The exact nature of the criterion varies, so that illiteracy must be defined in each case before the term can be used in a meaningful way. In 1930 the U.S. Bureau of the Census defined as illiterate any person over ten years of age who was unable to read and write in any language. By the next census (1940), however, the concept of "functional" illiteracy was adopted, and any person with less than five years of schooling was considered functionally illiterate, or unable to engage in social activities in which literacy is assumed.
Since that time, the concept of functional illiteracy has grown in popularity among American educators, but the standards of definition have changed with the increasing complexity of most social activities. Thus, by 1970, the U.S. Office of Education considered at least six years of schooling (and sometimes as many as eight) to be the minimum criterion for functional literacy. In 1990 over 5% of the adult population living in the United States did not meet that criterion.