Understanding anxiety in adults on the autism spectrum: An investigation of its relationship with intolerance of uncertainty,sensory sensitivities and repetitive behaviours

Abstract

Anxiety is present in high rates in both children and adults on the autism spectrum. An increasing number of studies have highlighted the potentially important role that intolerance of uncertainty may have in anxiety for those on the spectrum, as well as their interrelationships with sensory sensitivities and repetitive behaviours. In response to a lack of studies involving adults, this study examined self-report survey data regarding intolerance of uncertainty, sensory sensitivities, repetitive behaviours and anxiety in a sample of 176 adults on the autism spectrum (mean age = 42). Intolerance of uncertainty and anxiety were both found to be elevated relative to non-autistic adults (N = 116) and significant, positive correlations were found between intolerance of uncertainty, anxiety, repetitive behaviours and sensory sensitivities in those on the spectrum. Intolerance of uncertainty was found to be a significant mediator between sensory sensitivities and anxiety, as well as between anxiety and insistence on sameness behaviours. These results were not sensitive to age. Intolerance of uncertainty is an important factor to be considered in the conceptualisation and management of elevated rates of anxiety for adults on the autism spectrum.

Keywords

adults anxiety intolerance of uncertainty repetitive behaviours sensory sensitivities

High rates of anxiety are consistently reported for individuals on the autism spectrum. Recent estimates suggest lifetime rates of anxiety to be between 50% and 65% for autistic individuals of all ages (Buck et al., 2014; Joshi et al., 2013; Lever & Geurts, 2016; Lugnegard, Hallerback, & Gillberg, 2011). In efforts to better understand anxiety in autism spectrum disorder (ASD), researchers have examined and conceptualised the involvement of different underlying mechanisms in the development and maintenance of anxiety in this population.

Most frequently discussed is the role of intolerance of uncertainty (IU), which is a broad, dispositional factor characterised by the tendency to react negatively to situations and events that are unforeseen and/or unpredictable (Buhr & Dugas, 2006). It is found to have strong associations with clinically significant anxiety disorders in the general population, including generalised anxiety and social anxiety (Buhr & Dugas, 2006; Carleton, 2012; Carleton, Norton, & Asmundson, 2007). IU is found to be elevated in autistic individuals relative to those not on the spectrum (Boulter, Freeston, South, & Rodgers, 2014; Maisel et al., 2016; Vasa, Kreiser, Keefer, Singh, & Mostofsky, 2018). It is significantly associated with anxiety for those on the spectrum and more specifically acts as a mediator between autism and anxiety in children and young adults (Boulter et al., 2014; Maisel et al., 2016; Neil, Olsson, & Pellicano, 2016). Qualitative studies investigating the experience of anxiety in young autistic adults have suggested ideas resonant of IU, such as ‘uncertainty/making decisions’ and ‘unexpected or sudden change’, to be pertinent sources of anxiety (Trembath, Germano, Johanson, & Dissanayake, 2012, p. 217).

A number of other factors related to the ASD phenotype have also appeared in the literature alongside IU and anxiety in ASD. These include sensory sensitivities and repetitive behaviours (Boulter et al., 2014; Neil et al., 2016; Wigham, Rodgers, & South, 2015), which have been alternatively referred to in literature as ‘sensory processing atypicalities’ and ‘restricted and repetitive behaviours’, respectively (Rodgers & Ofield, 2018). An emerging body of qualitative and quantitative studies consistently find interrelationships between these factors, IU and anxiety in autistic populations (Black et al., 2017; Gillott & Standen, 2007; Joyce et al., 2017; Lidstone et al., 2014; Mazurek et al., 2013; Rodgers, Glod, Connolly, & Mcconachie, 2012; South & Rodgers, 2017; Trembath et al., 2012; Uljarević, Lane, Kelly, & Leekam, 2016; Wigham et al., 2015).

Sensory sensitivities, both hyper- and hypo-sensitivity, have primarily been conceptualised as a contributor to anxiety. Accordingly, one qualitative study of anxiety in young autistic adults reported ‘sound and light sensitivities’ to be key sources of anxiety (Trembath et al., 2012, p. 217), while quantitative studies find more severe sensory sensitivities in those with higher anxiety (Gillott & Standen, 2007; Uljarević et al., 2016). Numerous studies also report a significant positive association between repetitive behaviours and anxiety in autistic individuals (Joyce et al., 2017; Lidstone et al., 2014; Wigham et al., 2015). Qualitative evidence suggests that repetitive behaviours may be strategies for coping with anxiety and may also exacerbate anxiety (Joyce et al., 2017).

