In a competitive labor market, immigration affects native wages only through its impact on marginal products. Under the sole assumption of constant returns, we show that a larger supply of migrants (keeping their skill mix constant) must increase the marginal products of native-owned factors on average (an extension of the familiar “immigration surplus” result); and in the long run (if capital is supplied elastically), this surplus passes entirely to native labor. However, in a monopsonistic labor market, wages will also depend on any mark-downs applied by firms; and immigration may affect native wages through these mark-downs. We present a model of monopsony which generates testable restrictions on the null hypothesis of perfect competition, which we reject using US census data commonly studied in the literature. Our estimates suggest that the (negative) mark-down effect dominates the (by construction, positive) effect on marginal products for the average native. These findings shed new light on the interpretation of previous empirical estimates and the so-called “structural approach” to predicting wage effects
