In the 1970s the field developed through the writings of such authors as Robert Frager, Alyce and Elmer Green, Daniel Goleman, Stanley Krippner, Charles Tart, Roger Walsh, John Welwood, and Ken Wilber. Wilber emerged as a leading figure and major theoretician of the field. Another important contributor to the field, Michael Washburn, was drawing on the insights of Jungian depth psychology. According to Smith, Wilber and Washburn presented the major guiding theories of transpersonal development. The 1980s were also characterized by the work of Stanislav and Christina Grof, and their concept of "spiritual emergence" and "spiritual emergencies".