Based on existing qualitative and quantitative literature for children and young adults on the autism spectrum, Figure 1 presents an exploratory framework for the possible pathways involving these variables. This framework is closely based on the 2017 review by South and Rodgers (2017) regarding existing literature on the sensory, emotional and cognitive contributions to anxiety in ASD (p. 3). A number of studies have found evidence for components of this framework and offered similar models, all involving IU as a mediator of autism characteristics (sensory sensitivities, emotional processing difficulties) and anxiety (Boulter et al., 2014; Cai, Richdale, Dissanayake, & Uljarević, 2018; Wigham et al., 2015). Overall, it may be theorised that sensory sensitivities may exacerbate anxiety in autistic individuals via an aversion or reaction to uncertainty or unpredictability of the sensory aspects of an event or situation. Furthermore, repetitive behaviours may be both a manifestation of, and a coping mechanism for, anxiety, primarily by means of reintroducing certainty, control or consistency to the anxious situation. This would suggest that increasing an individual’s tolerance of uncertainty, which may reduce their anxiety, will likely have effects on their repetitive behaviours.

Figure 1.

Exploratory framework for possible pathways involving IU in the relationship between anxiety and key autism features.

While consistent evidence builds in this area, studies have predominantly focused on children and adolescents and there remains a lack of studies modelling these relationships in adults. One published study of 76 autistic adults and 75 community comparisons attempted to model anxiety in autistic adults and its relationship with IU, alexithymia (difficulty in describing internal emotional states) and emotional acceptance (Maisel et al., 2016). This study reported higher rates of IU and anxiety in autistic adults. No studies have attempted to model the relationships between IU, anxiety, sensory sensitivities and repetitive behaviours for adults over 25 years of age.

We also know little about the presentation of these factors in adulthood. A handful of studies report a general decrease in restricted, repetitive behaviours with age (Chowdhury, Benson, & Hillier, 2010; Esbensen, Seltzer, Lam, & Bodfish, 2009; Levy & Perry, 2011). A lack of evidence exists regarding the developmental trajectory of sensory sensitivities and IU for autistic individuals, though both are reported at high rates in adulthood (Crane, Goddard, & Pring, 2009; Robertson et al., 2018). It will be important to develop a better framework for understanding the presentation and interrelationships between these factors in autistic adults, in order to best develop targeted interventions for anxiety.

This study aimed to examine the relationships between IU, anxiety, sensory sensitivities and repetitive behaviours (referred to collectively as ‘the study variables’ herein) in autistic adults. Specific aims included the following:

To compare the levels of the IU, anxiety, sensory sensitivities and repetitive behaviours between autistic and non-autistic adults;

To examine relationships between these study variables and age; and

To examine the involvement of IU as a mediator between the sensory sensitivities, repetitive behaviours and anxiety, based on existing findings involving younger individuals on the spectrum (Figure 1).

It was hypothesised that autistic adults would have higher scores on the study variables relative to those from the general population, and that the relationships between these study variables as visualised in Figure 1, would be replicated for an adult sample. Given restricted available evidence, the only hypothesised relationship between these study variables and age was an expected decrease in both types of restricted, repetitive behaviours with age.

Methods

Participants

Participants were adults on the autism spectrum and a community comparison sample of non-autistic adults who participated in the first wave of the Australian Longitudinal Study of Adults with Autism (ALSAA) funded by the Cooperative Research Centre for Living with Autism (Autism CRC). The ALSAA is a questionnaire-based study including a comprehensive range of standardised and non-standardised measures of health, functioning and participation for autistic adults in Australia (ages 25 and over). Ethical approval for the study has been granted by the University of New South Wales Human Research Ethics Committee. More detail about the protocol for the ALSAA, including its inclusive research approach, is described elsewhere (Arnold et al., 2019).

ALSAA participants were recruited via promotional materials, including both paper and online flyers, which were dispersed via mail and email to a range of autism-specific organisations, aged-care and disability service providers, community newspapers, universities, medical centres and relevant online communities. Participants expressed interest via email, telephone or an online form. Interested participants were screened for eligibility before being sent either a paper questionnaire or a personalised link to the online questionnaire on Qualtrics (2018), a web-based survey platform. Both versions of the questionnaire were preceded by an information statement and consent form specifying that completing and returning the questionnaire implied informed consent.

Participants for this study included 214 autistic adults from the ALSAA who self-reported a clinical diagnosis of ASD with a score over 65 on the Autism-Spectrum Quotient-Short (AQ-short; Hoekstra et al., 2011) and 146 non-autistic adults. Inclusion criteria included being 25 years old or older, living in Australia and sufficient English proficiency to complete the questionnaire. Of this initial sample, 38 (18%) autistic participants and 30 (20%) non-autistic participants had incomplete data for the study variables. Chi-square tests and independent samples t-tests revealed no association between missing observations and being autistic (p = 0.40), age (p = 0.21), gender (p = 0.63) or scores on measures of IU (p = 0.23), anxiety (p = 0.96) or sensory sensitivities (p = 0.29). Therefore, only 176 autistic adults and 116 non-autistic adults with complete data were considered for analysis.

