Much of the “canned laughter” used on the soundtracks of sitcoms is
said to have come from tapes that were originally recorded during
broadcasts of “The Red Skelton Show” in the late 1950s and early ’60s.

It is eerie to realize that when we encounter a laugh track (or
“sweetening”) in a recent show, some of the people we hear laughing
may have been dead for decades. Yet their guffaws go on forever.

“For half a century, TV sitcoms have been seasoned with artificial
bursts of laughter and applause. It all goes back to 1953 when the
Laff Box was invented to beef up anemic reactions from live audiences
or when a show was taped without a live audience. The inventor of the
Laff Box, Charles Douglass, recently passed away at the age of 93…

According to legend, much of the laughter in Charlie’s Box came from
the Red Skelton Show. Since Red Skelton also did pantomimes, it was
easy for Charlie to get nice, clean recordings of laughter and
applause without disturbing dialogue.”

More here.