What does language include? When most people think of 'language,' they think immediately of the words and utterances that are spoken or written, but studying language as an entire system includes more than words and utterances because 'language' also includes the gestures we use to communicate. I'm not talking about gestures in terms of recognized signs in the world's signed languages--I'm talking about the gestures we use as we speak to show what we mean beyond the words we select.
For example, when you're speaking with someone, and they tell you some surprising news, you might respond with, "Wow." But more than that, you probably had some changes in your facial expression: wide eyes, open mouth, hands to cheeks--it depends on how surprising the news really was. In a different scenario, suppose someone says something that angers you; maybe your eyes narrow, your fists clench, your body moves a step forward... In written language, we like for the gestures to be included as part of texts: Imagine a novel being written with absolutely no descriptions given of how a character's body reacts in different situations.
A recent study on words and gestures (as reported by the NIDCD) shows that both these aspects of language are processed in the same area of the brain, further showing the connection between the two.
An interesting feature of gestures is that some are culturally conditioned (e.g., thumbs up, middle finger, "A-OK" sign where your thumb and first finger come together to form a circle) while others are more universally applied (e.g., taking a step backward when afraid, leaning toward someone you're interested in, bringing your eyebrows together when confused). Perhaps these more universal gestures are at the basest level of communication for humans--if we all share them, it would be counterintuitive to say that we have to learn them.
What gestures do you know of from other cultures? Or what gestures do you know of that are specific to Americans?