A slight shift in communication can make a completely different impression. A woman can raise the pitch and draw out the syllables, and suddenly the man talking to her feels he is smarter, stronger, and more protective. We don't know how long women have used a false voice to give the people around them a curated presentation of their personalities, but we have evidence going back as far as sound recording, and particularly movies. An example is given from a Saturday Night Live skit a few weeks ago that spoofed the TV show The Bachelor.
Most of the cast members depicting the contestants adopted a certain speaking style: monotonous, with elongated ending syllables and a lot of vocal fry, in line with the voice associated with “ditzy” girls today. But host Jessica Chastain’s interpretation was slightly different: her voice had a higher pitch and a little more musicality—more AMC than ABC. Though it sounded old-fashioned, it was clearly recognizable as part of a library of voices women have pulled from over the years to play silly, sappy, or simpering women.
A version of this voice has existed since sound met film and, in a way, since a little before that. Actresses of early film played mostly damsels in distress or wide-eyed young women, and by the time talkies took over, women were still portrayed as less headstrong, more head-in-the-clouds. “The 1920s had a serious case of the cutes,” notes Max Alvarez, a New York-based film historian. “There is a prevalence of childlike women in the popular culture [at the time] … Girlish figures, girlish fashion, girlish behavior.” Along with these girlish figures came a girlish voice—high-pitched, a bit breathy, and a little bit unsure, evident in Clara Bow’s pouty purr, and even Betty Boop’s singsong.
While the type of vocalizations that signal "sexy but dumb" have changed a bit over time, it is always recognized by the audience. Read about the 'ditz' voice and how it evolved in film at Atlas Obscura.