In a recent study, Glaeser and colleague Alberto Alesina demonstrated that roughly half the difference in social welfare spending between the US and Europe—Europe spends far more—can be attributed to the greater ethnic diversity of the US population. Glaeser says lower national social welfare spending in the US is a “macro” version of the decreased civic engagement Putnam found in more diverse communities within the country.
Economists Matthew Kahn of UCLA and Dora Costa of MIT reviewed 15 recent studies in a 2003 paper, all of which linked diversity with lower levels of social capital. Greater ethnic diversity was linked, for example, to lower school funding, census response rates, and trust in others. Kahn and Costa's own research documented higher desertion rates in the Civil War among Union Army soldiers serving in companies whose soldiers varied more by age, occupation, and birthplace.