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 potential therapeutic benefits in mental health treatment since the mid-20th century. Despite promising early findings, research momentum diminished in the 1970s due to political restrictions, concerns over recreational use, and methodological limitations (Nichols 2021). In recent decades, however, there has been a resurgence of scientific interest, with numerous clinical trials examining the use of psychedelics in treating 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 through glutamate modulation in cortical regions (Jiménez 2023). This process reopens critical periods—windows of heightened sensitivity to environmental stimuli typically limited to early development (Nardou 2023).

Alternatively, subjective experiences have been identified as a key factor influencing 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 perspective, 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 like 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 experiences—mystical experience, positive mood, transcendence of time and space, and ineffability—and has been shown to predict the therapeutic outcomes of psychedelic-assisted therapy, with higher MEQ scores correlating with greater symptom improvement in conditions such as depression and substance use disorders (Bogenschutz 2015; Palhano-Fontes 2015). The MEQ30 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 version, the MEQ-4, was developed to provide a brief yet effective assessment of mystical experiences through four core items, making it suitable for both clinical and research contexts (Strickland 2024). Given the cultural and linguistic variations 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 also a 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," with findings indicating that surrender is associated with ego dissolution and mystical experiences, while preoccupation correlates with adverse reactions (Russ 2019). Subsequent studies expanded on this by examining 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 Prediction 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 from 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 demonstrated that the IPPS can reliably predict various 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.Methods

Participants

The study included individuals aged 18 to 60 who had experienced a deep, meaningful encounter with a classic psychedelic substance within the past six months. The sample consisted of 325 self-reported psychedelic experiences provided by 325 unique participants. Among them, 185 identified as male (57%), 131 as female (40%), and 6 as non-binary (2%), with a mean age of 31 years (SD = 8.9). In terms of education, nearly half of the participants had higher education, while 40% held a secondary education degree.

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

Participants were asked whether they underwent their most recent experience in a group setting and/or with a sitter, and were accordingly redirected to the corresponding rapport items. Nearly half of the sample (147 observations) consisted of experiences that involved both a group and a sitter (referred to as the full rapport dataset), while the remaining experiences may or may not have included either factor (all dataset, N = 325).

To control for confounding variables, participants completed the Alcohol Use Disorder Identification Test (AUDIT) (Klimkiewicz 2021), a reliable screening tool (α = 0.90) developed by the World Health Organization (WHO) to assess excessive alcohol consumption. Participants who indicated alcohol misuse were excluded from further analyses.

The survey also explored participants' engagement with meditation practices. A total of 211 participants (65%) reported having experience with meditation. Among them, 41 participants (13%) practiced daily, 71 (22%) practiced several times a week, 62 (19%) practiced weekly, and 37 (11%) practiced monthly. Regarding the diversity of practices, 86 participants (26%) engaged in only one type of meditation, while 119 participants (37%) practiced more than one type. The most popular techniques were mindfulness with specific objects (143 participants, 44%), prayer (67 participants, 21%), somatic meditation (52 participants, 16%), and vipassana (47 participants, 14%).

Participants with a history of epilepsy, psychiatric disorders, or those undergoing pharmacotherapy at the time of the study were excluded to minimize health risks and ensure data reliability.

Psychological Measures

To assess the psychological constructs relevant to psychedelic experiences, several validated instruments were employed in their Polish adaptations.

Mystical Experience Questionnaire – MEQ-4

The Mystical Experience Questionnaire (MEQ-4) was used to evaluate the intensity of mystical-type experiences during psychedelic states (Strickland 2024). The MEQ-4 utilized the 6-point response scale (0 = "none; not at all," 1 = "so slight cannot decide," 2 = "slight," 3 = "moderate," 4 = "strong," and 5 = "extreme"), with each of its four items corresponding to one of the four core subscales. Specifically, Item 1 assessed the "sense of unity, insight into ultimate reality, or a feeling of sacredness," Item 2 measured "positive mood," Item 3 captured the "experience of transcending time and space," and Item 4 evaluated "ineffability" (i.e., the difficulty or impossibility of putting the experience into words). The MEQ-4 was developed as a brief version of the 30-item MEQ (MEQ-30) to reduce participant burden while retaining strong psychometric properties. In validation studies, the MEQ-4 demonstrated a high correlation with the full MEQ-30 (r = 0.89, p < 0.001), indicating excellent concurrent validity (Strickland et al., 2024). Correlations between the MEQ-4 and the MEQ-30 subscales ranged from r = 0.71 to r = 0.84 across the four dimensions. This strong correspondence supports the MEQ-4 as a reliable alternative for both research and clinical contexts where shorter assessments are necessary. Although Cronbach's alpha values for the MEQ-4 have not yet been reported, the MEQ-30—the scale from which it was derived—has consistently demonstrated excellent internal consistency. For instance, MacLean et al. (2012) found a Cronbach's alpha of 0.93 for the total MEQ-30 score, with subscales ranging from 0.83 to 0.94, underscoring the measure's reliability in capturing mystical-type experiences. The MEQ-4, designed to provide a briefer assessment while retaining core elements of the MEQ-30, reflects similar psychometric strengths through its strong correlations with the full scale (Strickland et al., 2024). Moreover, higher MEQ-4 scores have been linked to greater reductions in depressive and anxiety symptoms following psychedelic use, highlighting its predictive value in therapeutic contexts.