Measures

Sample characterisation

Information regarding age, gender, type of autism diagnosis and current anxiety diagnosis was collected as part of the ALSAA. Self-reported intellectual disability status and severity was also collected (none, mild, moderate, severe, profound). In addition, data from four self-reported measures were used in this study.

Intolerance of uncertainty

The intolerance of uncertainty scale-12 (IUS-12; Carleton et al., 2007) is an abridged version of the 27-item IU scale. Participants rate items regarding aversion and reactions to change or unforeseen events (e.g. ‘Unforeseen events upset me greatly’, ‘When it is time to act, uncertainty paralyses me’) on a 5-point Likert-type scale ranging from 1 ‘Not at all characteristic of me’ to 5 ‘Entirely characteristic of me’. Higher scores indicate higher levels of IU. In non-autistic adults, the scale has been found to have good test–retest reliability and construct validity against other symptom measures of worry, depression and anxiety (Buhr & Dugas, 2006). Internal reliability for this sample was strong for both samples (Cronbach’s α = 0.88–0.92).

Sensory sensitivities

The Glasgow Sensory Questionnaire (GSQ; Robertson & Simmons, 2012) is a measure of sensory sensitivities commonly reported by those on the spectrum. The 42-item measure contains an even split of questions measuring hyper- and hypo-sensitivity, with six questions dedicated to each of the seven common sensory modalities: visual; auditory; gustatory; olfactory; tactile; vestibular and proprioceptive. Participants respond to the frequency of certain sensory experiences on a 5-point scale (0 – Never to 4 – Always). Scores can be computed for total sensory sensitivity, hyper- and hypo-sensitivity, with higher scores indicating greater sensitivity. Internal reliability for the autistic (α = 0.93) and non-autistic adult (α = 0.91) samples were high.

Anxiety

The American Psychiatric Association’s (2013) Severity Measure for Generalised Anxiety Disorder (GAD) provides a dimensional assessment of GAD in adult populations as defined in the fifth revision of the Diagnostic and Statistical Manual of Mental Disorders. The measure consists of 10 items describing a state indicative of clinical GAD as of the past 7 days. These items describe a range of visceral, intellectual and behavioural reactions that evidence the state of anxiety (e.g. ‘I have felt moments of sudden terror, fear or fright’, ‘I sought reassurance from others due to worries’) Participants rate how frequently they experienced these items during the past 7 days on a 5-point scale (0 – never to 4 – all of the time). Internal reliability for the autistic (α = 0.90) and non-autistic adult (α = 0.90) samples were high. The scale has demonstrated good sensitivity (0.87) and specificity (0.73) and strong convergent and discriminant validity when compared with the previously validated 7-item scale (r = 0.80, p < 0.01; Beesdo-Baum & Knappe, 2012). A cutoff score of 14 indicates clinically significant anxiety.

Repetitive behaviours

Restricted and repetitive behaviours were measured via the Repetitive Behaviours Questionnaire-2 for adults (RBQ-2A; Barrett et al., 2015) which is an adaptation of the Repetitive Behaviour Questionnaire-2 (Leekam et al., 2007) for use in adults. The RBQ-2A consists of 20 items divided into five sections. Items in three of the sections are rated on a 4-point scale, while the remaining items use a 3-point scale. Higher scores indicate higher likelihood or frequency of engaging in restricted and repetitive behaviours. Previous work has found the measure to consist of two factors: repetitive motor behaviours and insistence on sameness. Insistence on sameness captures a preference for uniformity or consistency (e.g. ‘like to arrange items in rows or patterns’), while repetitive sensory–motor behaviours refer to behaviours which are repeated (e.g. ‘spin yourself around and around’). The RBQ-2A has displayed good psychometric properties in a sample of autistic adults (Barrett et al., 2015). Internal reliability for the autistic (α = 0.90) and non-autistic adult (α = 0.78) samples were high in this study.

Statistical analysis

Descriptive statistics, such as means, standard deviations and frequencies, were used to examine demographic characteristics and scores on the study variables for autistic adults and non-autistic adults. Chi-square tests and independent t-tests with Bonferroni-adjusted significance levels were run to examine differences in the study variables between autistic and non-autistic adults. Pearson’s correlations were used to explore associations between the study variables, age and gender. A series of linear regressions were used to further investigate significant associations between the study outcome variables and age. STATA statistical software version 14 (StataCorp, 2015) was used for the above analyses.

