Voters 'like low-pitched voices'
Voters prefer politicians with lower-pitched voices, according to Canadian researchers.
Archive recordings of US presidents were manipulated to create lower and higher-pitched versions of each voice and listeners were asked to rate them.
The Evolution and Human Behavior study found people consistently chose the lower-pitched versions.
One UK expert said a lower voice was linked with authority, and many politicians underwent voice training.
Experts have previously found that the lower-voiced candidates won every US presidential election between 1960 and 2000, while other studies had concluded that both men and women find lower-pitched voices more attractive.
The researchers carried out the study because, in evolutionary terms. low vocal pitch is linked to dominance and leadership.
Former Prime Minister Margaret Thatcher had vocal coaching to lower the pitch of her voice.'More dominant'
The researchers from McMaster University, in Ontario, played the altered recordings to 125 people and asked them to rate their perceptions of the speakers' attractiveness, leadership potential, honesty, intelligence and dominance.
They also asked subjects which version of the voice they would prefer to vote for, both in peacetime and wartime.
Though the motivations were different, in all cases they preferred candidates with lower-pitched voices.
While voice-pitch is not the only influence on voters, the researchers say their study shows it is clearly part of the decision-making process.
Cara Tigue, who worked on the paper, said: "Throughout our evolutionary history, it would have been important for our ancestors to pay attention to cues to good leadership, because group leaders affected a person's ability to survive and reproduce within a group.
"We're looking at it in a present-day, 21st-Century context."
McMaster psychology professor David Feinberg added: "People think we want to vote for men with lower-pitched voices because they're more attractive, but it's because people perceive them as better leaders and more dominant, not just because they're attractive."
UK psychologist Sue Lovegrove said: "Lower voices do tend to have an aura of authority.
"When people go too high-pitched, it sounds emotional and less trust-worthy."
She added: "Many politicians do have voice-coaching. It's not just about pitch, it's about things like where to place pauses and - if you really want to make a point, making sure your voice goes down at the end of the sentence."
But she said Margaret Thatcher went too far: "She lost credibility because it sounded patronising."