This study examined internal and external validity of the Thinking Styles Inventory (TSI) developed by Sternberg and Wagner (1991) within the framework of Sternberg's (1988) theory of mental self‐government. Participants were 96 adolescent students who were in a summer residential program at the time of testing. The results provide evidence of the external discriminant validity, but only lend partial support to the internal validity of the instrument, suggesting the need to theoretically clarify the relationships between and among posited thinking styles as well as empirically test the validity of these thinking styles measures. Implications of thinking styles for gifted education are discussed.