In order to replicate the relationships found between the study variables for existing studies examining younger autistic groups, five mediation analyses were conducted. All mediation analyses were conducted using the PROCESS macro in SPSS (Statistical Package for Social Science, version 22). Bootstrapping (1000 resamples) with bias-corrected and accelerated 95% confidence intervals (BCa) used to adjust for measurement error. The Sobel test was used to determine the significance of the mediation effect.

First, in order to replicate findings of past studies regarding IU as a key mediator of anxiety in autistic individuals, a mediation analysis was conducted with IU as a mediator of autism diagnosis and anxiety for the entire sample (both autistic and non-autistic adults; Figure 2(a); Neil et al., 2016; Wigham et al., 2015).

Figure 2.

(a) IU as a mediator between ASD diagnosis and anxiety, (b) IU as a mediator between hyper- and hypo-sensitivity and anxiety in autistic adults and (c) IU as a mediator between anxiety and repetitive behaviours (both repetitive sensory–motor behaviours and insistence on sameness) in autistic adults.

In addition to this, the mediating role of IU between the study variables and anxiety was tested for the autistic group only (Figure 2(b) and (c)). These were based on the exploratory framework presented in Figure 1.

Inclusive research approach

Following initial interpretation of the findings by the researchers, further insight and feedback regarding these findings and interpretations were sought from six autistic adults who were part of the ALSAA advisory panel. A plain English summary of the findings and possible interpretations were emailed to these advisors, who were given 1 month to give their feedback. Feedback could be given via reply email, video, telephone or video conferencing. Autistic advisors were offered reimbursement for their time. Insights gained from the feedback were incorporated into the discussion of the findings.

Results

Participant demographics are available in Table 1. Our sample included a total of 292 participants (176 autistic and 116 non-autistic adults). The mean age of autistic and non-autistic adults was comparable. Approximately 40% of autistic adults were male, while the non-autistic adult sample was predominantly female. A significantly higher proportion of autistic adults (57.7%) reported having a current diagnosis of anxiety than non-autistic adults (14.0%). Five participants had one item from the AQ missing and one participant had two items from the AQ missing. Total scores were imputed for these six participants by adding their total score to: (mean score for non-missing items × number of missing items). Significant differences were found on the GAD scores between those with and without self-reported anxiety diagnosis for both autistic (p < 0.001) and non-autistic adults (p < 0.001).

Table 1.

Demographic characteristics of autistic participants and non-autistic adults.

	ASD (N = 176)	Non-autistic adults (N = 116)	p
Age, M (SD; range)	41.7 (12.8; 25–78)	43.0 (13.2; 25–77)	0.390
Male, N (%)	67 (39.9)	25 (21.6)	<0.001
Intellectual disability, N (%)	8 (4.5)	1 (0.8)	0.092
AQ-short score, M (SD)	89 (9.42)	55 (11.6)	<0.001
Current anxiety diagnosis, N (%)	98 (57.7)	16 (14.0)	<0.001

ASD: autism spectrum disorder; PDD NOS: pervasive developmental disorder not otherwise specified.

‘Other’ diagnoses include autistic disorder, PDD NOS and infantile autism.

Between groups differences

Table 2 presents the scores on measures of IU, anxiety, sensory sensitivity and repetitive behaviours for autistic adults in comparison with non-autistic adults. The AQ scores for the non-autistic comparison sample were notably high. This may reflect the self-selecting sample, which may have included extended family and friends of those on the spectrum. Studies involving relatives of those on the spectrum suggest a significant subset to display ‘milder’, subclinical autism traits – described by the term ‘broad autism phenotype’ (Wallace, Budgett, & Charlton, 2016). Despite this, and in line with previous studies, autistic adults reported significantly greater levels of all variables of interest compared with non-autistic adults.

Table 2.

Differences between autistic participants and non-autistic adults on scores for IU, anxiety and sensory sensitivities.

	Autistic (N = 176)	Non-autistic adults (N = 116)	Possible score range	p
IU M (SD; range)	41.8 (10.4; 14–60)	24.3 (7.7; 12–50)	12–60	<0.001*
Total sensitivity M (SD; range)	78.4 (25.7; 1–147)	34.9 (17.3; 0–107)	0–168	<0.001*
Hypersensitivity M (SD; range)	43.2 (15.1; 0–79)	19.8 (9.7; 0–66)	0–84	<0.001*
Hyposensitivity M (SD; range)	33.2 (11.4; 1–65)	14.4 (8.4; 0–39)	0–84	<0.001*
Anxiety M (SD; range)	14.5 (8.4; 0–38)	5.6 (5.5; 0–29)	0–40	<0.001*
Total repetitive behaviours M (SD; range)	36.7 (8.4;19–56)	22.6 (3.5; 19–33)	19–57	<0.001*
Repetitive sensory–motor M (SD; range)	1.7 (0.49; 1–2.8)	1.1 (0.15; 1–1.8)	1–3	<0.001*
Insistence on sameness M (SD; range)	2.1 (0.48;1–3)	1.2 (0.26;1–2)	1–3	<0.001*

SD: standard deviation.

p < 0.0063 (Bonferroni-adjusted).

