Charisma is the lifeblood of politicians.

Nothing is more valuable, yet harder to quantify, than the ability of a candidate or elected official to draw others — a magnetism that often relies as much on how a message is delivered as what is actually being said.

For the past seven years, Rosario Signorello has been trying to measure just that quality. A phonetician and voice scientist at the Laboratoire de Phonétique et Phonologie in Paris, he has immersed himself in the acoustical patterns of political leaders. His goal: to figure out what in certain leaders' voices makes them particularly successful.

Signorello's latest findings may be particularly worrisome for any politician starting to sense a little tickle or hoarseness in their throat. He and fellow voice scientist Didier Demolin, of the Université Sorbonne Nouvelle, found that the more youthful a politician's voice sounds, the more charismatic he or she will seem to the public. The pair are presenting their findings Wednesday at a conference of the Acoustical Society of America.

“We rely on the voice to tell us so much. From just a few words you can tell if someone is from New York City or Los Angeles, their gender, their personality, ethnicity,” Signorello said in an interview. But age can be deceptive, and how old a person sounds does not always match with reality.

Donald Trump, for instance, is relatively close in age to Bernie Sanders. But compared to Sanders's gravelly acoustics, Trump's uses a much wider range of vocal pitch, which Signorello said makes him sound significantly younger. As with other politicians, such acoustical differences can influence how he is perceived.

To test this theory, Signorello and fellow researchers curated samples from two politicians whose voices had been drastically affected by disease or other factors to make them sound hoarse and older: Luiz Inácio Lula da Silva, former president of Brazil, whose larynx was damaged by throat cancer and chemotherapy treatments, and Italian party leader Umberto Bossi, whose vocal cords became partially paralyzed after a stroke. Both men's voices had grown quieter and raspier, with a reduced range of pitch — all qualities associated with an older voice.

The researchers played segments of the two leaders' speeches from before, during and after their afflictions and asked test subjects to describe each speaker and tell who they liked more. The audio clips were played to French-speaking participants who could not understand anything Lula and Bossi were saying in Portuguese and Italian, respectively. So the content of the politicians' speech was not a factor.

In the case of the former president, for example, listeners showed a strong preference for the Brazilian's pre-disease, younger-sounding voice, calling it charming, reassuring, creative. They described the older, post-disorder voice as not influential or persuasive, even annoying.

It was not someone they would vote for, participants said.

“In our studies with listeners, nothing changed except for the quality and pitch of the voice,” Signorello said. “It suggests that if you care about your political career, you need to do what you can to maintain your healthy voice.”

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