Imperial Psychedelic Predictor Scale – IPPS

To assess psychological preparedness and factors predicting individual responses to psychedelics, the Imperial Psychedelic Predictor Scale (IPPS) was used. The IPPS is a concise, 9-item tool designed to evaluate 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) (Angyus 2024). Items are rated on a 6-point Likert scale ranging from 0 ("not at all") to 5 ("extremely"). The set factor includes items assessing readiness, openness, and comfort toward the experience; the rapport factor focuses on the interpersonal connection and environmental trust; and the intention factor captures the participant's purpose for the psychedelic use. Factor analysis confirmed a three-factor structure with strong internal consistency: Cronbach's alpha was 0.81 for set and 0.76 for rapport, with an overall alpha of 0.83 for the total scale (Angyus 2024). Convergent validity was supported by significant correlations with the Surrender Scale (r = 0.455, p < .0001), underscoring the IPPS's utility in predicting salient aspects of acute psychedelic experiences. Notably, in controlled settings, set and rapport together accounted for 40% of the variance in mystical experiences, while set alone explained 16% of the variance in challenging experiences, and "rapport" accounting for 9% of the variance in emotional breakthroughs. These findings underscore IPPS as a valuable tool for anticipating acute responses to psychedelics, which can improve safety, improve therapeutic outcomes, and inform clinical practice and research. These findings highlight the IPPS as a valuable instrument for anticipating participants' responses to psychedelics, optimizing therapeutic outcomes, and ensuring safety in both clinical and research settings. Its brevity and strong psychometric properties make it suitable for use in various contexts, from controlled clinical trials to naturalistic settings (Angyus 2024).

MINDSENS questionnaire – KZU

To assess mindfulness-related skills and states, the KZU (pol. Kwestionariusz Zaawansowania w Uważności) — the Polish adaptation of the MINDSENS questionnaire (Soler 2014) adapted by Radoń (2020) — was employed. This instrument was developed to evaluate the advancement of mindfulness in both meditators and non-meditators. The KZU comprises three core subdimensions: Observing (awareness of internal and external experiences), Non-reactivity (the ability to allow thoughts and feelings to arise and pass without becoming entangled in them), and Decentering (the capacity to perceive thoughts and emotions as transient mental events rather than accurate reflections of reality or the self). The KZU consists of 19 items, each rated on a 5-point Likert scale ranging from 1 ("never") to 5 ("always"), measuring the frequency of mindfulness-related experiences and attitudes. Composite reliability values ranged from 0.74 to 0.93, and construct reliability (H) ranged from 0.82 to 0.93, reflecting strong internal consistency across the subdimensions. Although Cronbach's alpha values were not reported, the composite reliability estimates indicate similarly high internal consistency. The validated Polish version of the KZU is suitable for use across a broad age range (15–72 years) and can be applied in both clinical and non-clinical populations to assess mindfulness development. Recent research indicates that psychedelic substances can enhance mindfulness levels. For instance, studies have found that the use of psychedelics is associated with increased mindfulness, particularly in facets such as non-judgment of inner experience and non-reactivity (Murphy-Beiner 2020). Additionally, combining psilocybin administration with meditation practices has been shown to deepen meditation experiences and elevate trait mindfulness (Smigielski 2019). These findings suggest that psychedelics may facilitate mindfulness-related skills, thereby justifying the use of the KZU questionnaire in our adaptation to assess these effects.

Hood's Mysticism Scale – HMS

To assess mystical orientation and experiences beyond the psychedelic context, the Polish adaptation of the Hood's Mysticism Scale (HMS) was employed (Radoń 2014). Originally developed by Hood (1975) based on the theoretical framework of Stace (1987), the HMS measures individuals' predisposition toward mystical experiences. The scale encompasses three subdimensions: Interpretation (cognitive understanding of mystical experiences), Introvertive Mysticism (experiences of unity and self-loss beyond sensory perception), and Extrovertive Mysticism (a sense of unity with external objects and the environment). The Polish adaptation of the HMS was developed using the 12-item Mystical Experience (ME) version (Anthony 2010). The HMS includes 12 items, with responses provided on a 5-point Likert scale from 1 ("definitely not") to 4 ("definitely yes"), assessing the intensity of mystical orientation and experiences. Internal consistency was acceptable across subscales, with Cronbach's alpha values of 0.76 for Extrovertive Mysticism, 0.72 for Introvertive Mysticism, and 0.56 for Religious Interpretation. The Polish adaptation of the HMS provides a reliable and valid measure of mystical orientation suitable for both religious and secular populations, making it applicable in studies investigating mystical experiences in diverse contexts, including but not limited to psychedelic research.

