The laugh track has been used not just to sweeten but to replace audience reactions. The sitcom "Barney Miller" started out being taped before a live audience, but the actors, the majority of whom were stage actors (some from Broadway) strangely felt uncomfortable performing in front of the audience. After the series started, the audience was eliminated, the actors performed on a closed set in an otherwise empty studio, and a laugh track machine was used to provide the reactions.
Appraisal of 1953 "Laff Box" on "Antiques Roadshow":
http://video.pbs.org/video/1754622115/
Demonstration of a "Laff Box":
History of the "Laugh Track"
http://en.wikipedia.org/wiki/Laugh_track
The more time goes on, the more it seems reality is relative...