THE alleged involvement of two ethnic Chechen brothers in the deadly attack at the Boston Marathon last week should prompt Americans to reflect on whether we do an adequate job assimilating immigrants who arrive in the United States as children or teenagers. ..When asked “what do you like most about being here?” an 11-year-old Haitian boy in Cambridge told us, “There is less killing here.” His response was notably succinct, but not unique. ..Having just one friend who spoke English fluently was a strong predictor of positive academic outcomes. Yet more than a third of the students in our study...