Scale of Spiritual Transcendence – STD

To evaluate spiritual beliefs and experiences, we utilized the Scale of Spiritual Transcendence (STD), a Polish adaptation of Piedmont's (1999) original instrument, adapted by Piotrowski (2019). The STD assesses two core subdimensions: Core Transcendence (sense of connection with a greater reality beyond oneself) and Spiritual Awareness (awareness of spiritual aspects in everyday life). The STD comprises 22 items, each rated on a 4-point Likert scale ranging from 1 ("strongly disagree") to 4 ("strongly agree"), capturing spiritual beliefs and experiences related to transcendence.

The Polish version of the STD has demonstrated robust psychometric properties, with Cronbach's alpha coefficients of 0.87 for Core Transcendence and 0.78 for Spiritual Awareness, indicating satisfactory internal consistency (Piotrowski 2019).

Piedmont (1999) described spiritual transcendence as the capacity to move beyond immediate experiences of time and place, allowing individuals to view life from a wider and more objective perspective. The inclusion of the STD in our study is particularly pertinent given recent findings that psychedelic experiences can lead to significant shifts in metaphysical beliefs. Research indicates that psychedelics may induce lasting changes in individuals' perceptions of reality, often moving away from materialistic viewpoints towards more spiritual or non-physicalist perspectives (Griffiths 2019; Timmermann 2021). These ontological shifts underscore the importance of assessing spiritual transcendence when evaluating the psychological impact of psychedelics. By employing the STD, we aim to capture potential changes in spiritual beliefs and awareness, providing a comprehensive understanding of how psychedelic experiences may influence an individual's spiritual framework.

Religious Commitment Questionnaire – KZR

The Polish Religious Commitment Questionnaire (pol. Kwestionariusza Zaangażowania Religijnego – KZR), adapted by Polak (2017) from the Religious Commitment Inventory-10 (RCI-10) (Worthington 2003), is designed to measure the degree of religious involvement in individuals' daily lives and social relationships. The KZR consists of 5 items, each rated on a 5-point Likert scale ranging from 1 ("strongly disagree") to 5 ("strongly agree"). This instrument assesses a single dimension: Intrapersonal Religious Commitment, reflecting the internal importance and personal significance of religious beliefs and practices. Validation studies have demonstrated that the KZR exhibits strong psychometric properties, with Cronbach's alpha coefficients ranging from 0.82 to 0.95, indicating high internal consistency.

The inclusion of the KZR in our study is pertinent, as research indicates that psychedelic experiences can lead to significant changes in religious engagement. For instance, the Marsh Chapel Experiment demonstrated that psilocybin can occasion profound religious experiences, with participants reporting increased personal spirituality and religious commitment (Pahnke 1963) (Pahnke, 1963). Additionally, studies have shown that psychedelics can facilitate experiences that participants interpret as spiritual or religious, potentially leading to lasting changes in religious beliefs and practices (Griffiths 2008). By employing the KZR, we aim to assess how psychedelic experiences may influence intrapersonal religious commitment, providing valuable insights into the interplay between such experiences and personal religious engagement.

Together, these psychological measures allowed for a comprehensive evaluation of participants' mystical experiences, mindfulness capacities, mystical inclinations, and spiritual perspectives, providing insights into the interplay between these constructs and the acute effects of psychedelics.

3.Scales Adaptation and Validation

Datasets

Due to the nature of psychedelic experiences, which may or may not occur with a sitter/group setting, we put the rapport items in PPS as optional. Participants were asked whether they underwent the experience in a group and/or with a sitter and were accordingly redirected to the corresponding rapport items. Nearly half of the sample (147 observations) consisted of experiences undertaken in a group and with a sitter (both rapport items filled). The conducted analyses focused on two datasets: all experiences (N = 325, which may or may not have been accompanied by a sitter or a group, the all dataset) and experiences accompanied by both a sitter and a group (N = 147, the full rapport dataset).

Translation process

The translation and cultural adaptation process for both the IPPS and the MEQ-4 followed established guidelines for cross-cultural adaptation of psychological instruments (Beaton 2000). Initially, two independent bilingual experts translated the original English versions of both scales into Polish. Their translations were compared, and discrepancies were resolved through discussions to create a consensus version for each scale.

To ensure conceptual and linguistic equivalence, a back-translation process was subsequently conducted. Two different bilingual translators, who were not involved in the initial translation phase and were unfamiliar with the original versions, translated the Polish items back into English. This step aimed to verify that the adapted items preserved the original meanings without introducing cultural distortions 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 constructs intended by the original scale.

