We investigate whether selective intermarriage and endogenous ethnic identification interact to hide some of the intergenerational progress achieved by the Mexican-origin population in the United States. In part, we do this by comparing an "objective" indicator of Mexican descent (based on the countries of birth of the respondent and his parents and grandparents) with the standard "subjective" measure of Mexican self-identification (based on the respondent's answer to the Hispanic origin question). For third-generation Mexican-American youth, we show that ethnic attrition is substantial and could produce significant downward bias in standard measures of attainment which rely on ethnic self-identification.