Ethical Approval The research received a favourable ethical opinion from the relevant local Research Ethics Committee (ethics approval references P47603 and F1074).
Participants The autistic group comprised 308 adults (27% male gender) who self-reported a diagnosis of ASC from a trained clinician. The possibly autistic group comprised 113 adults (29% male gender) who self-reported that they suspected they were autistic but had not yet been diagnosed. The non-autistic group comprised 268 adults (31% male gender) who reported that they were not diagnosed autistic or suspected they might be autistic. The autistic and possibly autistic groups were significantly more likely to identify with a different gender than assigned at birth (16.3%) compared to the non-autistic group (3%) (X2(1)=28.79, p<.001; OR 6.24 CI 2.95 – 13.21). There was no significant difference in age between the three groups (F(688)=1.34, p=.263). Self-reported autistic traits (AQ Scores) were significantly different between the three groups (F(682)=601.59, p<.001). Bonferroni corrected t-tests showed that the autistic group self-reported significantly higher autistic traits (mean=22.86, SD=3.76) compared to the possibly autistic (mean=20.17, SD=4.57) and non-autistic group (mean=9.45, SD=5.67), and the possibly autistic group self-reported significantly higher autistic traits than the non-autistic group (all p<.001) (Table 1).
Suicide Behaviours Questionnaire – Autism Spectrum Conditions
Figure 1 shows the stages involved in the overall development and validation of the SBQ-ASC with and for autistic adults. The SBQ-ASC was adapted from the SBQ-R with permission of the tool developers . A previous study had explored how autistic compared to non-autistic adults interpret and respond to the SBQ-R , to inform how to adapt this tool for autistic adults. The adapted SBQ-ASC was subsequently refined through: a) Cognitive interviews with 9 autistic adults (who took part in the earlier study, ); and b) A survey completed by 251 autistic adults who provided qualitative feedback, and rated the clarity and importance of each item of the original SBQ-R and adapted SBQ-ASC (234 diagnosed, 17 awaiting assessment; 30.7% male; mean age=41.91, SD=13.44; mean age of ASC diagnosis=36.09, SD=14.03; 61.4% Asperger Syndrome diagnosis). Table 2 summarises the key issues identified with the SBQ-R by autistic adults across the interviews and online survey, and the subsequent adaptations incorporated into the SBQ-ASC to address these.
The penultimate version of the SBQ-ASC items tested in the online survey were rated as “clear” by at least 79% of the 251 autistic participants, and mean importance ratings ranged from 74.17 – 84.93 (out of 100) for each item (mean=78.89, SD=3.99). After minor corrections to grammar and wording to improve clarity, the final version of the SBQ-ASC used in the current study was given an overall mean clarity rating of 82.72 (out of 100, SD=22.9) by the combined autistic/possibly autistic group, and 88.05 (SD=21.13) by the non-autistic group. Analysis of the qualitative feedback across the two surveys showed that the issues identified with the penultimate version of the SBQ-ASC assessed in the earlier survey had been successfully addressed in the final version of the tool used in the current study.
The SBQ-ASC has 5 scored items (Table 2). Item 1 assesses lifetime experience of suicidal thoughts and behaviours from “Never” (0) to “I have attempted to end my life” (4). Item 2 assesses frequency of intense suicidal thoughts in the last 12 months from “Never” (0) to “1 or more times a day” (6). Item 3 assesses perseverative intense suicidal thoughts from “Not Applicable” (0), “Less than 5 minutes” (1) to “More than 8 hours” (5). Item 4 assesses likelihood of suicide attempt from “Not Applicable” (0), “No chance at all” (1) to “Very likely” (5). It is recommended that a visual aid, such as a measuring jug or thermometer, is offered to participants to help quantify each response option for item 4 if they think this could be useful to them. Item 5 assesses communication of future suicide intent and past suicide attempts to others. Responses are scored from “Not applicable” (0) / “No” (0), to “Yes, once” / “Yes more than once”. Endorsing either “Yes” item is scored 1 for suicidal thoughts, 2 for future suicide attempts, and 3 for past suicide attempts. Participants can endorse all the options giving a maximum score of 6 for item 5.
