66 Transpersonal Psychology Review, Volume 22, No. 2 (2020) Development and validation of a plateau experience psychometric to investigate the effect of shinrin-yoku on depression Scott Buckler & April Woodward A proposed direction for transpersonal psychology is the development of empirical research within the field, specifically in assessing the effects of transpersonal practices. The areas of self-actualisation, peak experience and flow have been subjected to research predominantly using psychometrics, however a range of theoretical and methodological issues within these concepts are evident. An alternate concept for future research was advocated by Maslow through his concept of self-transcendence, characterised by the plateau experience, yet in the past forty years, this concept has remained relatively obscure. This paper advocates further research into the plateau experience, through initially defining the concept, progressing to discuss the development of a plateau experience psychometric (PLEX), and using the PLEX within a validation study investigating the effects of shinrin-yoku (forest air breathing/bathing). Keywords: Plateau experience; PLEX; self-actualization; Maslow; shinrin-yoku. Introduction F OR transpersonal psychology to be deemed a science and in turn gain greater credibility as a field, there is a continued need to develop empirical research (Daniels, 2005; Friedman, 2002; Friedman & Harte- lius, 2007; MacDonald et al., 1994; Sutich, 1976). Furthermore, several authors have advocated the use of psychometrics as an essential element for the scientific approach to psychology (Edwards, 2003; Friedman & MacDonald, 1997; Kline, 2000; MacDonald et al., 1999; MacDonald et al., 1994). Within transpersonal psychology, the use of psychometrics has been deemed problematic, predominantly due to the limitations in development and validation of such measures (MacDonald et al., 1994). A further criticism relates to capturing the response of a participant while engaged with a transpersonal practice (Friedman, 2002). Daniels (2005) thus suggested that it is the effects on the person after such experiences which should be considered. An example of this is assessing the effect on a person who has experienced a near death experi- ence (NDE) illustrated through the work of Bonenfant (2004). This paper summarises research into one construct proposed by Maslow, that of the plateau experience. Specifically, this paper analyses why the plateau experience should be a focus for further research, providing a psychometric and validation study from which other research may evolve. Literature review Research into transpersonal practices has become increasingly diversified, although a tradition of research initially related to self-actualisation and the resultant peak experience. Other constructs such as the flow state have similarly received increased attention. The literature review describes research in this area while similarly high- lighting limitations, progressing to outline a viable alternative with a limited research tradition, the plateau experience. Self-actualisation and the peak experience One transpersonal domain that has received DOI:10.53841/bpstran.2020.22.2.66