How well do you know your neighbours? By sight? By name? Well enough to know the names of their children? In multicultural Britain we should be asking this question more often, for the answer matters a great deal.
All the main political parties except Ukip promote multiculturalism as a social model (or at least they did before the recent European elections). As Malory Nye pointed out in his Huffington Post blog on Thursday, it is already a reality on the ground: ethnic diversity is part of Britain's modern identity.
But what Nye and most politicians do not acknowledge is that stable societies require more than co-existence. Cooperation, particularly during times of economic or social insecurity, requires a level of solidarity that simply living side by side will never provide.
This is basic psychology: we are groupish animals who tend to favour our own. If diverse groups fail to integrate, the divisions between them can too easily be exploited by all sides. Multiculturalism without engagement is not just undesirable, it is also dangerous.
The idea that economic and social disquiet can exaggerate our groupish leanings and increase prejudice towards those who are different has been demonstrated many times by social psychologists and historians. In one classic study, a team at Yale University found that in the American Deep South between 1882 and 1930, levels of mob violence against black people rose and fell in line with the financial stability of the region's farmers. In a similar vein, support for the death penalty (often taken as a measure of intolerance) is known to be higher in the US in areas where there is greater inequality and where residents feel less safe.
The effect can play out across nations. In the early 1990s in east Germany, widespread unemployment caused by reunification with the West was one of the reasons for a ballooning of xenophobic sentiment, resulting in a dramatic rise in the number of attacks against asylum seekers, refugees and other foreigners. And in the UK, opinion polls show that attitudes towards immigrants were significantly more negative in 2012 and 2013 than they had been ten years previously, a hardening of attitudes likely caused by one of the worst recessions in decades.
It may be a biological inevitability that at times of threat or stress we take refuge in our in-groups and turn against the unfamiliar. Does this mean that multiculturalism is doomed? Far from it. The solution is to widen our circle of familiarity, to know those out-groups like we know our own. To learn the names of our neighbours' children (better still, to send our own children to the same schools).
How much social integration is necessary to immunize diverse communities against inter-group conflict and mistrust? One answer comes from the political scientist Ashutosh Varshney, who has studied why unrest between different religious and ethnic groups in India, Sri Lanka, Indonesia and Malaysia is more prevalent in some places than in others.
Varshney has found that a crucial condition for ethnic harmony is intercommunal engagement at a deep level. It is not enough for two groups merely to know each other as neighbours. They should be mixing in business associations, sports clubs, trade unions, political parties, community organizations, student unions and so on.
This kind of grassroots integration acts as a constraint on the polarizing strategies of leaders who are often all too keen to exploit differences for political gain. It gives communities an incentive to prevent sparks from becoming fires. In stable, mixed societies, engagement is built into the civic structure.
When we talk about the rights of communities in multicultural Britain to celebrate their differences, let us not forget the lesson from science, and from history: if you fail to account for innate group proclivities, and to mitigate them, when times get tough there will be very little to celebrate.