For decades, migration economics has stressed the effects of migration restrictions on income
distribution in the host country. Recently the literature has taken a new direction by estimating the
costs of migration restrictions to global economic efficiency. In contrast, a new strand of research
posits that migration restrictions could be not only desirably redistributive, but in fact globally
efficient. This is the new economic case for migration restrictions. The case rests on the possibility
that without tight restrictions on migration, migrants from poor countries could transmit low
productivity (“A” or Total Factor Productivity) to rich countries—offsetting efficiency gains from the
spatial reallocation of labor from low to high-productivity places. We provide a novel assessment,
proposing a simple model of dynamically efficient migration under productivity transmission and
calibrating it with new macro and micro data. In this model, the case for efficiency-enhancing
migration barriers rests on three parameters: transmission, the degree to which origin-country total
factor productivity is embodied in migrants; assimilation, the degree to which migrants’ productivity
determinants become like natives’ over time in the host country; and congestion, the degree to
which transmission and assimilation change at higher migrant stocks. On current evidence about
the magnitudes of these parameters, dynamically efficient policy would not imply open borders but
would imply relaxations on current restrictions. That is, the new efficiency case for some migration
restrictions is empirically a case against the stringency of current restrictions.