Polish adaptation and validation of the Imperial Psychedelic Predictor Scale (IPPS) and the four-item Mystical Experience Questionnaire (MEQ-4)

1.Introduction

Psychedelic substances such as psilocybin, LSD, and DMT have been investigated for their therapeutic potential in mental health treatment since the mid-20th century. Promising early findings notwithstanding, research momentum diminished in the 1970s due to political restrictions, concerns over recreational use, and methodological limitations (Nichols 2021). In recent decades, however, scientific interest has resurged, with numerous clinical trials examining psychedelics for conditions such as depression, anxiety, obsessive-compulsive disorder, eating disorders, and substance use disorders (Carhart-Harris 2018; Davis 2021; Goodwin 2025).

The mechanisms by which psychedelics improve mental health remain only partially understood. Some researchers emphasize neurochemical pathways, noting that psychedelics enhance structural and functional neuroplasticity, primarily via glutamate modulation in cortical regions (Jiménez 2023), a process that reopens critical periods—windows of heightened sensitivity to environmental stimuli typically limited to early development (Nardou 2023).

Alternatively, subjective experience has been identified as key to the therapeutic potential of psychedelics (Yaden 2021). Mystical-type experiences, ego dissolution, and expanded insight are proposed to underlie these effects, with stronger subjective experiences correlating with better clinical outcomes in psychedelic-assisted psychotherapy (Griffiths 2016; Kangaslampi 2023). Neuroimaging studies support this view, revealing distinct patterns of brain connectivity during psychedelic states (Singleton 2021; Luppi 2023). These experiences typically involve perceptual changes, mood shifts, and altered self-awareness, with phenomena such as unity, transcendence, and noetic insight associated with improved psychological well-being (Barrett 2017).

To systematically assess these subjective phenomena, several psychometric tools have been developed. Among the most prominent is the Mystical Experience Questionnaire (MEQ), initially introduced as a 43-item scale (Pahnke 1966) and later refined to the MEQ30 (Barrett 2015). The MEQ30 captures four core dimensions of mystical experience—mystical experience, positive mood, transcendence of time and space, and ineffability—and predicts therapeutic outcomes of psychedelic-assisted therapy, with higher scores correlating with greater symptom improvement in conditions such as depression and substance use disorders (Bogenschutz 2015; Palhano-Fontes 2015). It has been translated and validated in several languages, including Finnish, Portuguese, Japanese, French, and Dutch (Kangaslampi 2023; Yonezawa 2024), facilitating cross-cultural research. Recently, a shortened four-item version, the MEQ-4, was developed to provide a brief yet effective assessment suitable for both clinical and research contexts (Strickland 2024). Given cultural and linguistic variation in how mystical experiences and psychological states are perceived and expressed, rigorous translation and cultural adaptation are essential to ensure the validity and reliability of psychometric tools across populations (Beaton 2000).

While the MEQ and its variants assess the intensity and quality of mystical experiences following psychedelic use, there is growing interest in predicting how individuals might respond to psychedelics before administration. Initial research on predictive factors focused on constructs such as "surrender" and "preoccupation," finding that surrender is associated with ego dissolution and mystical experiences, while preoccupation correlates with adverse reactions (Russ 2019). Subsequent studies examined psychological preparedness, showing that higher readiness scores are linked to improved emotional outcomes post-psychedelic use (McAlpine 2024). Building on this line of inquiry, Haijen et al. (2018) developed a "pre-state" measure through a comprehensive prospective survey, revealing that individuals' psychological states prior to psychedelic use were positively correlated with mystical-type experiences and improvements in well-being, and negatively correlated with challenging experiences.

The Imperial Psychedelic Predictor Scale (IPPS) was developed to prospectively evaluate factors that influence the nature and intensity of psychedelic experiences (Angyus 2024). Refining and formalizing Haijen et al.'s (2018) "pre-state" scale, the IPPS draws on extensive datasets, including large-scale surveys and controlled trials, to ensure robust predictive validity (Kettner 2021; Carhart-Harris 2021). This concise, 9-item tool, designed by researchers at Imperial College London's Centre for Psychedelic Research, focuses on three primary domains: set (the individual's internal state), rapport (the quality of the relationship between the individual and the facilitator), and intention (the individual's purpose for using the psychedelic). Validation studies have shown that the IPPS reliably predicts aspects of the psychedelic experience, including mystical-type experiences, challenging experiences, and emotional breakthroughs (Angyus 2024).

