Why is the movement of population in the Americas almost wholly one-way? I pondered the question this week as I traveled from Florida to Guatemala and back. No Mexican politician has ever had to suggest building a wall to keep the gringos out. Around 160,000 Mexicans settle legally in the United States each year (it is, by definition, impossible to count the illegal ones), yet hardly any Americans make the reverse journey. Why?

On one level, the answer is obvious. Incomes in the U.S. are around seven times higher than in Mexico, and around 10 times higher than in most Central American countries. Americans take for granted things that are luxuries for many of their southern neighbors: clean water, steady electricity, decent schools, quality healthcare, noncorrupt judges, regular elections.