Deep voice important to voters
Former prime minister Gordon Brown had one vote-winning quality - his voice.
Research has shown that a rich, deep voice is attractive to voters, who associate it with strength.
Mr Brown, who was Labour leader and prime minister from 2007 to 2010, has a famously deep voice.
Andy Burnham and Jeremy Corbyn who are now both running for the Labour leadership do not - their voices are more tenor than baritone.
Scientists in the US who set out to investigate why people are drawn to political candidates with deep voices found it had little to do with age and experience.