Table A.1 presents the original items alongside their final Polish adaptations for both the IPPS and MEQ-4.

3.1 IPPS

Factor Structure Analysis

To assess whether the data are appropriate for conducting factor analysis, the Kaiser-Meyer-Olkin (KMO) measure of sampling adequacy and Bartlett's sphericity test were computed, yielding a high KMO value of 0.866 and a significant result for the Bartlett's test.

Initial exploratory factor analysis (EFA) was conducted to investigate the underlying factor structure of the Polish version. Analysis was performed using the factor-analyzer Python package with various oblique rotations (promax, oblimin, quartimin, geomin_obl) and two optimization methods, including minimum residual (minres) and maximum likelihood (ml) solutions. The EFA revealed a three-factor solution that diverged from the original IPPS structure. Rather than supporting distinct "set" and "intention" factors, the data suggested that these constructs merged into two new dimensions, which we termed "Readiness & Openness" and "Emotional Comfort." "Readiness & Openness" consists of original elements set1, set2, set3, and intention, that is items related to psychological preparedness, openness to the experience, and intentionality. Emotional items, originally integrated within "set" items (set4, set5, set6), emerged as a distinct factor named "Emotional Comfort," encompassing feelings of calmness and reduced anxiety before the experience. The third factor, "Rapport," remained consistent with the original scale, capturing the participant's sense of trust and connection with their sitter and/or group. Communalities and factor loadings indicated sufficient item contributions, although the "intention" item demonstrated lower communalities across datasets (0.19, 0.22), meaning it is not well explained by the common factors. This might indicate that it captures aspects—such as cultural nuances—that are not fully represented in the latent construct, suggesting potential cultural nuances in how the concept is understood in Polish contexts.

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

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

Convergent and discriminant validity

Convergent and discriminant validity were assessed through correlations (with the Bonferroni correction) with external psychological measures, including the MEQ-4, HMS, STD, and KZU. All following correlations proved to be significant. Readiness & Openness demonstrated 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. Notably, Readiness & Openness consistently emerged as the strongest predictor (as measured by standardized betas), reinforcing its centrality in shaping the subjective quality of psychedelic experiences. Emotional Comfort and Rapport, while contributing less variance, provided complementary insights into the role of emotional states and social dynamics in these experiences.

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, particularly in terms of its scope of usage. In English, "intention" is a broad term that can refer to various forms of planning, purpose, or intent, as well as actions related to the future. It can encompass both will and motivation behind a particular action, as well as what a person aims to achieve. In Polish, "intencja" is more closely associated with inner motivation or purpose, but it is also used in more specific contexts, such as philosophy, psychology, and law. Additionally, it 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.

3.2 MEQ-4

Factor Structure Analysis

To assess whether the data are appropriate for conducting factor analysis, the KMO measure and Bartlett's sphericity test were computed, yielding a good, suitable for factor analysis KMO value of 0.75 and a significant result for Bartlett's test.

EFA was conducted to examine the underlying factor structure of the Polish adaptation of the MEQ-4. Using the minimum residual (minres) solution with the promax rotation, an oblique rotation, a four-factor solution corresponding to the original subscales—transcendence, ineffability, positive mood, and mysticism—was obtained. This solution emerged exclusively under these conditions, as other optimization methods and rotation techniques predominantly indicated a single-factor solution, as reflected by the eigenvalues of the covariance matrix of the observed variables ([2.26, 0.79, 0.52, 0.44]). Communality values for the items, i.e. the proportion of variance in each observed variable that can be explained by the factors, ranged from 0.20 to 0.42 accordingly: transcendence–0.418, ineffability–0.397, positive mood–0.199, and mysticism–0.30. Such values are often deemed acceptable depending on the context of research, with the exception of the positive mood communality, which commonly is considered at the threshold in acceptability. Such a difference indicates that this variable either does not fully conform to the proposed factor structure or measures an aspect that is not entirely captured by the common factors of the model. This may suggest that the variable contains a significant portion of specific (unique) variance or that it is theoretically distinct from the other variables in the model.

Item loadings under the promax rotation and minres solution confirmed the distinctiveness of the four subscales: MEQ_transcendence loaded primarily on Factor 1 (a factor loading of 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). Overall, these values indicate that the items are reasonably good indicators of the latent construct. Although the first two items exhibit particularly strong associations, MEQ_positive_mood item—with a loading of 0.445—shows a comparatively weaker, albeit acceptable, relationship. However, when oblimin and quartimin rotations were applied, the transcendence dimension converged with mysticism, reducing the structure to three distinct factors. This suggests that, depending on the rotation method, these two subdimensions may overlap, i.e. share a considerable amount of common variance, implying that they might capture a similar underlying construct, potentially reflecting different facets of a broader latent variable.

