US Presidential candidate Rick Santorum has dropped out of the race for the Republican nomination, leaving the field open for his rival Mitt Romney, a man who has divided the Republican party.

Jonathan Haidt, professor of social psychology at the University of Virginia and author of the book The Righteous Mind: Why Good People Are Divided by Politics and Religion, told the Today programme's Evan Davis that it starts with differences in brain during childhood.

Some brains see opportunities and some see threats more easily, he explained, adding that it is a natural progression then that the group which likes to stay close to home and not try foreign food tend to vote Conservative.

Mr Haidt said that it is a basic design process in that the body has one system pushing one way and another pushing the other - the sympathetic and the parasympathetic.

He continued that politics follows in the same way in that Liberalism and Conservative are "well designed opposing forces to have a healthy society".

"We need both sides," he said, adding that "... we apply our reasoning to justify what we already believe... you need your opponents to do that... to improve your own thinking," he said.

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