In a study using 500 identical twins, two Princeton University economists found that for every additional year of schooling, the average person earns 9 to 16 percent more money. "The results of our study indicate that the economic returns to schooling may have been badly underestimated in the past," write Orley Ashenfelter and Alan Krueger.

Conventional estimates have placed the return on schooling at about 8 percent higher earnings per year completed. The only other study of identical twins, in 1980, put the difference as low as 3 percent.

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