Lajoie and Shapiro reviewed forty definitions of transpersonal psychology that had appeared in academic literature over the period from 1968 to 1991. They found that five key themes in particular featured prominently in these definitions: "states of consciousness"; "higher or ultimate potential"; "beyond the ego or personal self"; "transcendence"; and "the spiritual". Based upon this study the authors proposed the following definition of transpersonal psychology: "Transpersonal Psychology is concerned with the study of humanity's highest potential, and with the recognition, understanding, and realization of unitive, spiritual, and transcendent states of consciousness".