Well, I have no idea. But I attended a taping of a sitcom once, and the way it worked is they had mics hanging above all the parts of the audience. Before they show they had a warm up comic, which both put us in the laughing mood and gave them a chance to record our particular audience laughing really hard.

Then when the show was recorded, we actually did laugh pretty hard. You know how you laugh louder when you're at a comedy show or at a movie theater than when you're home alone watching the same thing? Because of peer pressure? It was like that. You laugh harder in the audience.

And then they would "enhance" the laughing by taking the recording of us from earlier and playing it over the spots where we laughed live, especially if they end up using a second or third take, since were didn't laugh as hard.

Also I remember in our episode there was a joke where as the live audience we could see the payoff right away, but on the TV the camera did a slow pull back to reveal the joke. They added in our recorded laughter for that. I remember because I laughed at home but not in the studio.

So it's sort of a combination. But except in those rare cases they don't really add in laughter where there was none. They just enhance the live audience.