Neither a four-factor nor a three-factor structure for MEQ-4 can be confirmed via CFA as the parameters for such models cannot be estimated given that the number of parameters exceeds the available information (i.e. the number of covariances, including the variances of the observed variables).

Reliability and validity

Cronbach's alpha was not calculated due to the scale's 1 item - 1 dimension structure, however test-retest reliability demonstrated good stability over time, with a correlation coefficient of r = 0.91.

All following correlations proved to be significant with the Bonferroni correction for the all dataset and all questionnaire scores and their dimensions (19). The correlations between the dimensions of the MEQ4 and the overall MEQ4 score were as follows: mysticism exhibited a very strong correlation (0.80), ineffability and transcendence showed strong correlations (0.77, 0.76), while positive mood demonstrated a strong to moderate correlation (0.61). The intercorrelations between the dimensions were moderate for: transcendence and mysticism (0.56), ineffability and mysticism (0.51), transcendence and ineffability (0.48). Weak correlations were observed between positive mood and the other dimensions - ineffability (0.27), transcendence (0.31), and mysticism (0.35), further highlighting positive mood's unique role in the MEQ-4 structure.

Convergent validity was assessed through correlations with external psychological measures, including the HMS, STD, and KZU. The significance of correlations between all questionnaire scores and their subdimension scores were calculated for the all dataset using the Bonferroni correction (for 19 variables).

The MEQ total score had the highest correlation with the HMS total score (0.79) and their subdimensions (correlations of 0.73–0.77). The HMS total score correlated most strongly with the mysticism dimension (0.65), followed by transcendence (0.58) and ineffability (0.55), and most weakly with positive mood (0.38). The HMS dimensions correlated moderately to strongly with MEQ items (except for positive mood), ranging from 0.48 to 0.63, while positive mood exhibited weak correlations, approximately between 0.30 and 0.40. Additionally, the MEQ-4 score showed a strong correlation with the STD total score (0.70) and the KZU total score (0.69). INTERPRETACJA (patrz niżej)

The questionnaire demonstrates good construct validity, as evidenced by its high correlations with HMS dimensions (excluding positive mood), indicating that it effectively measures the degree and nature of mystical experiences.

MEQ scores by psychedelic type

Table 7 displays mean MEQ-4 values by different substance used in the reported meaningful experience. LSD, psylocybin and DMT differed in MEQ mean scores, while DMT, ayahuasca and mescaline did not (LSD<Psilocybin<DMT=Ayahuasca=Mescaline).

The variation in MEQ scores across different substances aligns with their pharmacological profiles and the depth of subjective mystical experiences they induce. LSD, while capable of producing profound alterations in perception and cognition, typically results in lower MEQ scores compared to psilocybin, which is known for its more introspective, emotionally profound, and immersive mystical states. The highest MEQ scores are observed with DMT, ayahuasca, and mescaline, substances often associated with intense visionary states, deep spiritual insights, and a strong sense of ego dissolution and interconnectedness. This pattern likely reflects differences in serotonergic receptor binding affinities, particularly 5-HT2A agonism, as well as cultural and contextual factors, such as the ritualistic use of ayahuasca and mescaline in traditional ceremonies, which may amplify the mystical qualities of the experience.

4.Additional Analyses

PPS → (HMS, STD)

MEQ regression models

The dataset used in these analyses was the all dataset, with the exception of observations involving mescaline, as there were only 9 cases. The final sample size after exclusion was N = 316.

A commonly selected predictor for MEQ models was prior experience with mystical states, represented by the variable experienced user. In this context, an experienced user was defined as an individual with more than 10 psychedelic experiences, a threshold set based on the dataset's median lifetime psychedelic use, which proved to be a proper cut-off.

To evaluate model performance, MEQ values were considered for RMSE assessment. The average MEQ score was 3.75, with a median of 4.0 and 0 to 5 range.

MEQ psychedelics regression model

The initial model, which included only psychedelics (LSD, DMT, Ayahuasca) as predictors, with psilocybin as the reference psychedelic, exhibited poor fit, indicating that psychedelic use alone does not adequately explain variance in MEQ4 scores. To improve model performance, Readiness & Openness and Emotional Comfort were incorporated. Additionally, the inclusion of user experience with psychedelics further enhanced model fit, suggesting that both readiness and emotional comfort toward the upcoming psychedelic experience and accumulated psychedelic experiences contribute meaningfully to the prediction of MEQ4 scores.

The primary baseline psychedelic was psylocybin, but due to LSD's insignificance in various models, psylocybin was grouped into the same category as LSD. This reflects their similar pharmacological profiles and effects on mystical experiences.

Among binary predictors, Ayahuasca use has the strongest association with higher MEQ4 scores, followed by DMT use (both compared to the psylocibin and LSD group), and general psychedelic experience (Experienced user). The standardized beta coefficients (Figure 7) indicate that Readiness & Openness (RO) is the most influential variable affecting MEQ scores.

