The Seattle Longitudinal Study (SLS) begun in 1956, is considered to be one of the most extensive psychological research stu​dies of how adults develop and change through adulthood. The SLS is unique as a cohort-sequential longitudinal study, examining cognitive and psychosocial change in multiple birth cohorts over​ the same chronological age span. SLS participants represent a stratified random sample of enrollees in the Group Health Cooperative, a large HMO in Washington state. Over 6,000 adults from age 22 to 100+ years have participated in the study. 

Dr. K. Warner Schaie is the founding Director of the Seattle Longitudinal Study (SLS) and Dr. Sherry Willis joined the study in 1984 and became study co-director in 2005.