Participants and Procedure
Data was derived from two samples in Germany.
Sample 1 (Online Sample): The first sample comprised N=276 participants (67% female; Mage=32.08, SDage=10.73, range: 18–64 years) who took part in an online assessment between February and June 2020. One-hundred twenty-one participants (43.8%) reported lifetime suicide ideation, 69 participants (25%) reported some suicide ideation in the last four weeks (SSEV- score ≥1) and 26 participants (9.4%) indicated that they had attempted suicide at least once in their lifetime (range: 1-6). All participants were of Caucasian decent.
Data was collected through an anonymous online survey using the SoSci-server (https://www.soscisurvey.de/). Participants were recruited through postings at local universities. In order to take part in the study, participants had to be at least 18 years old and to give their consent to participation at the beginning of the study.
Sample 2 (Patient sample): The second sample comprised N=94 participants (73.4% female; Mage=49.26, SDage=13.32, range: 22–79 years) who completed the High Place Phenomenon Index  at a follow-up assessment after a one-day intensive CBT-treatment for flight phobia (cf. ) between August and October 2019. All participants suffered from clinically relevant fear of flying before treatment: 90.3% suffered from flight phobia, 8.6% from agoraphobia and 1.1% from height phobia according to the Diagnostic and Statistical Manual for Mental Disorders (DSM-5 ). At the follow-up assessment 80 patients were available for a second diagnostic interview. Of those n = 44 (55%) were classified as completely remitted. Diagnoses were assigned in individual diagnostic sessions conducted either by licensed psychotherapists or post-graduate clinical psychologists using a structured clinical interview (Short Interview for Mental Disorders, Mini-DIPS-OA ). Very good inter-rater reliability for anxiety disorders (κ = .94) has been reported for the long version of the DIPS . Two participants (2.1%) indicated some suicide ideation (BDI-suicide item ≥1) in the week before the assessment. All participants were of Caucasian decent.
Treatment and assessment took place at a university outpatient clinic in the Ruhr Area in Germany. In order to take part in the study, participants had to be at least 18 years old, suffer from a clinically relevant fear of flying, that is, suffer from a phobic disorder, and to give their consent to participation at the beginning of the study.
Prior to assessments, participants in both studies were informed about the purpose of the study, the voluntary nature of their participation, data storage and security. Both studies were approved by the responsible Ethics Committee.
The investigations of the two samples were planned independently. Therefore, not all questionnaires were used in both sub-samples. In the following, it is indicated which questionnaire was used in which sample.
High Place Phenomenon Index (HPPI; ). The HPPI assesses with three items how frequently participants have experienced the high place phenomenon in their lifetime, using a 6-point Likert type scale ranging from (0) never to (5) always: Item 1: When standing on the edge of a tall building or walking on a bridge, have you ever had the urge to jump? Item 2: When you see a tall building or are walking on a bridge, have you ever thought about what it would be like to jump off of it? Item 3: When you are inside a tall building have you ever imagined jumping out a window? The original scale evidenced good internal consistency: α = .85. The German version of the HPPI was developed by means of a translation-back-translation procedure according to relevant guidelines for the translation of psychometric instruments . Internal consistencies of the German HPPI were good in the online sample (α = .84) as well as in the patient sample (α = .86).
Depression Anxiety Stress Scales (DASS; ). The DASS is a 21-item questionnaire measuring depressive mood, anxiety and chronic tension/stress during the past week (“I was aware of dryness of my mouth”; “I couldn’t seem to experience any positive feeling at all.”) All items are rated on a 4-point (0-3) scale. Internal consistency (coefficient alpha) for the DASS has been shown to be good (range of α = 0.88 – 0.96; ). In the current study, only the DASS-Depression and the DASS-Anxiety subscales were used. Internal consistencies of both subscales were good: α = .91 (DASS-Depression) and α = .84 (DASS-Anxiety) in the online sample, and sufficient in the patient sample: α = .77 (DASS-Depression) and α = .71 (DASS-Anxiety).
Anxiety Sensitivity Index (ASI; ). The ASI is a 16-item self-report inventory designed to measure the degree to which individuals are concerned about the potential negative effects of experiencing anxiety symptoms (“It scares me when I feel faint”, “Unusual body sensations scare me”). Respondents are asked to indicate the degree to which each item applies to them using a 5-point Likert type scale ranging from (0) very little to (4) very much. The scale has been found to have strong internal consistency and test-retest reliability (e.g., ). The ASI was only used in the online sample. Internal consistency was good within this sample: α = .88.
Fear of Flying Scale (FFS; [14, 15]). Fear of Flying (FoF) was assessed using the German version of the FFS consisting of 21 items with 5-point Likert scales (0 = no fear; 4 = very strong fear). The instrument provides a dimensional assessment of FoF severity and has shown to have good internal consistency: α = .90 . The FFS was only used in the patient sample. Internal consistency of the FFS was excellent within this sample: α = .96.
Suicide Ideation and Behavior Scale – Suicide Ideation (SSEV-SI; . The SSEV-SI assesses with eight items the frequency of suicide ideation in the past four weeks (e.g., “During the past four weeks, … I thought it would be better if I wasn’t alive, … I’ve been thinking about killing myself, … I have seriously considered killing myself”). All items are answered on a 6-point Likert scale ranging from “0=never” to “4=many times every day”, with higher scores indicating greater severity of suicidal ideation. Internal consistency was good in the current online sample (α = .87). Occurrence (“In the course of my life I have tried to kill myself (and I really wanted to die)”) and number of lifetime suicide attempts (“How many times have you tried to kill yourself?”) are assessed with two further SSEV-items. Finally, lifetime suicide ideation was assessed using a screening item from the Perseverative Thoughts about Suicide Questionnaire : “In my lifetime I have seriously considered suicide”. The SSEV was only used in the online sample.
Beck Depression Inventory – Suicide item (BDI-SI; ). Suicide ideation was measured using the respective item from the BDI (Item 9) in the patient sample. The suicide item of the BDI has repeatedly been used in clinical studies  and has been shown to significantly correlate with the Beck Scale for Suicide Ideation .
Statistical analyses were conducted with SPSS 24. Descriptive statistics and zero-order bivariate correlations between the investigated variables were calculated. Furthermore, group differences between (lifetime/current) suicide ideators and non-ideators were investigated using t-tests. To identify significant predictors of experiences with the high place phenomenon, hierarchical regression analyses – including age, gender, lifetime suicide ideation, depressive symptoms (DASS-D), anxiety symptoms (DASS-A) and anxiety sensitivity (ASI) as independent variables and HPPI scores as dependent variable – were calculated using the online sample. There was no violation of the multicollinearity assumption as all values of tolerance were >.25, and all variance inflation factor values were <5.