IN BAD economic times the temptation to bash immigration is overwhelming. “Get the stench out of Greece,” runs a slogan of Golden Dawn, an increasingly popular anti-immigrant party there. David Cameron has pledged to more than halve annual net migration into Britain by 2015. In America Republicans are wondering how much anti-immigration rhetoric contributed to Mitt Romney’s defeat in the presidential election. A change of political tune is badly needed. Evidence suggests that increased flows of people across borders could ignite global growth.