Optional follow-up items which are not scored are also included in the SBQ-ASC. For those who endorse lifetime suicide attempt, these items address presence of plans, impulsivity and access to means. For those who have communicated suicidality to others, follow up items gather information on who was told (e.g. friend, family member or professional). For those who have never told anyone about their suicidality, follow up items gather information on why (e.g. I had no one to tell, I was afraid to). Item 6 also captures lifetime experience of non-suicidal self-injury (Yes/No). Therefore, alongside item 1, the SBQ-ASC can be used to classify lifetime experience of self-harm, with or without intent to end life.
Suicide Behaviours Questionnaire – Revised (SBQ-R)
The SBQ-R  is a four-item self-report questionnaire measuring suicidality. Item 1 assesses lifetime suicidal behaviour (on a scale from “Never” to “I have attempted to kill myself, and really hoped to die”). Item 2 assesses suicide ideation over the past 12 months (on a scale from “Never” to “Very Often (5 or more times)”). Item 3 assesses communication of suicidal intent to others (on a scale from “No” to “Yes, more than once, and really wanted to do it”). Item 4 assesses likelihood of a suicide attempt someday in the future (on a scale from “Never” to “Very likely”). The SBQ-R has been validated for use in general population samples to reliably distinguish people who have, from people who have not attempted suicide [18, 29]. The SBQ-R is widely used in research with moderate-strong evidence in support of internal consistency, structural validity, and criterion validity in research with general population samples . The SBQ-R has also been utilised in research with autistic adults [e.g. 3, 27], with evidence that the structure and interpretation of the SBQ-R is different in autistic compared to non-autistic adults . Cronbach’s alpha for whole scale: Autistic group a=.739, possibly autistic group a=.755, non-autistic group a=.734.
Autism Spectrum Quotient – Short (AQ-S)
The AQ-short  is a 28-item subset of the full 50 item Autism Spectrum Quotient . The AQ-28 measures the number of self-reported autistic traits, with high scores indicating more autistic traits. Items such as ‘it does not upset me if my daily routine is disturbed’ and ‘I find it easy to work out what someone is thinking or feeling’ are rated on a 4-item response scale from 1 “definitely agree” to 4 “definitely disagree” . In the current study, responses endorsing autistic traits were given a score of 1, giving a total range from 0-28. A systematic review showed satisfactory evidence in support of the AQ-S factor structure, internal consistency, test–retest reliability and convergent validity as rated by a validated research tool (COSMIN) . Using the dichotomous scoring method, scores at or above a clinical cut-off of 16 have showed acceptable sensitivity and specificity in distinguishing autistic from non-autistic adults . Cronbach’s alpha for whole scale: Autistic group a=.765, possibly autistic group a=.791, non-autistic group a=.854.
Camouflaging Autistic Traits Questionnaire (CAT-Q)
The Camouflaging Autistic Traits Questionnaire (CAT-Q) is a 25-item self-report questionnaire assessing the extent to which a person engages in social camouflaging behaviours, validated in autistic and non-autistic adults with equivalent factor structure between the groups . The CAT-Q captures three domains of social camouflaging: (1) “compensation” (behaviours used to compensate for autism-related difficulties in social situations); (2) “masking” (behaviours used to hide autistic characteristics or present a non-autistic personality to others); and (3) “assimilation” (behaviours used to fit in better with others and not “stand out” from the crowd). Participants rate each of the 25 questions on a seven-point Likert scale between “Strongly Agree” to “Strongly Disagree”. Responses are scored between 1 and 7, with higher scores for items which endorse presence of social camouflaging behaviour. Cronbach’s alpha for whole scale: Autistic group a=.919, possibly autistic group a=.9, non-autistic group a=.931.
Anxiety Scale for Autism (Adults) (ASA-A)
The ASA-A  is a 20-item self-report measure of anxiety designed with and for autistic adults, adapted from the Anxiety Scale for Autism Spectrum Disorder (ASC-ASD) (REF). The ASA-A measures four components of anxiety: Social Phobia (e.g. ‘I worry what other people think of me’), Anxious Arousal (e.g. ‘All of a sudden I feel really scared’), and Uncertainty (e.g. ‘I am anxious about unfamiliar things, people or places’). Each item is rated on a scale from “Never” (0) – “Always” (3), with total scores ranging from 0-60. Scores at or above 28 indicate clinically significant levels of . The ASA-A has strong evidence in support of its measurement properties (factor structure, internal consistency, test re-test reliability, convergent and divergent validity) in autistic adults . Cronbach’s alpha for whole scale: Autistic group a=.921, possibly autistic group a=.926, non-autistic group a=.943.
