It is hard to pull off a convincing fake laugh. You'd have to be conscious about the facial muscles to pull to come up with a very genuine look. But we can't help it. We all fake laugh, all the time. And the reason behind it is almost always because we want to appear polite.

Here's the kicker: chances are, we're fooling no one. According to research from UCLA, fake laughter is mistaken as real only a little more than 1/3 of the time. The 2/3 of the time, your audience is quite aware you didn't really find the joke funny.

As study author and associate professor at UCLA Greg Bryant points out, "Quite a few fake laughs sound pretty good, but listeners seem to pay attention to certain acoustic features that are really hard to fake."

Fake laughter tends to be quite a bit slower than the real laughter, something that 63% of listeners are able to pick up on.

From the UCLA Newsroom:
For the study, Bryant recorded the spontaneous conversations of college roommates. From these recordings, he collected 18 spontaneous laughs, which he considered to be genuine. He then enlisted a different group of co-eds to laugh on command. From this exercise, he recorded 18 fake laughs of the same length as the real ones.

With Athena Aktipis, a research scientist at UC San Francisco, he then played the recordings to three groups of UCLA undergraduates. In the first round, the participants were asked to determine whether the laughs were real or fake, and the students could usually tell the difference. But they were fooled by 37 percent of the fake laughs.
Here's an example from The Big Bang Theory: