In the old days, 'huddled masses' were willing to give up their homeland traditions in exchange for freedom and opportunity. Today's immigrants have different expectations.
This is what caused the rumpus recently when the town council of Hamtramck, in Michigan, decided to allow a mosque to broadcast live calls to prayer in Arabic over loudspeakers five times a day. The population of 23,000 was transformed in the 1990s by immigrants from China, Bosnia, Bangladesh and Somalia. Now, 41 per cent of inhabitants were born overseas, and one-third speak less than perfect English.
Meanwhile in Europe, there was an outburst of anti-immigrant sentiment following reports that refugees regularly apply to a number of countries for asylum and weigh up the respective levels of freedom and material advantages offered in each.
In Hong Kong, immigration policies are tough, and ethnic self-segregation is the rule among both transient and embedded communities. Some young Hongkongers are even reluctant to learn English - the city's acknowledged social glue. But the fact remains that everyone comes from somewhere else, or their ancestors did.
The thing is, the developed world is moving from relative homogeneity towards increasing diversity - in terms of ethnicity, values, customs, languages and lifestyles. Naturally, tensions arise and people are torn between building bridges and fences.
Alliances changed more slowly in the past. People, for the most part, 'knew their place'. But at today's fast pace, forcing together in-groups and out-groups, as psychologists call them, can lead to individuals not having a clear idea of where they belong, and therefore where their loyalties should lie. However, although a society's complex web of in-group-out-group relationships can be severely disrupted by an inflow of strangers, it cannot be eliminated: people are hard-wired to favour their own. In-groups get people organised and foster security. But they also necessarily exclude out-group members.