SCIENTIFIC Transpersonal psychology provides a heterodox approach to psychological science. Although it is currently marginalized from mainstream psychology, it offers potential for advancing the discipline of psychology, as well as for human betterment. It currently suffers from rampant romanticism, but it also challenges narrow scientism prevalent within mainstream psychology. The construct of self-expansiveness and its related measure, the Self-Expansiveness Level Form (SELF), are presented as examples of a responsible scientific approach within transpersonal psychology. Self-expansiveness refers to the malleable nature of self-concept, and how it can expand in various, including transpersonal, ways. Research supporting the validity of the SELF and its use in a variety of transpersonal studies are discussed, as well as are its implications for developing scientific transpersonal psychology. (PsycINFO Database Record (c) 2018 APA, all rights reserved)