This study aims to translate, culturally adapt, and validate the Polish versions of the MEQ-4—a concise, four-item variant of the MEQ30—and the IPPS. By providing reliable and culturally appropriate instruments for assessing both the occurrence of mystical experiences, the factors predicting individual responses to psychedelics, and preparedness for altered states of consciousness, this research seeks to contribute to both national and international efforts to better understand the therapeutic potential of psychedelics while promoting culturally sensitive approaches in psychedelic research.

2.Materials and methods

Participants

The study sample comprised 325 unique participants, aged 18 to 60, who reported a deep, meaningful encounter with a classic psychedelic substance within the past six months. Each participant contributed one self-reported experience, yielding 325 total experiences. Of the sample, 185 identified as male (57%), 131 as female (40%), and 6 as non-binary (2%); mean age was 31 years (SD = 8.9). Nearly half of participants held higher education, while 40% had completed secondary education.

Among the 325 meaningful experiences, psilocybin accounted for 137 (42%), LSD for 88 (27%), DMT for 49 (15%), ayahuasca for 42 (13%), and mescaline for 9 (3%). When participants were asked about any psychedelic use within the past six months regardless of whether the experience was meaningful, 222 (68%) reported psilocybin use, 159 (49%) LSD, 103 (32%) DMT, 52 (16%) ayahuasca, and 16 (5%) mescaline.

To control for confounding variables, participants completed the Alcohol Use Disorder Identification Test (AUDIT) (Klimkiewicz 2021), a WHO-developed screening tool with strong reliability (α = 0.90); those indicating alcohol misuse were excluded from further analyses. Participants also reported on meditation engagement: 211 (65%) had some meditation experience, of whom 41 (13%) practiced daily, 71 (22%) several times weekly, 62 (19%) weekly, and 37 (11%) monthly. Eighty-six participants (26%) practiced a single meditation type and 119 (37%) practiced more than one; the most common techniques were mindfulness with specific objects (143, 44%), prayer (67, 21%), somatic meditation (52, 16%), and vipassana (47, 14%). Individuals with a history of epilepsy, psychiatric disorders, or current pharmacotherapy were excluded to minimize health risks and ensure data reliability.

Datasets

Because rapport items in the IPPS depend on whether the experience occurred with a sitter and/or in a group, participants were redirected to the corresponding items based on their responses. Nearly half of the sample (147 observations) involved experiences with both a group and a sitter present; these constitute the full rapport dataset (N = 147). The remaining experiences may or may not have included either factor and, together with the full rapport observations, form the all dataset (N = 325). All analyses were conducted separately for these two datasets.

Translation and cultural adaptation

The IPPS and the MEQ-4 were translated and culturally adapted following established guidelines for cross-cultural adaptation of psychological instruments (Beaton 2000). Two independent bilingual experts first translated the original English versions of both scales into Polish; discrepancies between their translations were resolved through discussion to produce a consensus version for each scale.

To verify conceptual and linguistic equivalence, a back-translation was then performed by two different bilingual translators who had not participated in the initial translation and were unfamiliar with the original scales. This step confirmed that the adapted items preserved the original meanings without introducing cultural distortion or unintended shifts in interpretation. For the IPPS, the back-translated version was submitted to the original authors, who confirmed that the Polish adaptation accurately reflected the intended constructs. Table A1 presents the original items alongside their final Polish adaptations for both the IPPS and the MEQ-4.

Psychological measures

The Mystical Experience Questionnaire (MEQ-4) assessed the intensity of mystical-type experiences using four items on a 6-point scale (0 = "none; not at all" to 5 = "extreme"), each corresponding to one of four subscales: unity/sacredness, positive mood, transcendence of time and space, and ineffability. Cronbach's alpha for the MEQ-4 itself has not been reported, but it correlates highly with the parent 30-item MEQ (r = 0.89, p < 0.001), which shows excellent internal consistency (alpha = 0.93) (Strickland 2024) (see Appendix B for full description).