The model is statistically significant and explains 42.5% of the variance (R² = 0.425), indicating a moderate to strong relationship between these factors and mystical experiences. RMSE (0.614) suggests a reasonable level of error. Model assumptions are well met. Residuals are approximately normal, as indicated by D'Agostino tests, Omnibus, and Jarque-Bera tests. Max VIF value of 1.67 further supports model fit, indicating very low multicollinearity. Durbin-Watson statistic (1.537) suggests no severe autocorrelation, supporting the model's validity.

Overall, this model effectively captures the influence of preparedness for the psychedelic experience and substance use on mystical experiences.

MEQ meditation regression model

The purpose of this model was to examine the meditation frequency influence on MEQ. The model that included only the meditation frequency variables (R² = 0.226) violated the assumptions of linear regression. Therefore, the relationship between mystical experiences (MEQ4) and PPS dimensions, meditation frequency, and psychedelic experience was examined to ensure a more robust and reliable model. In this model, the reference group consists of individuals who either do not meditate or meditate approximately once a month (due to variable's strong insignificance in various meditation models). The results (Table A.13) indicate that individuals who meditate daily or several times a week show the strongest positive association with MEQ4 scores, suggesting that more frequent meditation is linked to a higher likelihood of mystical experiences. Those who meditate weekly also exhibit a significant but weaker association.

Additionally, Readiness & Openness and Emotional Comfort remain important predictors, indicating that individual differences in one's readiness and openness to experience and emotional comfort during the experience contribute to mystical experiences. Furthermore, having extensive psychedelic experience is associated with higher MEQ4 scores, reinforcing the role of past psychedelic use in shaping mystical-type experiences.

The model is statistically significant, demonstrates strong explanatory power (R²_adjusted = 0.457) and meets the key assumptions of linear regression (Table A.12), including normality of the residuals, very low correlation of features (max VIF value 1.87) and lack of severe residual autocorrelation. Overall, these findings suggest that frequent meditation, alongside emotional comfort, and readiness and openness to the upcoming experience, contributes to variations in mystical experiences.

The Role of Religiosity in Mystical Experiences

The aim of this analysis was to examine the role of religiosity (KZR) in the relationship between preparation for experience (PPS) and mystical experiences. Specifically, the study sought to determine whether religiosity moderates the effect of preparation for experience and emotional comfort before the experience on mystical experiences (MEQ4).

Linear mystical models incorporating different combinations of variables (RO, EC, KZR) and moderation effects of KZR with RO and EC were found to be poorly fitted, as they violated the normality assumption of residuals. This suggested that the models might have been missing a key predictor. To address this, meditation-related variables (identified in the previous subsection), and experienced user variable were introduced and the RO*KZR interaction was removed due to its consistent non-significance across multiple models. After these adjustments, the model met the assumptions of a linear regression model.

The effect of EC on mystical experience is weaker for higher values of KZR—the higher the KZR, the weaker the effect. However, this effect remains positive across all levels of KZR. Additionally, the strength of this effect also decreases as KZR increases.

The effect of KZR on MEQ is positive but only for individuals with low emotional comfort before the experience (EC). For individuals with sufficiently high emotional comfort, the effect of KZR on MEQ becomes negative. As EC increases, KZR actively reduces the likelihood of profound mystical experiences. When EC < 222, the effect of KZR on MEQ weakens as EC increases, indicating that KZR becomes progressively less relevant in predicting MEQ. At EC = 222, both the strength and the effect of KZR on MEQ reach their lowest value. For individuals with EC > 222, a further increase in EC leads to a stronger negative effect of KZR on MEQ. KZR has the strongest effect on MEQ for EC < 144, and this influence becomes stronger as EC decreases.

The model is statistically significant, demonstrates strong explanatory power (R²_adjusted = 0.461) and meets the key assumptions of linear regression (Table A.14), including normality of the residuals, and lack of severe residual autocorrelation.

Studies such as Pahnke's 1962 "Good Friday" experiment with theology students showed that psilocybin, when used in a religious context, could lead to profound mystical experiences that participants deemed spiritually significant (Pahnke 1963). Similarly, Griffiths (2008) found that psilocybin-induced mystical experiences had long-lasting personal and spiritual meaning. These studies suggest that religiosity can serve as a facilitator of mystical experiences in specific contexts. However our findings further suggest that religiosity may act as a buffer for individuals with low emotional comfort, helping them achieve deeper mystical experiences by providing a structured interpretative framework. This aligns with the idea that religiosity can function as a coping mechanism, offering meaning and structure that facilitate a positive mystical experience. Such insights are particularly relevant for psychedelic-assisted therapy, where preparation and support are crucial factors. On the other hand, for individuals with high EC, religiosity appears to impose constraints—potentially through rigid interpretative frameworks or expectations—that inhibit full openness to the experience. This unexpected finding that religiosity can have a limiting effect under certain conditions sheds new light on psychedelic research and highlights the importance of considering individual differences.

