What impact did childhood environment have for US migrants?

By linking data on Ellis Island arrivals with the 1940 US Census Zach Ward and I estimate the importance of childhood environment for immigrant outcomes. Looking only at male siblings, a brother who arrived in the US at a younger age had higher adult incomes than their brother who arrived at an older age. The high return to human capital acquired during childhood contrasts with a low rate of assimilation during this period. We further show that the quality of childhood environment is a primary determinant of a source country’s skill gap with natives, and may be more important than other factors like selection.