The Imperial Psychedelic Predictor Scale (IPPS) is a 9-item measure of psychological preparedness spanning three domains (set, rapport, intention), rated on a 6-point scale from 0 ("not at all") to 5 ("extremely"). It showed strong internal consistency (overall alpha = 0.83; set = 0.81, rapport = 0.76) and convergent validity with the Surrender Scale (r = 0.455, p < .0001) (Angyus 2024) (see Appendix B for full description).

Mindfulness was assessed with the KZU, the Polish adaptation of the MINDSENS questionnaire, covering Observing, Non-reactivity, and Decentering. Mystical orientation beyond the psychedelic context was measured with the Polish adaptation of Hood's Mysticism Scale (HMS), covering Interpretation, Introvertive Mysticism, and Extrovertive Mysticism. Spiritual beliefs and experiences were assessed with the Scale of Spiritual Transcendence (STD), covering Core Transcendence and Spiritual Awareness. Religious involvement was measured with the Religious Commitment Questionnaire (KZR), a Polish adaptation of the Religious Commitment Inventory-10. Full descriptions of all measures are provided in Appendix B.

Statistical analysis

Exploratory factor analysis (EFA) and confirmatory factor analysis (CFA) were conducted on the Polish IPPS to examine its factor structure across the all and full rapport datasets. Factor extraction and oblique rotations were performed using the factor-analyzer Python package; CFA models were tested in SPSS, comparing the three-factor solution against an alternative higher-order factor model. Internal consistency was assessed via Cronbach's alpha, and temporal stability via test–retest reliability. Convergent and discriminant validity were assessed through correlations (with the Bonferroni correction) with external psychological measures. Associations between factor scores and relevant variables were examined using ordinary least squares (OLS) regression.

Ethical considerations

This study was conducted in accordance with the Declaration of Helsinki. [Ethics / IRB status — DO POTWIERDZENIA] Participation was voluntary and fully anonymous, and all participants provided written informed consent online before beginning the survey. As the study was a non-interventional, anonymous survey, no clinical-trial registration was required.

3.Results

3.1. IPPS-PL

Factor structure analysis

Sampling adequacy was confirmed by a high Kaiser-Meyer-Olkin value (KMO = 0.866) and a significant Bartlett's test of sphericity.

Exploratory factor analysis (EFA), conducted in Python (factor-analyzer) using oblique rotations (promax, oblimin, quartimin, geomin_obl) with minres and ml solutions, revealed a three-factor solution diverging from the original IPPS structure. Rather than distinct "set" and "intention" factors, items merged into "Readiness & Openness" (set1, set2, set3, intention; psychological preparedness, openness, and intentionality) and "Emotional Comfort" (set4, set5, set6, originally part of "set"; calmness and reduced anxiety before the experience). "Rapport" remained consistent with the original scale, capturing trust and connection with the sitter and/or group. Communalities and loadings were generally adequate, though "intention" showed low communalities (0.19, 0.22) across datasets, possibly reflecting cultural nuances in how the construct is understood in Polish contexts.

Confirmatory factor analysis

Confirmatory factor analysis (CFA) was subsequently conducted to validate the EFA-derived structure. CFA models were tested using the SPSS statistical package, comparing the three-factor solution with an alternative second-order factor model. The results supported the hierarchical structure, with "Readiness & Openness," "Emotional Comfort," and "Rapport" loading onto a higher-order latent factor representing overall psychological preparedness. Despite the second-order model's slightly higher constraints, the differences in model fit were not statistically significant, indicating the appropriateness of interpreting the scale both as multidimensional and as reflecting a single overarching construct. High correlations between the three first-order factors further supported the existence of this general factor. The conducted CFA analyses proved that new dimensions better fitted the factor structure for the data than the original dimensions (higher CFI, GFI, AGFI, NFI, ATLI, lower RMSEA and logLik).

Reliability

Cronbach's alpha demonstrated reasonable internal consistency across the IPPS-PL subscales. In the full dataset (N = 325), Readiness & Openness and Emotional Comfort yielded alphas of 0.774 and 0.771, respectively; in the subset of experiences undertaken with both a group and a sitter (full rapport dataset, N = 147), alphas were 0.79, 0.711, and 0.838 for the three subscales. Removing the "intention" item would marginally improve internal consistency in the full dataset but decrease it in the full rapport dataset, further justifying its retention.