Overall, the influence of religiosity on mystical experiences is complex and depends on emotional comfort before the experience. For those who already feel emotionally secure, religiosity tends to disrupt rather than deepen mystical states. Its effect is weakest at moderate levels of emotional comfort, where it becomes less relevant. However, at the lowest levels of emotional comfort, religiosity has the strongest impact, significantly enhancing the intensity of mystical experiences.

STD Spiritual Openness' moderators

In additional analyses, the role of interpersonal relationships in shaping spiritual openness was examined. Specifically, the effects of Readiness & Openness and Emotional Comfort on the Spiritual Openness dimension of the STD questionnaire were analyzed, with Rapport included as a mediating variable. Using a stepwise significant-mediation inclusion approach, an OLS model was obtained in which Readiness & Openness significantly moderated the effect of Rapport on Spiritual Openness. Emotional Comfort had been added to the models, but was excluded due to its insignificance (p > 0.05). The model was statistically significant, with unautocorrelated residuals (|DW| > 1.5) that followed a normal distribution (nonsignificant Omnibus and Jarque–Bera tests). The proportion of the dependent variable's variance explained by the model was R² = 33.9%. Among the predictors (Readiness & Openness, Rapport, and their interaction), the Rapport variable was significant (p < 0.05), as well as the interaction between Readiness & Openness and Rapport (p < 0.001). Standardized betas indicated that the interaction between Rapport and RO has the strongest impact on predicting Spiritual Openness.

The results suggest that Rapport influences Spiritual Openness, with Readiness & Openness acting as a moderating factor. This finding highlights the importance of interpersonal dynamics in shaping an individual's receptivity to spiritual experiences. Specifically, while Rapport—characterized by trust, emotional connection, and shared understanding—directly contributes to Spiritual Openness, its effect is significantly enhanced when an individual is psychologically prepared and open to transformative experiences. The significant interaction between Readiness & Openness and Rapport suggests that a supportive relational context alone may not be sufficient to foster spiritual receptivity; rather, the individual's intrinsic willingness to engage with new perspectives plays a crucial role. These findings align with previous research on the role of psychological set and social environment in facilitating mystical and transcendent states, particularly in contexts such as meditation, psychedelics, and spiritual practice. Furthermore, the well-fitting model, with an explained variance (R² = 33.9%) notably high for this field, underscores the interplay between internal disposition and external relational factors in the cultivation of spiritual openness.

5.Discussion

The present study aimed to validate the Polish versions of the Imperial Psychedelic Prediction Scale (IPPS) and the Mystical Experience Questionnaire (MEQ-4), refining their factor structures and ensuring their applicability for psychedelic research in Polish-speaking populations. The results confirm their strong psychometric properties while also highlighting key modifications that align with cultural and linguistic nuances.

One of the most significant findings was the restructuring of the IPPS, where the original "Set" and "Intention" dimensions were replaced by "Readiness & Openness" and "Emotional Comfort." This refinement suggests that, in the Polish context, psychological preparedness for a psychedelic experience is better understood as a combination of openness to experience and emotional security. Future research on psychedelics should consider these revised constructs when investigating predictors of positive or challenging experiences. The findings further highlight the importance of distinguishing between cognitive and emotional readiness, suggesting that interventions designed to enhance psychedelic preparation should address both aspects separately.

The significant role of rapport in moderating spiritual openness also provides valuable insight into the mechanisms underlying mystical-type experiences. Future studies should explore how social and relational factors influence psychedelic experiences in different cultural contexts, particularly in structured therapeutic settings versus more naturalistic environments. Additionally, since the study demonstrated that Readiness & Openness has a stronger predictive effect on mystical experiences than Emotional Comfort, further research should examine how specific preparatory techniques (e.g., meditation, psychological priming, expectation setting) enhance these dimensions.

Practical Applications for Researchers and Clinicians

The revised scales have important applications for both researchers and clinicians working in psychedelic science and therapy. The IPPS-PL can be used before psychedelic-assisted therapy or research studies, to identify individuals who may require additional psychological preparation to optimize their experience. In clinical practice, assessing Readiness & Openness and Emotional Comfort can help tailor preparatory interventions, ensuring that patients are adequately prepared for their sessions. Therapists could use these measures to personalize pre-session discussions, emphasizing areas where the participant may need additional support.

Similarly, the MEQ-4-PL remains a reliable and efficient tool for measuring mystical-type experiences following psychedelic use. Given its brevity and strong psychometric properties, it can be integrated into clinical protocols to evaluate the intensity and therapeutic relevance of psychedelic-induced mystical experiences. Researchers conducting cross-cultural studies on psychedelic experiences may also find the MEQ-4-PL particularly useful in comparing mystical experiences across different linguistic and cultural groups.

