The recent empirical literature on the impact of migrant clustering on socio-economic welfare indicators shows inconclusive and often even contradictory results. In this paper we argue that there is not an unambiguous empirical outcome of migrant or ethnic diversity, but that it depends on the level of migrant or ethnic composition itself. A low degree of socio-economic and cultural diversity may be beneficial for neighbourhoods, whereas an excessive degree of diversity may be harmful. We test this hypothesis by (i) constructing a migrant clustering index for all neighbourhoods in the Netherlands based on a gamma index; and, subsequently, (ii) incorporating it in a regression framework to assess three relevant socio-economic outcomes: neighbourhood income, number of students, and average housing value. We show that there is apparently an optimal level of migrant clustering, and that it is remarkably robust. For the Netherlands as a whole and for the ten largest Dutch cities as well, it is striking that largely similar effects were found. Our results suggest that population composition in neighbourhoods may vary up to about 40 per cent from the national average before migrant clustering generates negative effects. © 2010 Elsevier Ltd.