Patient Health Questionnaire – 9 Item (PHQ-9)
The Patient Health Questionnaire-9 item (PHQ-9)  is a 9-item self-report scale used to assess severity of current depressive symptoms in line with DSM-V diagnostic criteria . Scores range from 0 to 27 with scores at or over 10 indicating moderate, 15 moderately severe, and 20 severe depression. A recent systematic review showed that the PHQ-9 was extensively used in general population research, with strong evidence for its psychometric properties as rated by a validated research tool (COSMIN) , and more recently, evidence in support of total scores being comparable between autistic and non-autistic adults . Cronbach’s alpha for whole scale: Autistic group a=.907, possibly autistic group a=.904, non-autistic group a=.909.
Interpersonal Needs Questions – 15 Item (INQ-15)
The Interpersonal Needs Questionnaire (INQ-15) is a 15-item self-report questionnaire assessing ‘thwarted belongingness’ (e.g. ‘These days, I often feel like an outsider in social gatherings’) and ‘perceived burdensomeness’ (e.g. ‘These days, I think I am a burden on society’) . The INQ-15 has been validated in young non-autistic adults  and has been used in previous research with autistic adults and those with high autistic traits [26-27, 41]. Cronbach’s alpha for whole scale: Autistic group a=.927, possibly autistic group a=.933, non-autistic group a=.936.
Participants were asked to report on their age, sex, gender, employment, education, living situation, diagnoses (developmental, mental health and other), ASC diagnosis (clinically confirmed, suspected but not yet unconfirmed, and not autistic or suspected to be autistic), and for those with clinically confirmed diagnosis, the age of ASC diagnosis and the type of professional they were diagnosed by (e.g. paediatrician, psychologist, psychiatrist).
Participants were invited to complete an online survey using Qualtrics aiming to adapt mental health assessment tools for autistic adults. Participants were informed that anyone 18-years or over could participate, regardless of autism diagnosis, experience of mental health problems or suicidal thoughts or behaviours. Participants were fully briefed about the nature of the research, that they could skip questions and sections of the survey that made them feel uncomfortable, stop the survey at any time and complete it later. Participants were also provided information about relevant support services before taking part in the study, after each section of the study, and after taking part in the study in a downloadable debrief sheet. After providing consent, participants completed the demographics questions, AQ-S, CAT-Q, INQ-15, ASA-A, PHQ-9, SBQ-R and SBQ-ASC. The order of the SBQ-R and SBQ-ASC were randomised between participants. Participants were then asked for consent to complete the SBQ-R and SBQ-ASC again in two weeks. Subsequently participants were provided with a full debrief including information about further information and support, followed by a positive mood induction procedure (a doodle page with jokes, puzzles and cute animal videos) which has proved effective in previous research exploring similar topics .
The autistic and non-autistic samples were split in two, stratified by gender and age. The first half was utilised in the exploratory factor analysis (‘exploratory sample’ n = 292), and the second half utilised in the confirmatory factor analysis (‘confirmatory sample’ n = 295). The two samples did not significantly differ in age (partial η2=.001), autistic traits (partial η2=.001), birth sex (OR=.84) or gender (Cramer’s V=.008). The confirmatory factor analysis model in the autistic group was subsequently tested in the whole possibly autistic group (n=113). Analyses were conducted in SPSS version 26 and measurement invariance analysis conducted in SPSS AMOS version 24. 1319 participants initially accessed the survey. Of these, 748 participants who met eligibility criteria opted to see the SBQ-ASC questions, 689 of these participants completed all SBQ-ASC items with no missing data, and 686 also completed at least one additional measure with no missing items. Only measures with complete data for all items were included in the analysis.
