An online survey investigating sensory processing sensitivity, transliminality, and boundary-thinness as predictors of anomalous experience and belief

Introduction:
Sensory Processing Sensitivity (SPS) is a temperament trait identified by deeper processing of information, being easily overwhelmed by stimulation, greater empathy and emotional reactivity, and being able to sense subtleties in the environment (Greven et al., 2019). Aron and Aron (1997) devised a questionnaire called the Highly Sensitive Person Scale to measure high sensitivity and the Highly Sensitive Person (HSP) is used to describe an individual who experiences SPS. Preliminary research suggests that HSPs may be more likely to experience anomalous experiences (AEs), and that further research is warranted to establish if there is a relationship between AEs and SPS (Irwin et al., 2014; Williams et al., 2021). Sensitivity is also part of the transliminality personality construct (Thalbourne & Delin, 1993). Highly transliminal people are susceptible to the occurrence of large amounts of imagery, thought, and emotion and tend to pay more attention to their inner processes, which resonates with the characteristics of SPS. Another personality concept associated with sensitivity is Hartmann’s (1991) ‘boundary thinness’, which is characterised by openness and ease of entering an altered state of consciousness, and significantly thinner boundaries have been identified in persons reporting AEs (Rabeyron & Watt, 2010; Simmonds-Moore, 2009). We might expect HSPs to have thinner boundaries and be highly transliminal given their differential sensitivity to external and internal stimuli. Further, studies have found an association between transliminality, schizotypy, and paranormal belief (Dagnall et al., 2010) and that transliminality and transpersonal self-expansiveness predicts paranormal belief (Rock et al., 2021), but have not yet explored transliminality or boundary thinness alongside SPS in relation to anomalous experiences, beliefs, and ability. As such, this study consisted of an online mixed methods survey including standardised measures of SPS, boundary-thinness, and transliminality as independent variables and a measure of anomalous experiences, beliefs, and ability as the dependent variable to predict whether sensitivity/personality variables contribute significantly to reporting of anomalous experiences, beliefs, and ability. We also conducted correlations between the sub-scales of the HSP-12 (Ease of Excitation, Low Sensory Threshold, and Aesthetic Sensitivity) and each of the AEI scales. In addition, three mediation analyses were carried out, with HSP as the predictor variable, RTS as the mediator and anomalous experience, belief and ability for each moderation. Open-ended responses gathered qualitative data from participants about their personal anomalous experiences and beliefs, and their experience of SPS if they identified as HSP.

Methods:
This was an exploratory study pre-registered with the Koestler Parapsychology Unit (ref#1064) and ethical approval was received from the Ethics Committee at Canterbury Christ Church University and the University of Derby. The survey included a demographic section, a measure of sensitivity (Highly Sensitive Person Scale - Brief Version (HSP-12; Pluess et al., 2020), a measure of anomalous experiences, beliefs, and abilities (Anomalous Experiences Inventory; Gallagher et al., 1994), a measure of transliminality (Revised Transliminality Scale; Lange et al., 2000), a measure of boundary thinness (Boundary Questionnaire Short-Form; Kunzendorf et al., 1997), and open-ended questions on sensitivity and AEs. Information about the survey was distributed to staff and students at both universities, posted on social media and Sensitivityresearch.com, and shared on the Society for Psychical Research website. Data collection continued until 200 participants had completed the survey. There were 151 females, 41 males, 6 non-binary and 2 preferred not to say, with an age range of 18 to 80 years (M = 32.23 years, SD = 15.28 years).

Results:
The internal consistencies (Cronbach's alpha) of the AEI subscales were anomalous beliefs, a=0.81; anomalous experiences, a=0.82; anomalous abilities, a=0.84; HSP-12: sensory processing sensitivity, a=0.86; BQ-18: boundary thinness, a=0.77; RTS: transliminality scale, a=0.83. There were significant correlations between all variables apart from the EOE subscale of the HSP-12 and anomalous abilities. Linear multiple regression utilised a forced entry approach to examine whether scores on the three sensitivity measures (i.e., HSP-12, BQ-18, and RTS) significantly predicted scores on the AEI sub-scales of belief, experiences and ability. The model explained 28.3% of the variance and was a significant predictor of AEI-Belief. However, the analysis showed that only RTS (t(198) = 5.51, p < 0.001) and BQ-18 (t(198) = 2.53, p < 0.05) were the significant predictors, with no contribution made from HSP-12 (t(198) = -0.65, p = 0.52).The model explained 43.8% of the variance and was a significant predictor of AEI-Experiences. However, analysis showed that RTS (t(198) = 9.74, p < 0.001) was the only significant predictor and that both BQ (t(198) = 1.28, p = 0.20) and HSP (t(198) = -1.56, p = 0.12) did not make any significant contribution to the model. The model explained 33.2% of the variance and was a significant predictor of AEI-Abilities. However, analysis showed that RTS (t(191) = 7.45, p < 0.001) was the only significant predictor and that both BQ (t(191) = 0.63, p = 0.55) and HSP (t(191) = -0.03, p = 0.97) did not make any significant contribution to the model. In addition, three mediation analyses were carried out, with HSP as the predictor variable, RTS as the mediator, and anomalous experience, belief and ability for each moderation. All three analyses show that transliminality significantly mediates between HSP and anomalous experience, belief and ability. In response to the open-ended question ‘Do you think being HSP makes you more likely to have anomalous experiences? If so, please state why that might be’, participants mentioned the following: “Because we pick up on things other people might miss”, “Can detect subtle changes in environment”, “Heightened state of awareness”, “I think if you are open to things, you are more likely to experience them”, “Senses are always active”, “I think I am more aware of my surroundings when I get around other people, and I think it is almost chemical, as if I smell a change in mood, danger, or someone”.

Discussion:
Correlation analysis showed a clear linear relationship between each of the three personality predictor variables and the subscales of the AEI apart from anomalous abilities and the EoE subscale of the HSP-12. EoE is characterised by a tendency to feel overwhelmed by internal and external stimuli, which may indicate that this dimension needs to be low (i.e. reducing internal and external stimuli) for people to report anomalous abilities. This aligns with the ‘noise reduction’ approach, which proposes that psi information is subtle and likely to remain nonconscious unless overwhelming sensory inputs are reduced (Honorton, 1977/1986). The finding that SPS correlates with AEs is consistent with previous research (Irwin et al., 2014; Williams et al., 2021). Both transliminality and boundary thinness positively predicted anomalous beliefs with transliminality being the stronger, which is a similar finding to Rock et al., (2021) and Dagnall et al., (2022). However, only transliminality predicted anomalous experiences and anomalous abilities. Therefore, it is proposed that transliminality plays a mediating role between SPS and anomalous experiences, belief and abilities, and is a possible explanation for the relationship between SPS and anomalous experience, belief and ability.