However, social mobility in the US is lower than in some European Union countries if it is defined by income movements. American men born into the lowest income quintile are much more likely to stay there than similar people in the Nordic countries or the United Kingdom. Many economists, such as Harvard economist N. Gregory Mankiw, however, state that the discrepancy has little to do with class rigidity; rather, it is a reflection of income disparity: "Moving up and down a short ladder is a lot easier than moving up and down a tall one."