(a) All dataset  ·  (b) Full rapport dataset

Convergent and discriminant validity

Convergent and discriminant validity were assessed through Bonferroni-corrected correlations with external psychological measures, including the MEQ-4, HMS, STD, and KZU; all reported correlations were significant. Readiness & Openness showed the strongest correlation with the MEQ-4 total score (r = 0.62 for the all dataset; r = 0.59 for the full rapport dataset), underscoring the significance of psychological preparedness in predicting mystical-type experiences. Emotional Comfort and Rapport also showed moderate to strong associations with MEQ-4 and other scales, reinforcing their relevance in capturing experiential predictors. The correlation between the IPPS-PL total score and KZU (r = 0.71) highlighted the connection between mindfulness-related traits and readiness for psychedelic experiences.

Regression analyses further explored the predictive utility of the IPPS-PL. Ordinary least squares (OLS) linear regressions indicated that the three-factor model explained substantial variance in MEQ-4 scores. In the all dataset, the combination of Readiness & Openness and Emotional Comfort accounted for 39% of the variance (p < 0.001 for RO; p = 0.014 for EC). The full rapport dataset revealed similar patterns, with the three dimensions collectively explaining 36% of MEQ-4 variance. Readiness & Openness consistently emerged as the strongest predictor (as measured by standardized betas), reinforcing its centrality in shaping the subjective quality of psychedelic experiences, while Emotional Comfort and Rapport contributed complementary, smaller effects reflecting the role of emotional states and social dynamics.

3.2. MEQ-4-PL

Factor structure analysis

The Kaiser-Meyer-Olkin measure indicated the data were suitable for factor analysis (KMO = 0.75), and Bartlett's test of sphericity was significant.

EFA using the minimum residual (minres) solution with promax rotation yielded a four-factor solution corresponding to the original subscales—transcendence, ineffability, positive mood, and mysticism—with eigenvalues of 2.26, 0.79, 0.52, and 0.44; this structure emerged only under these conditions, as other optimization and rotation methods predominantly indicated a single factor. Communalities ranged from 0.20 to 0.42 (transcendence = 0.418, ineffability = 0.397, positive mood = 0.199, mysticism = 0.30); positive mood's communality fell at the threshold of acceptability, suggesting it does not fully conform to the proposed structure and may reflect variance not shared with the other items.

Item loadings confirmed the four subscales' distinctiveness: MEQ_transcendence loaded on Factor 1 (0.640), MEQ_ineffability on Factor 2 (0.626), MEQ_mysticism on Factor 3 (0.511), and MEQ_positive_mood on Factor 4 (0.445). Under oblimin and quartimin rotations, however, transcendence and mysticism converged into a single factor, yielding a three-factor structure and indicating these subdimensions share considerable common variance.

Neither the four-factor nor the three-factor structure could be confirmed via CFA, as the number of parameters exceeded the available information (i.e., the covariances, including the variances, of the observed variables).

Reliability and validity

Cronbach's alpha was not computed given the scale's one-item-per-dimension structure; however, test-retest reliability was good, with a correlation coefficient of r = 0.91.

All correlations reported below were significant after Bonferroni correction for the all dataset across all questionnaire scores and dimensions (19). Among the MEQ-4 dimensions, mysticism showed the strongest correlation with the total score (0.80), followed by ineffability and transcendence (0.77, 0.76) and positive mood (0.61). Intercorrelations were moderate for transcendence-mysticism (0.56), ineffability-mysticism (0.51), and transcendence-ineffability (0.48); positive mood correlated weakly with the others—ineffability (0.27), transcendence (0.31), mysticism (0.35)—underscoring its distinct role.

Convergent validity, assessed against the HMS, STD, and KZU with Bonferroni correction (19 variables), was supported: the MEQ-4 total score correlated most strongly with the HMS total score (0.79) and its subdimensions (0.73–0.77), especially HMS mysticism (0.65), followed by transcendence (0.58) and ineffability (0.55); positive mood correlated weakest (0.38). HMS dimensions correlated moderately to strongly with MEQ-4 items other than positive mood (0.48–0.63), whose correlations remained weak (approximately 0.30–0.40). The MEQ-4 total score also correlated strongly with the STD (0.70) and KZU (0.69) totals, supporting the questionnaire's construct validity as a measure of the degree and nature of mystical experience.

