The evidence that diversity and solidarity are negatively correlated (controlling for many potentially confounding variables) comes from many different settings:
• Across workgroups in the United States, as well as in Europe, internal heterogeneity (in terms of age, professional background, ethnicity, tenure and other factors) is generally associated with lower group cohesion, lower satisfaction and higher turnover (Jackson et al. 1991; Cohen & Bailey 1997; Keller 2001; Webber & Donahue 2001).
• Across countries, greater ethnic heterogeneity seems to be associated with lower social trust (Newton & Delhey 2005; Anderson & Paskeviciute 2006; but see also Hooghe et al. 2006).
• Across local areas in the United States, Australia, Sweden, Canada and Britain, greater ethnic diversity is associated with lower social trust and, at least in some cases, lower investment in public goods (Poterba 1997; Alesina et al. 1999; Alesina & La Ferrara 2000, 2002; Costa & Kahn 2003b; Vigdor 2004; Glaeser & Alesina 2004; Leigh 2006; Jordahl & Gustavsson 2006; Soroka et al. 2007; Pennant 2005; but see also Letki forthcoming).
• Among Peruvian micro-credit cooperatives, ethnic heterogeneity is associated with higher default rates; across Kenyan school districts ethno-linguistic diversity is associated with less voluntary fundraising; and in Himalayan Pakistan, clan, religious, and political diversity are linked with failure of collective infrastructure maintenance (Karlan 2002; Miguel & Gugerty 2005; Khwaja 2006).
• Across American census tracts, greater ethnic heterogeneity is associated with lower rates of car-pooling, a social practice that embodies trust and reciprocity (Charles & Kline 2002).
• Within experimental game settings such as prisoners-dilemma or ultimatum games, players who are more different from one another (regardless of whether or not they actually know one another) are more likely to defect (or ‘cheat’). Such results have been reported in many countries, from Uganda to the United States (Glaeser et al. 2000; Fershtman & Gneezy 2001; Eckel & Grossman 2001; Willinger et al. 2003; Bouckaert & Dhaene 2004; Johansson-Stenman et al. 2005; Gil-White 2004; Habyarimana et al. 2006).
• Within the Union (northern) Army in the American Civil War, the casualty rate was very high and the risks of punishment for desertion were very low, so the only powerful force inhibiting the rational response of desertion was loyalty to one’s fellow soldiers, virtually all of whom were other white males. Across companies in the Union Army, the greater the internal heterogeneity (in terms of age, hometown, occupation, etc.), the higher the desertion rate (Costa & Kahn 2003a).