over several decades of factor-analytic study, cattell and his colleagues gradually refined and validated their list of underlying source traits. the search resulted in the sixteen unitary traits of the 16pf questionnaire. these traits have remained the same over the last 50 years of research. in addition, the 16pf questionnaire traits are part of a multi-variate personality model that provides a broader framework including developmental, environmental, and hereditary patterns of the traits and how they change across the life span (cattell, 1973, 1979, 1980).