Table 1 Pattern matrix for Study 1 following exploratory factor analysis for the 42 items selected to be retained for the final version of the CATI, together with internal consistency for each of the subscales (Cronbach alpha), total scale (Cronbach alpha stratified and McDonald’s omega hierarchical), and bifactors identified in Study 2 (Cronbach alpha)

Social interactions [SOC] (α = .93)

 Social interaction is easy for me*

 I generally enjoy social events*

 I find social interactions stressful

 Social occasions are often challenging for me

 I am confident and capable when meeting new people*

 In social situations, I try to avoid interactions with other people

 I find it difficult to make new friends

Communication [COM] (α = .83)

 I can tell how people feel from their facial expressions*

 Reading non-verbal cues (e.g. facial expressions, body language) is difficult for me

 I find it easy to sense what someone else is feeling*

 Metaphors or ‘figures of speech’ often confuse me

 I rarely use non-verbal cues in my interactions with others

 I have difficulty understanding the 'unspoken rules' of social situations

 I have difficulty understanding someone else’s point-of-view

Repetitive behaviour [REP] (α = .85)

 I often find myself fiddling or playing repetitively with objects (e.g. clicking pens)

 There are certain objects that I fiddle or play with that can help me calm down or collect my thoughts

 I often rock when sitting in a chair

 There are certain repetitive actions that others consider to be 'characteristic' of me (e.g. stroking my hair)

 I have a tendency to pace or move around in a repetitive path

 I engage in certain repetitive actions when I feel stressed

 I have certain habits that I find difficult to stop (e.g. biting/tearing nails, pulling strands of hair)

Social camouflage [CAM] (α = .86)

 Sometimes I watch people interacting and try to copy them when I need to socialise

 I look for strategies and ways to appear more sociable

 Before engaging in a social situation, I will create a script to follow where possible

 I rely on a set of scripts when I talk with people

 I try to follow certain 'rules' in order to get by in social situations

 I expend a lot of mental energy trying to fit in with others

 When interacting with other people, I spend a lot of effort monitoring how I am coming across

Cognitive rigidity [RIG] (α = .81)

 I like to stick to certain routines for every-day tasks

 I like my belongings to be sorted in certain ways and will spend time making sure they are that way

 There are certain activities that I always choose to do the same way, every time

 I often insist on doing things in a certain way, or re-doing things until they are ‘just right’

 I feel discomfort when prevented from completing a particular routine

 I like to arrange items in rows or patterns

 It annoys me when plans I have made are changed

Sensory sensitivity [SEN] (α = .80)

 I am over-sensitive to bright lighting

 I am sensitive to flickering lights

 I react poorly to unexpected loud noises

 There are times when I feel that my senses are overloaded

 I am over-sensitive to touch

 I am over-sensitive to particular tastes (e.g. salty, sour, spicy, or sweet)

 Sometimes the presence of a smell makes it hard for me to focus on anything else

Total scale (α = .97^a, ω^h = .83^b)

Social bifactor [SOC, COM, CAM] (α = .92)

Non-social bifactor [REP, RIG, SEN] (α = .90)