Exploratory Factor Analysis of the SBQ-ASC
Principle components analyses were performed on the exploratory half of the autistic (n=153), and non-autistic (n=132) subsamples, and both the autistic and non-autistic groups combined (n=291). The sample size was sufficient for EFA, with over 7 participants per item, and over 100 participants total . Items with loadings below 0.4, or with cross-loadings of greater than 0.4 were excluded .
Confirmatory Factor Analysis
The Chi-square statistic was used an indicator of fit , alongside other fit indices given that chi-square is affected by sample size . The χ2/df ratio should be close to zero , root mean square of approximation (RMSEA) close to 0.06 , the comparative fit index higher than 0.9 , and Tucker-Lewis Index (TLI) values over 0.9 . CFA was conducted on the confirmatory half of the autistic (n=155), and non-autistic (n=136) subsamples. The model identified in the autistic group was then tested in the whole possibly autistic group (n=113). Groups that showed acceptable fit to the same model were combined, and the model re-run to test fit in the combined sample(s). Sample size was sufficient for CFA (>7 participants per item, and >100 participants total) .
The exploratory and confirmatory samples were re-combined, and multi-group confirmatory factor analysis used to determine whether the SBQ-ASC had a similar structure between the groups: autistic (n=308), possibly autistic (n=113) and non-autistic (n=268). Data was combined across groups which showed evidence for measurement invariance. Further analysis subsequently explored whether the structure was equivalent in males and females, and those who did and did not request visual aids for item 5 of the SBQ-ASC.
Measurement invariance analysis tests a series of nested models, with increasingly strict constraints, to assess evidence for increasingly strict levels of measurement invariance (i.e. equivalence) between groups [50-51]: 1) configural invariance tests whether sets of items measure the same latent variable in both groups; 2) metric invariance tests whether the strength of the relationship between items are the same for both groups; 3) scalar invariance tests whether the total scores result from similar responses to individual items across groups; 4) residual invariance tests whether scale items measure the latent constructs with the same amount of measurement error across groups. In order to compare total scores from a measure between different groups, evidence for scalar invariance must be shown across the groups, as this suggests that total scores on the measure consists of similar performance on individual items . Increase in RMSEA (>.015) and reduction in CFI (>.01) at each level were used as indicators of a significant degradation in fit, given that the chi-square statistic is strongly influenced by sample size .
Reliability and Validity
In each group, internal consistency was measured using Chronbach’s alpha for total scores. Spearman’s correlations, intraclass co-efficient, and ANOVA assessed test re-test reliability of SBQ-ASC total scores between time one and time two in each group. Spearman’s correlations also assessed convergent validity between the SBQ-ASC with the original version of the tool (SBQ-R), and other measures of autistic traits (AQ), anxiety (ASA-A), depression (PHQ-9), lifetime NSSI (item 6 of the SBQ-ASC), thwarted belongingness and perceived burdensomeness (INQ-15). Divergent validity was assessed using z-tests to compare the strength of the correlation coefficients. Specifically: a) whether the SBQ-ASC was more strongly correlated with autism relevant constructs (AQ, CAT-Q and ASA-A) compared to the original version of the tool (SBQ-R); b) whether the correlation between the SBQ-ASC with the original version of the tool (SBQ-R) was larger compared to other proximal risk markers for suicide (thwarted belongingness, perceived burdensomeness, depression and anxiety); and c) whether the SBQ-ASC was more strongly correlated with more proximal mental health risk markers for suicide (depression and anxiety) compared to more distal risk markers (autistic traits and camouflaging autistic traits).
Receiver Operating Curve (ROC) analysis was used to establish an indicative cut-off score for the SBQ-ASC discriminating those who have from those who have not attempted suicide in their lifetime (using item 1 of the SBQ-R as the criterion). Kruskal Wallis analyses compared the SBQ-ASC items between groups, and total scores between groups (with evidence of measurement invariance at the scalar or residual level). Significant main effects were followed up with Mann Whitney U to test focused comparisons with partial eta squared calculated as a measure of effect size. Chi-square analyses compared frequency of sub-questions probing characteristics of suicidality, NSSI and above cut-off scores between the groups, with phi calculated as an estimate of effect size for multiple group comparisons, and odds ratios calculated as a measure of effect size for focused comparisons, with an alpha level of p < .01 to correct for multiple comparisons.