IN THE 1930S AND ’40S, Hollywood had a way of tracking the popularity of its movie stars. Studios would sift through the quarter of a million or so fan letters that arrived each month and sort them into separate bags by actor name. Then studio employees would heave these bulging bags onto a scale, according to industry researcher Leo Handel. A big spike in weight meant the star was trending up. A sharp decline suggested the star was on the way to becoming yesterday’s news.

As measurements go, this was pretty crude. Even back then, the people who would take the time to write a letter represented a tiny subset of the population, usually teenagers motivated by an excess of adoration (or antipathy). So, in time, movie executives would follow the lead of their counterparts in radio, television, and advertising and adopt the techniques of opinion research to understand what their audiences wanted.