Correlations

Pearson’s correlations between age, gender and study variables are displayed for autistic and non-autistic adults in Tables 3 and 4, respectively. For autistic adults, moderate to strong positive correlations were found between the study variables (r = 0.27–0.73). In general, the strongest associations were found among repetitive behaviours (total and subscales) and sensory sensitivities (total and subscales; r = 0.58–0.73). Strong associations were found within scores on the total and subscale scores for sensory sensitivities (r = 0.76–0.96). For repetitive behaviours, a smaller association was found between the two subscales, reflecting the distinct two-factor structure of this scale reported in previous studies (Barrett et al., 2015). IU had the strongest associations with anxiety (r = 0.55) and insistence on sameness (r = 0.53), and weaker associations with hyposensitivity (r = 0.27) and repetitive sensory–motor behaviours (r = 0.34). Regarding demographic variables, age of autistic adults was found to have significant negative associations with hyposensitivity (r = −0.26), total repetitive behaviours (r = −0.28) and repetitive sensory–motor behaviours (r = 0.38). Female gender had moderate positive associations with total- and hyper-sensitivity.

Table 3.

Pearson’s correlations between age, gender and study variables for autistic participants (N = 176).

	Age	Gender^a	IU	Anxiety	Total ss	Hyper-ss	Hypo-ss	Total repetitive behaviours	Repetitive sensory–motor	Insistence on sameness
Age	1
Gender^a	−0.15	1
IU	−0.13	0.09	1
Anxiety	−0.19	0.12	0.55*	1
Total ss	−0.20	0.32*	0.34*	0.38*	1
Hyper-ss	−0.14	0.40*	0.36*	0.38*	0.96*	1
Hypo-ss	−0.26*	0.16	0.27*	0.33*	0.92*	0.76*	1
Repetitive behaviours	−0.28*	0.15	0.50*	0.49*	0.73*	0.65*	0.73*	1
Repetitive sensory–motor	−0.36*	0.16	0.34*	0.44*	0.67*	0.58*	0.69*	0.84*	1
Insistence on sameness	−0.21	0.13	0.53*	0.46*	0.64*	0.59*	0.63*	0.95*	0.65*	1

Positive indicates female gender.

p < 0.005 (Bonferroni adjustment).

Table 4.

Pearson’s correlations between age, gender and study variables for non-autistic adult participants (N = 116).

	Age	Gender^a	IU	Anxiety	Total ss	Hyper-ss	Hypo-ss	Total repetitive behaviours	Repetitive sensory–motor	Insistence on sameness
Age	1
Gender^a	−0.03	1
IU	−0.15	−0.01	1
Anxiety	−0.04	0.14	0.65*	1
Total ss	0.04	−0.11	0.30*	0.28*	1
Hyper-ss	0.03	0.06	0.32*	0.28*	0.93*	1
Hypo-ss	0.04	−0.25	0.26*	0.23	0.93*	0.73*	1
Total repetitive behaviours	−0.13	−0.09	0.43*	0.42*	0.59*	0.48*	0.62*	1
Repetitive sensory–motor	−0.17	−0.07	0.37*	0.39*	0.47*	0.41*	0.45*	0.60*	1
Insistence on sameness	−0.10	−0.08	0.37*	0.35*	0.48*	0.37*	0.53*	0.94*	0.32*	1

Negative indicates male gender.

p < 0.005 (Bonferroni adjustment).

A similar pattern of associations was found for non-autistic adults. Weak to moderate correlations were found between study variables (r = 0.26–0.65), the highest association being that between IU and anxiety (r = 0.65). Similar to the autistic group, high associations were found between the total score and subscale scores for sensory sensitivities (r = 0.73–0.93) and a smaller association between the two RBQ-2A subscales. No significant correlations were found between these variables and demographic variables of age and gender.

Associations between age and study variables

Given the significant associations found between age and the study variables presented in Table 3, a series of linear regressions were conducted with age as the predictor of hyposensitivity, repetitive behaviours and repetitive sensory–motor behaviours. Hyposensitivity (b = −0.24, 95% CI = −0.37, −0.11, p = 0.001), total repetitive behaviours (b = −0.18, 95% CI = −0.28, −0.08, p = <0.0001) and repetitive sensory–motor behaviours (b = −0.14, 95% CI = −0.02, −0.01, p < 0.0001) were all predicted by age.