MEQ scores by psychedelic type

Table 6 presents mean MEQ-4 scores by substance associated with the reported meaningful experience; scores differed among LSD, psilocybin, and DMT, but not among DMT, ayahuasca, and mescaline (LSD < Psilocybin < DMT = Ayahuasca = Mescaline). Figure 4 depicts this pattern across substances; detailed models are reported in the Supplemental Material.

Beyond the core validation reported above, a series of exploratory analyses examined regression models of MEQ-4 scores as a function of substance, meditation frequency, and prior psychedelic experience, as well as the moderating role of religiosity. Across these analyses, Readiness & Openness consistently emerged as the strongest predictor of mystical experience. The full models and accompanying text are reported in the Supplemental Material.

4.Discussion

The present study aimed to validate Polish versions of the Imperial Psychedelic Predictor Scale (IPPS-PL) and the Mystical Experience Questionnaire (MEQ-4-PL), refining their factor structures to ensure applicability for psychedelic research in Polish-speaking populations. The results confirm strong psychometric properties for both instruments while revealing modifications aligned with cultural and linguistic nuances. The most significant finding was the restructuring of the IPPS, in which the original "Set" and "Intention" dimensions were replaced by a revised two-factor solution, "Readiness & Openness" and "Emotional Comfort," each supported by strong reliability; in the Polish context, psychological preparedness for a psychedelic experience appears better understood as a combination of openness to experience and emotional security than as separate set-and-intention constructs. The MEQ-4-PL, by contrast, retained its four-factor structure and remained a reliable, efficient tool for capturing mystical-type experiences. Readiness & Openness emerged as the central predictor of mystical experiences, showing a stronger predictive effect than Emotional Comfort, while the significant role of rapport in moderating spiritual openness points to relational mechanisms underlying such experiences. These findings underscore the value of distinguishing cognitive from emotional readiness, suggesting that preparatory interventions should address both aspects separately and that future work should test how specific techniques (e.g., meditation, psychological priming, expectation setting) strengthen these dimensions.

The restructuring of the IPPS reflects both empirical findings and culturally specific interpretations of the psychological states that precede psychedelic use, refinements that also enhanced the scale's psychometric properties. Polish-speaking participants appeared to conceptualize psychological preparedness differently from English-speaking populations, where "intention" is typically treated as a separate construct; because no prior cross-cultural adaptations of the IPPS exist, direct comparisons with other languages are not yet possible. The term "intencja" in Polish carries a more formal or metaphysical connotation than its English counterpart, which likely explains why the intention item merged with Readiness & Openness rather than remaining a distinct factor. The prominent role of rapport may similarly reflect a Polish conceptualization of spirituality that integrates secular and religious perspectives and emphasizes relational, communal meaning-making over the more solitary, self-transcendent framing common in individualistic cultures (see Appendix C for the full discussion of cultural context).

The revised scales carry important applications for researchers, clinicians, and harm-reduction practitioners. The IPPS-PL can be administered before psychedelic-assisted therapy or research studies to identify individuals needing additional psychological preparation, with Readiness & Openness and Emotional Comfort scores used to tailor preparatory interventions, personalize pre-session discussions, and train facilitators and therapists to assess and strengthen client readiness. In harm-reduction contexts, it can be incorporated into educational programs so that individuals using psychedelics in non-clinical settings can evaluate their own readiness and emotional state beforehand, reducing the likelihood of challenging experiences. The MEQ-4-PL, given its brevity and strong psychometric properties, is well suited to clinical protocols and post-session assessments, allowing clinicians to evaluate the intensity and therapeutic relevance of mystical experiences and track their depth over time; it may also prove useful in cross-cultural comparisons and public health surveys assessing the prevalence of such experiences within Polish-speaking populations.

Limitations and future directions

Several limitations should be acknowledged. The study relied on self-reported, retrospective accounts of psychedelic experiences, which may introduce recall bias; future studies should consider prospective designs or experience-sampling methods to improve data accuracy. Although the sample size was adequate for factor analyses, larger and more diverse samples would improve generalizability. The sample also consisted primarily of individuals already familiar with psychedelics, limiting applicability to first-time users or clinical populations undergoing psychedelic-assisted therapy, since experienced users may show greater psychological readiness and a different interpretive framework for mystical experiences; future research should include more diverse participants, particularly those preparing for a first psychedelic experience or undergoing structured therapeutic interventions. Future work should also examine additional moderating variables, such as personality traits and therapeutic context, and investigate the mechanisms through which interpersonal relationships, including therapeutic alliances and group dynamics in guided sessions, shape the psychedelic experience, given the significant role of rapport in facilitating spiritual openness. Longitudinal designs, assessment of predictive validity in therapeutic settings, and comparative studies with other language versions would further clarify cultural influences on psychedelic experiences.

