In highly diverse Los Angeles or San Francisco, for example, roughly 30 percent of the inhabitants say that they trust their neighbours 'a lot', whereas in the ethnically homogeneous communities of North and South Dakota, 78-80 percent of the inhabitants say the same. In more diverse communities, people trust their neighbours less.
Professor Putnam summarised his research as follows:
Diversity does not produce ‘bad race relations’ or ethnically-defined group hostility, our findings suggest. Rather, inhabitants of diverse communities tend to withdraw from collective life, to distrust their neighbours, regardless of the colour of their skin, to withdraw even from close friends, to expect the worst from their community and its leaders, to volunteer less, give less to charity and work on community projects less often, to register to vote less, to agitate for social reform more, but have less faith that they can actually make a difference, and to huddle unhappily in front of the television.
This effect was seen for both conservatives and liberals.
Now another major study, undertaken by two Michigan State University researchers (Zachary Neal and Jennifer Watling Neal), has come to similar conclusions:
Their simulations of more than 20 million virtual “neighborhoods” demonstrate a troubling paradox: that community and diversity may be fundamentally incompatible goals. As the authors explain, integration “provides opportunities for intergroup contact that are necessary to promote respect for diversity, but may prevent the formation of dense interpersonal networks that are necessary to promote sense of community.”
...After 20 million-plus simulations, the authors found that the same basic answer kept coming back: The more diverse or integrated a neighborhood is, the less socially cohesive it becomes, while the more homogenous or segregated it is, the more socially cohesive.
It seems that you can have ethnic diversity or you can have a close sense of community - you just can't have both together.
So why then are liberals so wedded to diversity as a moral aim? The reasons no doubt intersect. First, liberals believe that the goal of politics is to maximise individual autonomy, meaning a freedom to be self-constituted or self-defined or self-directed. Therefore, a predetermined quality like our ethnicity is thought of negatively as something that constrains us and therefore has to be made not to matter. Liberals therefore don't want to discriminate on the basis of ethny or race, even if it is for an important purpose such as maintaining community.
A related reason is that liberals think of the act of individual choice as being the key expression of morality, rather than what is actually chosen. There is no moral "outside" for liberals, only the act of choosing and allowing others to do likewise. Therefore, liberal morality is based more on qualities that demonstrate a willingness not to interfere with the choices of others (except for those who fall outside the liberal schema), such as non-discrimination, tolerance, openness and support for diversity.
It's also the case that liberals tend to want to manage society in a "technocratic" way, either through the markets or state regulation, and this is more readily achievable when people are stripped of "opaque" loyalties, such as those to family or ethny, that provide direction and authority outside of the technocratic systems. In a liberal society, people tend to become interchangeable units of the markets or bureaucracies, and obviously those with power in these systems feel comfortable with this outcome.
It's also true that diversity can be used as a weapon against whoever is the existing majority ethnic group. It can be used as such either by disloyal members of the majority group or else by members of minority groups.
So what can be done? One important achievement would be to undermine the dominance of liberalism as a political philosophy in the West, as this is a significant source of the idea that diversity is a moral aim. Another achievement would be to undermine the dominance of the corporate and bureaucratic elites. This can be done by making people aware of the bias of these elites (a process already partly completed) and by building up alternative sources of media, education and culture.
Finally, the research of Jonathan Haidt shows that liberals do care about the "harm" principle of morality. So it might also have some effect to show that individuals are harmed (by a loss of community) when diversity is forced upon communities.