Mediation models

Results of the five mediation models (Figure 2(a) to (c)) are presented on separate rows in Table 5. Of interest are the indirect effects for each of the mediation models, with a significant indirect effect (p < 0.05) suggesting that the observed relationship between the predictor and outcome variable is at least partially explained by the mediator (IU). IU was found to be a significant mediator in all models except in the relationship between anxiety and repetitive sensory–motor behaviours (Model 4). The lack of a significant direct effect in Model 1 indicates that the relationship between autism status and anxiety may be fully accounted for (mediated by) IU. For Models 2–5, a sensitivity analysis was conducted for age using two age groups (<40 and 40+; N = 87 and 88). No differences were found for any of the models in the two age groups regarding the significance of IU as a mediator.

Table 5.

Direct and indirect effects from the mediation models involving IU.

Model	Sample	Predictor	Mediator(s)	Outcome	R square	Total effect (B), SE [95% CI]	Direct effect (B), SE [95% CI]	Indirect effect (B), SE [95% CI], p
1	ASD & non-autistic adults	ASD	IU	Anxiety	0.51	17.15, 1.15[14.90, 19.41]***	0.80, 0.96[−1.08, 2.69]	7.77, 0.82[6.19, 9.40]***
2	ASD	Hypersensitivity	IU	Anxiety	0.38	0.22, 0.04[0.13, 0.29]***	0.11, 0.04[0.04, 0.18]***	0.10, 0.02[0.06, 0.15]***
3	ASD	Hyposensitivity	IU	Anxiety	0.37	0.26, 0.05[0.16, 0.36]***	0.15, 0.05[0.05, 0.24] **	0.11, 0.03[0.05, 0.18]***
4	ASD	Anxiety	IU	Repetitive sensory–motor	0.30	0.03, 0.004[0.02, 0.03]***	0.02, 0.005[0.01, 0.03]***	0.005, 0.003[0.001, 0.01]
5	ASD	Anxiety	IU	Insistence on sameness	0.34	0.03, 0.004[0.02, 0.04]***	0.01, 0.004[0.006, 0.023]**	0.013, 0.003[0.008, 0.019]***

CI: bootstrapped confidence interval; B: beta coefficient; ASD: autism spectrum disorder.

Models 2–5 were adjusted for age and gender.

p < 0.01; ***p < 0.001.

Discussion

Overall, these findings support the central role of IU in the interrelationships between anxiety and autism traits for autistic adults aged 25 and older. Significant positive correlations between these study variables replicate similar findings in children and younger adults (Neil et al., 2016; Wigham et al., 2015). Also consistent with the findings in children (Boulter et al., 2014; Neil et al., 2016), adults on the spectrum had significantly higher IU and anxiety than those not on the spectrum. Self-reported rates of anxiety were high in this sample (58%) and similar to those in published studies of autistic adults (Buck et al., 2014; Joshi et al., 2013). Sensitivity analyses suggest that, although a decline in repetitive behaviours is evident, these relationships between the study variables hold for both older (40+) and younger (<40) adults.

As with existing studies, a decline in repetitive behaviours was found with age (Levy & Perry, 2011). Specifically, a significant decrease was found in sensory–motor behaviours, hyposensitivity and repetitive behaviours overall. Although statistically significant, the effect sizes for hyposensitivity and total repetitive behaviours are small when considered in light of the scales, that is, two autistic adults 10 years apart in age are expected to have a difference of 2.4 on their hyposensitivity as measured by the GSQ (possible score range 0–128). The observed decline in repetitive sensory–motor behaviours was arguably more substantial. Autistic individuals may learn with experience or as a result of targeted therapies, to ‘mask’ these behaviours to appear more socially acceptable (Cage, Di Monaco, & Newell, 2018; Dean, Harwood, & Kasari, 2017). Despite a decline, these behaviours are still apparent in adulthood.

Given the heterogeneity of the autism spectrum, further work is needed to clarify the reasons for this apparent decline as well as whether and why certain adults may have reduced repetitive behaviours while others do not, especially in older adults for whom research is scarce. An alternative explanation may be that autistic adults may develop other means of fulfilling their need for repetition through other activities. Experiences of behavioural intervention may also contribute to this decline. Taken together, these findings agree with existing beliefs that despite evidence of some reduction in traits over time, most autistic individuals remain on the spectrum in adulthood (Howlin & Magiati, 2017; Levy & Perry, 2011).

