By Sean Coughlan
BBC News education reporter
Different parts of the brain are activated by male and female voices
A university research team says it has discovered why most people "hearing voices" in hallucinations say they hear male voices.
Dr Michael Hunter's research at the University of Sheffield says that male voices are less complex to produce than female.
As such, when the brain spontaneously produces its own "voices", a male voice is more likely to have been generated.
Among both men and women, 71% of such "false" voices are male.
"Psychiatrists believe that these auditory hallucinations are caused when the brain spontaneously activates, creating a false perception of a voice," says Professor Hunter of the university's psychiatry department.
"The reason these voices are usually male could be explained by the fact that the female voice is so much more complex that the brain would find it much harder to create a false female voice accurately than a false male voice," he says.
Such imaginary voices are typically likely to be middle-aged and carry "derogatory" messages.
The research, published in NeuroImage, shows how the brain interprets information from human voices - and how female and male voices activate different parts of the brain.
"The female voice is more complex than the male voice, due to differences in the size and shape of the vocal cords and larynx between women and men, and also due to women having greater natural 'melody' in their voices.
"This causes a more complex range of sound frequencies than in a male voice," says Professor Hunter.
These gender differences in voices trigger responses in different parts of brain - and as the male version is simpler, both men and women who hear voices, are on average more likely to produce a male-sounding voice.
The research says that "auditory verbal hallucinations" are a symptom of schizophrenia and "occur in 40% to 60% of patients who suffer from the condition".