We analyze dynamic incentives in pension systems created by the use of a small set of final years of earnings to compute benefits. Using social security records and household surveys from Uruguay, we show that self-employed workers and some employees of small firms respond to these incentives by increasing reported earnings in the benefit calculation window. We find evidence that suggests that these responses are explained by changes in earnings reporting and not in total earnings or labor supply. Back-of-the-envelope calculations indicate that this behavior increases the cost of pensions by about 0.2% of the GDP.