The first mediation analysis was able to replicate findings in younger samples regarding the central role of IU in the anxiety of those on the spectrum in comparison with those from the general population (Boulter et al., 2014; Neil et al., 2016). Further, the results suggest IU to be an important mechanism underlying the relationship between anxiety and sensory sensitivities (both hyper- and hypo-sensitivity), as well as between anxiety and insistence on sameness. Of note was our finding of a lack of a mediation effect of IU on the relationship between anxiety and repetitive sensory–motor behaviours. In line with this, existing literature has converged on ideas that directly link repetitive sensory–motor behaviours with sensory sensitivities (e.g. modulation of sensory arousal) and anxiety (e.g. coping with anxiety; Joyce et al., 2017; Leekam et al., 2007).

Implications

For adults on the autism spectrum, anxiety and anxiety-provoking experiences are common in day-to-day life (Cage et al., 2018; Hare, Gracey, & Wood, 2016). These may be the effect of a range of factors, including a lack of self-efficacy in navigating healthcare systems (Maloret & Scott, 2018; Nicolaidis et al., 2013), where a range of barriers such as lack of clinician training exist (Warfield, Crossman, Delahaye, Der Weerd, & Kuhlthau, 2015), as well as negative experiences in employment, including underemployment and employment instability (Eaves & Ho, 2008; Tobin, Drager, & Richardson, 2014). While past studies present important suggestions for environment and system-level changes to improve the mental health of autistic adults, the present findings also present the potential for addressing an individual-level factor, IU, as a method of managing elevated anxiety levels in this population. This may offer a more sustainable method of decreasing anxiety than efforts to minimise uncertainty in the autistic individual’s life environment.

The findings suggest that interventions for reducing anxiety in autistic adults should target IU, which has been the target of interventions that have successfully reduced anxiety severity in non-autistic adults and children (Boswell, Thompson-Hollands, Farchione, & Barlow, 2013; Buhr & Dugas, 2009; Dugas et al., 2010). In autistic individuals, IU is receiving increased attention. One study of autistic children reported IU to impact treatment response to modified cognitive behavioural therapy, with higher levels of pre-intervention IU predicting higher anxiety both pre- and post-intervention (Keefer et al., 2017). One manualised treatment programme targeting IU has been examined for a small sample of autistic adolescents (Rodgers et al., 2017) and adults (Rodgers, Herrema, Honey, & Freeston, 2018) with promising results regarding effectiveness and acceptability. The present findings suggest that such interventions may have a follow-on effect on insistence on sameness behaviours in autistic adults and may have additional benefits for those with sensory sensitivities above interventions that do not address IU.

In developing such interventions, it is important to consider the practical, psychological and communicative factors implicated in delivering effective mental healthcare to adults on the spectrum. Emerging research suggests the presence of alexithymia, a difficulty in recognising and describing one’s own emotional states, is increased in autistic individuals and is directly related to anxiety (Maisel et al., 2016). In a similar vein, the presence of psychiatric comorbidities, such as attention-deficit hyperactivity disorder (ADHD) in this population (Lever & Geurts, 2016) may further complicate the delivery of interventions. Accordingly, interventions should be appropriately designed to account for such differences and careful consideration of the effect of these conditions on the therapy should be considered.

More broadly, the comfort and effectiveness of the intervention experience may be improved through accommodations such as involvement of a support person and allowing diverse methods of communication (Nicolaidis et al., 2015; Robertson et al., 2018). One recent study observed psychotherapy outcomes in a college counselling setting for 76 autistic adults and concluded that while desirable outcomes may be achieved with psychotherapy, autistic adults may require more extended treatment periods and have unique communication needs compared with non-autistic adults (Anderberg et al., 2017). Such findings are important for considering the impact of session caps and counselling techniques for autistic populations.

Further, it is important to acknowledge the heterogeneity in the precipitators and presentation of sensory sensitivities, repetitive behaviours and anxiety in autistic individuals (Barrett et al., 2015; South & Rodgers, 2017; Uljarević et al., 2017). Autism is a spectrum condition, and qualitative literature also attests to the variety of experiences related to anxiety that autistic individuals are exposed to (Robertson et al., 2018). As such, a one-size-fits-all approach to intervention will be limited in its effect. Further work, especially that which focuses on the different trajectories of these factors into middle and late adulthood, will be useful for targeting interventions appropriately.

Limitations and future work

This study is the first examination of the theoretical relationships between IU, anxiety, sensory sensitivities and repetitive behaviours for adults on the autism spectrum. The findings are most representative of those on the spectrum without intellectual disability. At present, findings are inconsistent and sparse regarding the prevalence of anxiety, and the prevalence of other study variables in those with intellectual disability versus without (Crane et al., 2009; Sterling, Dawson, Estes, & Greenson, 2008; van Steensel, Bogels, & Perrin, 2011) with a noted lack of studies exploring the former (Rodgers & Ofield, 2018). Further studies focusing on those with intellectual disability are needed.

