Singing Loudly

Caveat: I admit that my understanding of both psychology and anything dealing with human instincts is far below par as you will soon see.

Yesterday I picked up some great Mexican food from restaurant that I love. While I was waiting on the food, I was trying to entertain myself by looking at all of the nick-nacks on the wall; however, I became bored of that very quickly. I noticed that one table of guys weren't making any noise except for frequent laughs. Of course, being curious and nosy, I decided to watch them to see what was going on that was so funny. As it turns out, they weren't talking at all because they were all deaf.

While it was just really cool to watch six 30-40 year old guys having a conversation in sign language, I wondered something about laughter. Why do we laugh? I always guessed that it was a way of showing others our emotions. I'm not sure if it is instinctual or a learned behavior? I know that babies often laugh when their parents are cooing and smiling. Is that imitation?

I'm curious because a laugh doesn't seem to do much more than a good smile unless it's an auditory signal to others that you think something is funny. Yet, all of these guys were laughing despite the fact they weren't meaning to use auditory signals for others. Is that enough to show that laughter is instinctual?