Overall, the validation of the IPPS-PL and MEQ-4-PL provides essential tools for assessing psychedelic experiences in Polish-speaking populations, bridging a gap in cross-cultural psychedelic research. By demonstrating strong psychometric properties and refining factor structures to align with cultural context, this study contributes to both the theoretical understanding of set and setting and the practical assessment of psychedelic preparedness, and it underscores Readiness & Openness as a central, culturally grounded predictor of mystical experience.

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References

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Appendix A. Scale items (English and Polish)

Appendix B. Full descriptions of the psychological measures

MEQ-4

Six-point response scale (0 = "none; not at all" to 5 = "extreme"), four items mapping to the four core subscales. Developed as a brief version of the MEQ-30; correlates r = 0.89 with the MEQ-30 (excellent concurrent validity); subscale correlations r = 0.71–0.84. The MEQ-30 shows Cronbach's alpha 0.93 (MacLean et al. 2012), subscales 0.83–0.94. Higher scores are linked to greater symptom reduction after psychedelic use.

IPPS

Nine-item tool over three domains (set, rapport, intention), 6-point Likert. Three-factor structure with Cronbach's alpha 0.81 (set) and 0.76 (rapport), overall 0.83; convergent validity with the Surrender Scale (r = 0.455). In controlled settings, set and rapport together explained 40% of variance in mystical experiences, set alone 16% of challenging experiences, and rapport 9% of emotional breakthroughs.

KZU (MINDSENS)

Polish adaptation (Radoń 2020) of the MINDSENS questionnaire; three subdimensions — Observing, Non-reactivity, Decentering; 19 items, 5-point scale. Composite reliability 0.74–0.93; suitable across ages 15–72.

HMS (Hood's Mysticism Scale)

Polish adaptation (Radoń 2014); 12-item Mystical Experience version; three subdimensions — Interpretation, Introvertive Mysticism, Extrovertive Mysticism; 5-point scale. Cronbach's alpha 0.76 / 0.72 / 0.56.

STD (Spiritual Transcendence)

Polish adaptation (Piotrowski 2019) of Piedmont's instrument; Core Transcendence and Spiritual Awareness; 22 items, 4-point scale; alpha 0.87 / 0.78.

KZR (Religious Commitment)

Polish adaptation (Polak 2017) of the RCI-10; single dimension (Intrapersonal Religious Commitment); 5 items, 5-point scale; alpha 0.82–0.95.

Appendix C. Refinement of factor structure and cultural context

The refinement of the factor structure from the original IPPS—replacing "Set" and "Intention" with "Readiness & Openness" and "Emotional Comfort"—reflects both empirical findings and culturally specific interpretations of psychological states preceding psychedelic use. These adjustments not only enhanced the scale's psychometric properties but also ensured greater resonance with the lived experiences of Polish-speaking individuals. Participants' feedback underscored the importance of emotional readiness and relational factors (captured by the Rapport dimension), particularly when experiences occurred in group settings with facilitators.

Intention

The unique nature and the poor communalities of the intention item can be justified by the fact that the term "intention" has slightly different connotations in Polish and English. In English, "intention" broadly refers to planning, purpose, or intent, encompassing both will and motivation as well as what a person aims to achieve. In Polish, "intencja" is more closely associated with inner motivation or purpose, but is also used in specific contexts such as philosophy, psychology, and law, and can carry a more formal or metaphysical connotation.

IPPS-PL adaptation summary

In conclusion, IPPS-PL demonstrates robust psychometric properties and cultural appropriateness, making it a valuable tool to assess predictors of psychedelic experiences in Polish-speaking populations. The adapted factor structure provides nuanced insights into the psychological and interpersonal factors that shape these experiences, offering both theoretical contributions to the understanding of set and setting and practical applications in psychedelic-assisted therapy and research contexts.