A 2018 study in the "American Sociological Review" found that within racial groups, most immigrants to the United States had fully assimilated within a span of 20 years. Immigrants arriving in the United States after 1994 assimilate more rapidly than immigrants who arrived in previous periods. Measuring assimilation can be difficult due to "ethnic attrition", which refers to when ancestors of migrants cease to self-identify with the nationality or ethnicity of their ancestors. This means that successful cases of assimilation will be underestimated. Research shows that ethnic attrition is sizable in Hispanic and Asian immigrant groups in the United States. By taking account of ethnic attrition, the assimilation rate of Hispanics in the United States improves significantly. A 2016 paper challenges the view that cultural differences are necessarily an obstacle to long-run economic performance of migrants. It finds that "first generation migrants seem to be less likely to success the more culturally distant they are, but this effect vanishes as time spent in the US increases."