Unique Cultural Aspects Influencing the Results

The findings align with prior adaptations of the MEQ in other languages but also introduce important cultural distinctions. The restructuring of the IPPS suggests that Polish-speaking individuals conceptualize psychological preparedness in a slightly different way than English-speaking populations, where "intention" is typically considered a separate construct. However, as no prior cross-cultural adaptations of the IPPS have been documented, direct comparisons with other languages are not possible at this time.

In contrast, previous adaptations of the MEQ-30 in languages such as Japanese and Brazilian Portuguese followed standard translation and cultural adaptation procedures, but no specific linguistic refinements regarding ineffability and transcendence have been documented.

Cultural factors unique to Poland may have influenced how participants engaged with the constructs measured in this study. The Polish conceptualization of spirituality often integrates both secular and religious perspectives, potentially shaping the way individuals interpret mystical experiences. This may explain the prominent role of rapport in influencing spiritual openness, as relational and communal aspects are frequently emphasized in Polish cultural narratives about transformative experiences. Unlike in highly individualistic cultures, where mystical experiences are often framed in solitary or self-transcendent terms, Polish participants may be more inclined to relate such experiences to shared meaning-making and interpersonal connection.

Moreover, the meaning of "intention" in Polish carries a more formal or metaphysical connotation than its English counterpart, which might explain why it merged with Readiness & Openness rather than remaining a distinct factor. This suggests that adaptations of psychedelic assessment tools must carefully consider cultural connotations to ensure conceptual equivalence across languages.

Real-World Applications Beyond Research

Beyond academic research, the adapted scales have significant practical applications in clinical, therapeutic, and harm reduction settings. In psychedelic-assisted therapy, the IPPS-PL can be used to screen patients for psychological preparedness, helping clinicians identify individuals who may benefit from additional preparation before undergoing treatment. Likewise, it can serve as a tool for training facilitators and therapists, enabling them to assess and enhance client readiness.

For harm reduction initiatives, the IPPS-PL can be incorporated into educational programs aimed at individuals using psychedelics in non-clinical settings. By encouraging users to assess their psychological readiness and emotional state beforehand, the tool may help reduce the likelihood of challenging experiences and promote safer, more meaningful engagements with psychedelics.

The MEQ-4-PL also has utility in monitoring and evaluating outcomes in both therapeutic and self-exploratory psychedelic use. Its brief format makes it suitable for integration into post-session assessments, allowing researchers and clinicians to track the depth and impact of mystical experiences over time. In public health contexts, the MEQ-4-PL could be used in large-scale surveys assessing the prevalence and characteristics of psychedelic-induced mystical experiences within Polish-speaking populations.

6.Summary and conclusions

The validation of the IPPS-PL and MEQ-4-PL provides essential tools for assessing psychedelic experiences in Polish-speaking populations, bridging a crucial gap in cross-cultural psychedelic research. By demonstrating strong psychometric properties and refining factor structures to align with cultural contexts, this study contributes to both theoretical understanding and practical applications. The results highlight Readiness & Openness as a central predictor of mystical experiences, reinforcing its importance in shaping psychedelic outcomes. Future research should explore longitudinal changes in psychological preparedness, assess the predictive validity of these measures in therapeutic settings, and conduct comparative studies with other language versions to deepen our understanding of cultural influences on psychedelic experiences. These efforts will further enhance the applicability of psychometric tools in both research and clinical practice.

7.Limitations and Future Directions

While this study provides robust evidence supporting the validity and reliability of the IPPS-PL and MEQ-4-PL, several limitations should be acknowledged. First, the study relied on self-reported retrospective accounts of psychedelic experiences, which may introduce recall bias. Future studies should consider using prospective designs or experience sampling methods to enhance data accuracy. Additionally, while the sample size was adequate for factor analyses, larger and more diverse samples could improve generalizability.

Furthermore, the sample primarily consisted of individuals already familiar with psychedelics, which may limit the applicability of findings to first-time users or clinical populations undergoing psychedelic-assisted therapy. Experienced users might exhibit greater psychological readiness and a different interpretative framework for mystical experiences compared to those with no prior exposure. Future research should aim to include more diverse participant groups, particularly individuals preparing for their first psychedelic experience or those undergoing structured therapeutic interventions, to ensure broader applicability of the adapted measures.

Future research should also explore additional moderating variables, such as personality traits, and therapeutic context, to better understand individual differences in psychedelic responses. Moreover, given the significant role of Rapport in facilitating spiritual openness, future studies could investigate specific mechanisms through which interpersonal relationships shape the psychedelic experience, including the influence of therapeutic alliances and group dynamics in guided sessions. Understanding these factors could provide valuable insights for optimizing therapeutic outcomes in psychedelic-assisted therapy.

Acknowledgements

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References

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Appendix A. Original vs. final adapted items of IPPS

Appendix B. Correlation plots