Drawing on the work of British anthropologist Robin Dunbar, he argues that "for most of history, humans lived in groups of about 150 people" – a figure known as "Dunbar's number".However, many of us now live in much large communities.
Such groups were big enough for some specialisation, but small enough for everyone to know and trust everyone else. People were born, mated, hunted and died within their small community.
But while hunkering down in the face of difference might have been a useful evolutionary strategy in the past, the growth of cities changed the equation, Leigh argues.Gittins and Leigh argue that the some populist politicians stoke our natural distrust of outsiders for political gain.
Cities are bound together by not by familial relationships, but by rules and norms of acceptable behaviour.
For hundreds of years, the most productive cities have been those that welcome visitors. In a primitive tribe, a dislike of difference can keep you alive. In a city, it's likely to just make you poorer.
"In this sense, a distrust of diversity is a bit like wisdom teeth – an evolutionary vestige that once helped us grind up plants, but now are more likely to take us on a trip to the dentist's chair."
In a seminal study of the politics of hatred, the Harvard authority on urban economics Edward Glaeser noted that the key to building a powerful coalition around hate is to focus voters' anger on an "out group" that is sufficiently large to be taken seriously as a threat, but too small to be electorally decisive.They argue that one of the four main drivers of the growth of populism is inequality.
First, slow growth in living standards when the proceeds of economic growth haven't been shared.
"In societies where prosperity is broadly shared, a cosmopolitan outlook steadily replaces traditional values of religion, deference to authority, and an exclusive focus on the security of our family and tribe," he says.