... nodifference in skill gain between those who happened to cross the cutoff between not speaking and speaking the language and those who did not cross the cutoff. In that case, the estimated return using the ... differencing the data would lead to lower estimates of the return to speaking the language. In essence, the people who crossed the critical boundary between speaking and not speaking the language ... as investment in other forms of education. Of course, we do not mean to suggest that the choice is either to invest in language skills or in otherforms of education. In fact, one of the key findings...