while some americans can trace their ancestry back to a single ethnic group or population in europe, africa, or asia, these are often first- and second-generation americans. generally, the degree of mixed heritage increases the longer one's ancestors have lived in the united states (see melting pot). in theory, there are several means available to discover the ancestry of the people living in the united states, including genealogy, genetics, oral and written history, and analysis of federal population census schedules. in practice, only few of these have been used for a larger part of the population.