Italy’s Northern League former leader Umberto Bossi had a deep, harsh voice but after a stroke, it became ‘very flat’. Photo: Alessandro Garofalo/Reuters

If you want to scare others into submission, speak with a deep, dynamic voice that varies widely in pitch.

On the other hand, vocal tones that are higher on average with a narrower pitch range are likely to mark you out as sincere and trustworthy.

While leaders throughout history have instinctively known how to manipulate people with their voices, scientists are now learning the secrets of the dark art.

In 2004 Umberto Bossi, former leader of the Northern League Party, suffered a severe stroke that permanently altered his speech.

Whereas before he had been perceived as dominant and authoritarian, suddenly the Italian firebrand came over as strangely benevolent.

The reason was that after the stroke his voice became ‘flat’, varying little in pitch.

Rosario Signorello, from the University of California at Los Angeles, said: “I collected speeches of him before and after the stroke, and I discovered that before the accident, he was perceived as an authoritarian leader, because his voice was characterised by low average offundamental frequency, normal modulation of the pitch contour, a wide pitch range, a lot of perturbation in voice and a lot of creakiness and harshness.

“The stroke caused him to have a very flat pitch contour, so even if he had the harshness, even if he had the creakiness, his pitch contour was very flat.

“I submitted his voice to the listeners and he was perceived as a benevolent and competent leader, which is very different from the authoritarian perception. In that case, the pitch contour played a very important role.”

The hypothesis is that the biological function of charismatic voice is also cross-species

Signorello’s team conducted further research using a technique called ‘delexicalisation’ which strips out words from a speech while retaining acoustic properties.

The scientists found that one of the most important charismatic influences was fundamental frequency, or ‘F0’ − the lowest average rate of vocal cord vibration.

Another was the range of frequency variation in a voice.

Comparisons were made between the way the voices of Italian, French and Portuguese politicians − namely Luigi de Magaistris, François Hollande and Luiz Inacio Lula da Silva − were perceived by listeners.

Signorello concluded that someone who speaks with a low average F0 and wide pitch range is seen as dominant and threatening. Conversely, a higher F0 and narrow pitch range conveys the idea of “sincere and reassuring” leadership.

However, the way different people responded to leaders’ voices was also affected by cultural factors. “The Italians seem to need a low-pitched voice, and the French a high-pitched one, because of cultural reasons,” said Signorello.

“What we want to do is understand how the use of the F0 helps the non-human primate individuals to emerge and be recognised by the group and understand how these individuals use their voice behaviour to create different patterns and convey leadership,” he said.

“The hypothesis is that the biological function of charismatic voice is also cross-species.”