Given equal amounts of positive and negative information about a candidate, the overall impression of voters is skewed towards the negative, says a review of the data available in this month's Psychiatric Bulletin.
Dr Nicholas Beecroft, registrar at the Maudsley Hospital, London, who conducted the review, said that people have a generally positive view of others with the result that negative information is seen as more salient.
"If one expects the average person to be basically decent, honest and polite then if someone is presented as dishonest this would give a more marked impression," he said.
In one study a single negative sentence was enough to sway voters against a candidate, but different shortcomings were given different weight. People were presented with neutral information about two candidates which differed only in what the candidate was accused of.
"This single sentence was enough to sway the voters against the candidate. Adultery was less negative than corruption. [And] it did not matter whether the accusation came from a partisan or independent source," he added.
Dr Beecroft said that he personally thought the Tories' "demon eyes" campaign had been the most effective image in the run-up to this election: "It was very clever. It reminded people of last time when it looked like Labour were going to win and then at the last minute long standing fears made people change. Excessive fear is counterproductive.
"Although most people see negative campaigning for what it is and find it unethical, they still find it more informative."
But he warned that no politician could hope to win an election just by smearing their opponents. Emotions also played a larger part in how people voted than their beliefs about them. "If you look at past elections - Thatcher, Reagan - there are great emotional factors involved, the fact that someone was a great leader or they had a great story to tell. It is a phenomenon that is very difficult to pin down."Reuse content