This is an argument for immigration restriction that specifically focuses on the diversity of immigrants and their visible (racial, ethnic, cultural, and linguistic) differences with the native population. It is argued that when a society gets more heterogeneous, then, overall, there is a decline of social capital, in particular of trust, between the members. People turn more inward. Further, it is argued that this decline occurs even within individual communities, not just across communities.

This restrictionist argument is based on the research of Robert Putnam, author of Bowling Alone. Below are some links.

• E Pluribus Unum: Diversity and Community in the 21st Century (PDF), a five-year study of the effect of immigration on social capital. Putnam found negative effects of immigration on social capital due to an increase in diversity. He was more optimistic about the long run impact.
• Interviw of Robert Putnam on immigration and social capital (text).
• Bowling With Our Own, an article by John Leo for City Journal discussing the implications of Putnam’s findings.
• Robert Putnam: Diversity Is Our Destruction by Steve Sailer for VDARE.

Relevant research:

• Barbara Arneil’s Diverse Communities argues that Putnam’s measured “decline” may simply be a “change” that is not entirely bad. Reviews suggest she also questions directly the view that social capital is an unadulterated good.
• Abdolmohammad Kazemipur’s Social Capital and Diversity attempts to reproduce Putnam’s research in Canada, and is unsuccessful. In the area of social trust, he actually finds that more diverse Canadian communities have more social capital.
• Irene Bloemraad reviews Kazemipur in the Canadian Journal of Sociology. This review gives an overview of Kazemipur’s findings and notes some caveats.
• Jong-Sung You’s Corruption and Inequality as Correlates of Social Trust finds that after controlling for a few variables, diversity has no discernible relationship with social capital:
  

  Based on a multi-level analysis using the World Values Surveys data that cover 80 countries, I find that (1) freedom from corruption, income equality, and mature democracy are positively associated with trust, while ethnic diversity loses significance once these factors are accounted for; (2) corruption and inequality have an adverse impact on norms and perceptions of trustworthiness; (3) the negative effect of inequality on trust is due to the skewness of income rather than its simple heterogeneity; and (4) the negative effect of minority status is greater in more unequal and undemocratic countries, consistent with the fairness explanation.

• The British Home Office’s Migration Advisory Committee issued a study in 2012, The Impacts of Migration on Social Cohesion and Integration. (Their findings on social cohesion are on pages 47 through 52 of the report.) They studied social outcomes at the local authority (roughly equivalent to municipality) level, and found that migrant settlement and diversity are negatively but weakly correlated with social cohesion. They also found that economic deprivation is a much stronger predictor of social cohesion, and that models which control for all 3 variables (migrant settlement, diversity, and deprivation) find statistically insigificant relationships for all 3, suggesting that the effect, if any, of migration on social cohesion is minimal.

Some responses by open borders advocates:

• Robert Putnam, social capital, and immigration by Nathan Smith on the Open Borders blog, December 19, 2012.