is born with a repertoire of instinctive cries and then learns a language in addition to it. This enables us to do more than scream and cry: we can also talk about it. Which is a good thing, for otherwise this answer could not have been written. 

Originally written in Dutch by Mark Dingemanse and published in "Kennislink Vragenboek"
Translated by Katrien Segaert and Judith Holler