When I watched the Brady Bunch as a youngster, there was one particular deep guffaw that always caught my attention. I knew the laughs were pre-recorded but always assumed that there was just a laugh track tape and they'd press play at the appropriate times. I liked (and still like) the faux communal experience that laugh tracks provide when watching old sitcoms.
Turns out, that the rise of the laugh track was due to Charles Douglass (1910-2003), a Navy-trained electronics engineer/maker who went on to build a custom "Laff Box" of several dozen tape loops triggered by keys and dials. After its initial use on the Jack Benny Program, the machine, officially called the "Audience Reaction Duplicator," took the TV industry by storm. Douglass "played" the Laff Box like a proto-sampler and for years had the monopoly on TV laugh tracks. It was a process that the TV show producers and Douglass himself liked to keep secret. (You can learn more about Douglass in this episode of the 99% Invisible podcast.)
The video above profiles audio engineer Carroll Pratt who worked for Douglass in the 1953 doing "laugh sweetening." According to TV Crew Guy, "by the 1980s Pratt's company was providing prerecorded laughter for about 80% of all sitcoms on TV in the U.S."