I collated these from the following video (time 3:53 - 6:03), and checked the sources for myself, picking out key quotes. I hope the following is informative for many of you, and at least provides ammunition in debate against people who sing the praises of diversity.
https://www.ncbi.nlm.nih.gov/pubmed/24913947 : Ethnic diversity, trust, and the mediating role of positive and negative interethnic contact: a priming experiment.
"Ethnic diversity is associated with lower trust for both natives and immigrants."
"For both, the cognitive salience of ethno-cultural diversity causally reduces trust."
https://onlinelibrary.wiley.com/doi/abs/10.1111/juaf.12015 : Ethnic diversity and its impact on community social cohesion and neighborly exchange
- "Our findings indicate that social cohesion and neighborly exchange are attenuated in ethnically diverse suburbs"
https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1465-7287.2010.00215.x : ETHNIC DIVERSITY AND TRUST
- "I find a negative relationship between ethnic polarization and trust and a U‐shaped relationship between ethnic fractionalization and trust."
http://journals.sagepub.com/doi/abs/10.1177/0190272515612403 : Effects of Heterogeneity and Homophily on Cooperation
- "The results show that heterogeneity hampers between-group cooperation at the dyadic level. In addition, endogenous sorting mitigates this negative effect of heterogeneity on cooperation."
https://academic.oup.com/esr/article-abstract/21/4/311/556895 : Predicting Cross-National Levels of Social Trust: Global Pattern or Nordic Exceptionalism?
- "Cause and effect relations are impossible to specify exactly but ethnic homogeneity and Protestant traditions seem to have a direct impact on trust, and an indirect one through their consequences for good government, wealth and income equality."
https://www.researchgate.net/publication/300799700_The_Geography_of_Ethnic_Violence : The Geography of Ethnic Violence
- "Our analysis supports the hypothesis that violence between groups can be inhibited by both physical and political boundaries."
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0095660 : Good Fences: The Importance of Setting Boundaries for Peaceful Coexistence
- "Our analysis shows that peace does not depend on integrated coexistence, but rather on well defined topographical and political boundaries separating groups, allowing for partial autonomy within a single country."
https://academic.oup.com/esr/article/32/1/54/2404332 : Does Ethnic Diversity Have a Negative Effect on Attitudes towards the Community? A Longitudinal Analysis of the Causal Claims within the Ethnic Diversity and Social Cohesion Debate
- "Studies demonstrate a negative association between community ethnic diversity and indicators of social cohesion (especially attitudes towards neighbours and the community), suggesting diversity causes a decline in social cohesion."
https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1467-9477.2012.00289.x : Trust in a Time of Increasing Diversity: On the Relationship between Ethnic Heterogeneity and Social Trust in Denmark from 1979 until Today
- "The results suggest that social trust is negatively affected by ethnic diversity."
https://academic.oup.com/sf/article-abstract/93/3/1211/2332107?redirectedFrom=fulltext : Ethnic Diversity, Economic and Cultural Contexts, and Social Trust: Cross-Sectional and Longitudinal Evidence from European Regions, 2002–2010
- "The results show that across European regions, different aspects of immigration-related diversity are negatively related to social trust. In longitudinal perspective, an increase in immigration is related to a decrease in social trust."
https://www.npr.org/templates/story/story.php?storyId=12802663&t=1540581035853 : Political Scientist: Does Diversity Really Work?
"A study conducted by Harvard political scientist Robert Putnam suggests that diversity hurts civic life and that differences can actually translate into distrust. The political scientist and author explains his findings on the flip-side of cultural diversity."
"The more diverse the group around us, ethnically, in our neighborhood, the less we trust anybody, including people who look like us. Whites trust whites less. Blacks trust blacks less, in more diverse settings."
http://archive.is/xFWbh : The downside of diversity
"Harvard political scientist Robert Putnam -- famous for "Bowling Alone," his 2000 book on declining civic engagement -- has found that the greater the diversity in a community, the fewer people vote and the less they volunteer, the less they give to charity and work on community projects. In the most diverse communities, neighbors trust one another about half as much as they do in the most homogenous settings. The study, the largest ever on civic engagement in America, found that virtually all measures of civic health are lower in more diverse settings."
"The extent of the effect is shocking," says Scott Page, a University of Michigan political scientist."