This article summarizes results from studies of the organization and development of cognitive abilities in adults aged 20 to 60 years old. Theories of intelligence stipulating a hierarchy of intellectual functions, with fluid and crystallized intelligence at the top, are supported. Six conclusions on age differences are offered. (Author/RD)

Summarizes the conduct and results of the Seattle Longitudinal Study, an investigation that has assessed mental abilities in more than 5,000 adults and has followed some for as long as 35 years. Integrative findings are presented on age changes, cohort differences, factor structure, individual differences, and interventions. (SLD)