These findings also rely on the accurate self-reporting of symptoms by ALSAA survey participants. Self-report is arguably most suitable for measuring repetitive behaviours, sensory processing and IU in adults. Regarding anxiety, given the use of psychometrically sound scales and no need for distinction between clinical or sub-clinical levels of anxiety, self-report was also deemed sufficient for the present analysis. Clinically assessed or validated anxiety scores may be of interest for future studies, particularly those that investigate the effects of intervention.

Clinical validation of the autism sample was not feasible for the present nationwide longitudinal study and the findings are thus limited by the self-reporting of autism status. Only self-reported intellectual disability status was collected. Although there was reliance on self-report, participants were required to explicitly indicate their receipt of a formal diagnosis at the commencement of the survey, with other options including ‘I do not have a formal diagnosis but think I have autism’. Autistic adults were then required to report which year they received this diagnosis, the specific diagnosis they received, and the name and profession of the diagnosing clinician(s). This approach, combined with the time-intensive (2–3 h) nature of the survey and absence of participant reimbursement, minimised the likelihood of participation by those who did not meet inclusion criteria for the study. Furthermore, participants who expressed that they did not receive a formal diagnosis, and those with scores below the AQ cutoff, were not included in this study. This method of self-report coupled with the AQ-short was deemed appropriate by the autistic advisory panel, and has also been argued as appropriate for research that may be generalised to all adults who identify as autistic (Bal & Lounds Taylor, 2019). This approach allows for the ‘lost generation’ of middle-aged and older autistic adults (Lai & Baron-Cohen, 2015) who experience significant barriers to diagnosis.

A gender bias towards females was apparent in this sample, which is contrary to the established gender ratio for autistic samples (Halladay et al., 2015). This is a commonly evidenced phenomenon in online research with self-selecting samples (Guo, Kopec, Cibere, Li, & Goldsmith, 2016) and has been true of other online survey studies involving those on the spectrum (Gilmour, Schalomon, & Smith, 2012; Nicolaidis et al., 2013). Studies have inconsistently reported gender differences regarding sensory sensitivities and repetitive behaviours (Antezana et al., 2019; Bitsika, Sharpley, & Mills, 2018), and further research may be useful in understanding their presentation and relationship to anxiety. The present analyses were adjusted for the effects of gender.

Moving forward theoretically, studies that are experimental in nature with improved ecological validity are needed in order to disentangle the causal relationships between the study variables. Importantly, to increase the validity of such work, focused investigations of the psychometric properties of IU and anxiety measures, and their validation for use with autistic adults, are needed. This study was a cross-sectional examination of the role of IU in the present levels of anxiety and autism characteristics in the sample. History of involvement in therapeutic interventions for anxiety, and relevant medication use was not examined. Future studies, especially those that may consider the effect of interventions for anxiety should consider the effect of medication use and other therapies.

Finally, there will be a wealth of lived experience on the part of autistic adults and carers who have experienced these symptoms over many years, developed their own coping mechanisms and experienced a range of therapies and health systems over their lifetimes. These will be important to consider for designing and implementing effective, appropriate and accessible interventions for the reduction of anxiety in future.

Conclusion

IU is an important factor to be considered in the conceptualisation and management of elevated rates of anxiety for adults on the autism spectrum, as is the case for children and young adults. Sensory sensitivities and repetitive behaviours are also implicated in anxiety. Continued development of targeted interventions for managing IU in autistic adults will be useful for managing anxiety. Future work should involve those with lived experience, and should focus on the validation of measures for autistic adults as well as experimental investigations of the causal relationships between study variables. These will help inform the development and adaptation of interventions for adults on the spectrum.

Supplemental Material

AUT868907_Lay_Abstract – Supplemental material for Understanding anxiety in adults on the autism spectrum: An investigation of its relationship with intolerance of uncertainty, sensory sensitivities and repetitive behaviours

Supplemental material, AUT868907_Lay_Abstract for Understanding anxiety in adults on the autism spectrum: An investigation of its relationship with intolerance of uncertainty, sensory sensitivities and repetitive behaviours by Ye In (Jane) Hwang, Samuel Arnold, Preeyaporn Srasuebkul and Julian Trollor in Autism

Footnotes

Acknowledgements

We are grateful for the contribution and participation of all adults on the autism spectrum and their carers involved in the ALSAA.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This work was supported by the Cooperative Research Centre for Living with Autism (Autism CRC), established and supported under the Australian Government’s Cooperative Research Centres Program.

ORCID iD

Ye In (Jane) Hwang

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