A 1969 recording of Hillary Clinton — then Hillary Rodham and freshly possessed of a bachelor’s degree in political science from Wellesley — went viral shortly after the Democratic presidential candidate’s speech on Thursday night at the Democratic National Convention. Part of the reason for the video’s resonance is that Hillary Rodham delivered an impassioned, impressive address. Part of the reason is that she sounded crazy different than Hillary Clinton.

The internet freaks out over stuff like this.

Young Hillary is earnest, her voice higher and more lilting. Rodham’s is eloquent in an East Coast, liberal arts college way, choosing her words carefully, even through the controversial impromptu section that kicked off her speech. There are notes of hesitancy, where she falters a bit and peppers her speech with um’s, and you can almost hear the nervousness tinging young Hillary’s first big break with public speaking.

Hillary the Clinton and candidate sounds much more controlled, confident. Her voice is also unquestionably deeper. In voice science circles, Hillary’s gotten a more “chest” voice, says Ingo Titze, director of the National Center for Voice and Speech and a vocologist, or a voice scientist, at the University of Iowa.

Titze says that voices change as we age, tending towards a higher octave for men and a lower register for women. That’s because our vocal cords have a layered structure. The outermost layer is skin; air passes through that layer as we breathe. Next, the air slides into a loose and pliable gel-like layer, then is absorbed through ligament, finally smacking into muscle. “Unlike a violin string, we have layers of material,” Titze said — so your voice is constantly adjusted based on the amount of air that passes through and the gel part’s moisture levels. It explains why your voice is squeaky when you first wake up, normal during the day, and bedraggled by the time you topple into bed.

As we age, that gel layer gets thinner and thinner, and the cords aren’t able to vibrate as cleanly as before. Titze uses the example of a flag: “When it waves in the wind, it buckles in and out and keeps changing its shape,” Titze explains, comparing the flag to how your voice vibrates in the prime of your youth. But on a still day, even the slightest breeze doesn’t really move a flag — comparable to what happens to your voice as you age.

Another way Clinton’s voice has probably changed physiologically is her muscle bulk. Yes, even your vocal cords have muscle, and as time wears on, those fibers get thinner, the tissue gets smaller, and you get characteristic grandparent voice, thanks to a process known as atrophy.

That explains the depth of Clinton’s voice. But what about the fact that she’s gotten a lot hoarser and, as Titze puts it, has somehow adopted a “chest” voice? That’s due to a combination of public speaking — and, potentially, a good old dash of sexism.

The public speaking aspect first: Titze co-conducted a study where researchers looked at the voice quality of frequent speakers, looking at teachers who spoke five days a week for several hours a day. Over time, their voice quality crumbled and became hoarser, their vocal cords “fatigued.” (When your favorite pop star cancels a concert because their vocal cords are fried, be nice to them.) The fact that Clinton has been speaking not just on the campaign trail for endless hours for the majority of her career — which spans decades — means her voice has naturally probably gotten ripped apart to the point where she can never recover young Hillary’s earnestness ever again.

Titze thinks that society’s got a big role to play with Clinton’s 2016 voice, however. Vocologists call the lower register that’s becoming increasingly common among women “corporate,” which relies on speaking from the caverns of your chest and lends itself to a naturally deeper voice. Women have classically had “flutier” voices — higher voices are tinnier because they don’t rely on the chest as much — but the vocologist community has noticed a gradual decline in the “fluty” quality of female voices. In other words, Clinton probably learned that the male-dominant world of politics would take her more seriously if she sounded more like a man, leading to Clinton’s voice du jour. Ah, patriarchy.

Photos via Getty Images / Justin Sullivan

Tanya Basu is the Science editor at Inverse. Her writing focuses on the social sciences and behavior. Now based in Brooklyn, she will always call Chicago home and never be too full for one more taco.