17 evidence showing that immigration has no effect on violent crime and no, or only a small effect, on crime (Adelman 2017; Bell, Fasani, and Machin, 2013; Bianchi, Buonanno, and Pinotti, 2012; Spenkuch, 2013). The instances where immigration contributes to crime tend to be linked to cases where migrants are not able to work. For instance, an increase in the number of asylum seekers who are generally prevented from seeking employment can result in a slight increase in property crime (Bell, Fasini, and Machin, 2013).

Probably issues related to national security and loss of national identity pose the more powerful, but also the more intractable reasons against a liberal migration policy (Collier, 2013).<sup>36</sup> Indeed, the literature indicates that non-economic factors such as racial prejudice and concerns about crime and cultural impacts are correlated with attitudes towards migration.<sup>37</sup> Direct surveys of attitudes in the United States and Europe show that both low- and high-skilled locals have a preference for high- over low-skilled immigrants, which runs contrary to the predictions of the labor competition model. Rich and poor locals are also found to be opposed to low-skilled immigration (Hainmueller and Hiscox, 2010). Mindful of backlashes against free trade, the concerns and voices against labor mobility in the host countries cannot be ignored. The political economy and real reasons behind them must be properly understood and addressed, and winners and losers duly identified, acknowledged and compensated. This is a recurrent theme in the history of migration (UNDP, 2009; ch2.2).

In terms of domestic migration, the types of concerns are similar. Fast urban population growth challenges cities to keep up their capital base. While Africa’s fast urban population growth (4.9 percent during 1960-2010) follows mainly from its rapid urban natural increase (2.9 percent), it is compounded by rural-urban migration.<sup>38</sup> As a result, cities struggle to capture the economies of agglomeration from density (Lall, Henderson and Venables, 2017). High rates of unemployment, strained infrastructure and environmental degradation are widespread instead. It is, however, the natural increase of the urban population, and not rural-urban migration, that is found to be associated with indicators of congestion (Jedwab, Christiaensen and Gindelsky, 2017).<sup>39</sup> How best to manage the speed of urban population expansion is an important policy challenge, with the speed and destination of rural-urban migration (cities versus towns) providing one of the policy entry points. Similarly, signs are emerging that more fertile rural areas in Africa tend to be poorer (Wantchekon and Stanig, 2016), especially when poorly serviced by road infrastructure. With fertile land attracting migrants, overcrowding may result when not accompanied with the necessary investment in infrastructure and market access, an important hypothesis for further investigation.

> 36 The concerns and evidence are similar when it comes to the impact of protracted refugee situations on hosting communities (Verwimp and Maystadt, 2015).

> 37 See Mayda (2006); O’Rourke and Sinnott (2006); Dustmann and Preston (2007); Facchini and Mayda (2012).

> 38 At 2.9 percent, a city’s population doubles every 24 years. At 4.9 percent, it doubles almost every 14 years. Despite a decline in Sub-Saharan Africa’s natural urban increase, at 2.4 percent during 2000-2010, it remains high, as does overall urban growth (4.1 percent during 2000-2010). During this period, urban population growth in Asia stood at 2.3 percent, with urban natural increase only 1.1 percent. 39 The authors find indications that this is linked to the demographic composition of the migrant population, i.e. their larger share of working age people.