Unheard Voices: A Systematic Literature Review of Studies Using Self-Report Methods to Gather the Perspectives of Autistic Adults with Intellectual Disability

Abstract

Background:

Autistic individuals with intellectual disability remain significantly underrepresented in research, particularly in studies that capture their lived experiences through self-report methods. This systematic review critically examines existing research employing self-report methods in studies where the focus is on autistic adults who have intellectual disability, evaluating the scope of this research, the self-report measures used, and the extent of community involvement.

Methods:

We conducted a systematic literature review in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. We searched PsycINFO, Scopus, MEDLINE, ERIC, CINAHL, and Web of Science for studies published between January 2013 and July 2023. We conducted a narrative synthesis to evaluate study characteristics, the use of self-report measures, and the involvement of autistic individuals and their communities in the research process.

Results:

Only 14 studies met the inclusion criteria. The majority focused narrowly on evaluating interventions, with little attention paid to exploring the broader lived experiences of autistic adults with intellectual disability. We identified 24 self-report measures, yet only two had been validated specifically for individuals with intellectual disability. Furthermore, less than half of the remaining measures reported the use of any adaptations or accommodations such as visual aids or simplified language. Alarmingly, community involvement was almost entirely absent, with no studies incorporating co-production with autistic individuals.

Conclusions:

This review reveals a critical and urgent need for autism research that genuinely includes and represents autistic adults with intellectual disability. It is imperative that future research prioritizes the development and rigorous validation of accessible self-report tools, expands its focus beyond intervention outcomes, and commits to the meaningful involvement of autistic individuals throughout the research process. Addressing these profound gaps is essential for building an equitable and inclusive body of knowledge that fully recognizes and responds to the diverse experiences within the autistic community.

Community Brief

Why is this an important issue?

Autistic people with intellectual disabilities are often left out of research that tries to understand the experiences of autistic individuals. Because of this, their unique needs and challenges might not be fully understood or considered when creating support services. Most research relies on the opinions of parents, teachers, and clinicians instead of hearing directly from autistic people, especially those with intellectual disabilities.

What is the purpose of the review?

This study aimed to look at research that directly asks autistic adults with intellectual disabilities about their own experiences. The researchers wanted to see how much research exists, what tools have been used to collect views, and whether autistic individuals and their communities are involved in the research process.

What did the researchers do to review the literature?

The researchers reviewed studies from 2013 to 2023 that focused on autistic adults with intellectual disabilities. They looked at how the studies were done, the tools used to gather information, and how much autistic individuals were involved in the research.

What studies did the authors find?

The review found 14 studies that met the criteria. Most of these studies were aimed at evaluating specific programs or treatments rather than exploring broader life experiences. Out of 24 tools used, only 2 were specifically designed for people with intellectual disabilities. There was very little involvement of autistic individuals in shaping the research. Many studies did not explain how they made the tools easy to use for people with intellectual disabilities.

In summary, what did those studies show?

This study shows that there is still a big gap in research that includes autistic adults with intellectual disabilities. More work is needed to create better tools for gathering their views, and they should be more involved in the research process to make sure their voices are heard.

What are the remaining gaps in the literature?

With just 14 studies identified that used self-report measures with autistic adults, it is clear that we need to know more about their views on a range of areas especially their support needs and priorities, and how to improve their quality of life, from their perspective.

Based on this review, what do the authors recommend?

Future research should include these adults’ own perspectives to understand their needs and priorities better. This means using methods that allow them to share their experiences directly instead of relying mainly on caregivers or others to speak on their behalf. Researchers also need to use self-report measures more often, especially for topics such as quality of life and well-being, where it’s essential to capture the person’s own viewpoint.

Introduction

Autistic people have historically had limited representation in autism research, with studies predominantly relying on reports from informants such as parents, teachers, and clinicians.¹ While parental reports can provide valuable insights, they often do not capture the full scope of autistic individuals’ experiences, as there can be significant differences between the perspectives of autistic people and their parents.^2–5 Roche et al.⁶ underscore the necessity of including diverse stakeholder perspectives in autism research, emphasizing that research priorities often differ between researchers, parents, and autistic individuals themselves. Their systematic review reveals a significant preference among stakeholders for applied research that brings real-world benefits to the autism community, further highlighting the importance of engaging directly with autistic individuals as research participants to ensure that research outcomes are relevant and impactful.

Historically, self-report data from autistic individuals have been primarily collected using verbal methods such as interviews and focus groups^7,8 or surveys and questionnaires.^9–13 This means that research studies that utilize self-report data do not capture the perspectives of all autistic individuals due to poor accessibility. Although there have been recommendations made as to how data collection methods could be more accessible to the diverse autistic community¹⁴ and a call for a broader range of participation options such as written rather than verbal responses,^15,16 reviews continue to highlight a significant lack of self-report data and an underrepresentation of autistic individuals with intellectual disability across autism research.^8,17–20 This underrepresentation is particularly troubling given recent estimates that approximately 31%–50% of autistic people also have an intellectual disability.^21,22 Despite these statistics, their inclusion in autism research remains disproportionately low. Russell et al.²³ conducted a meta-analysis that revealed a significant selection bias against individuals with intellectual disability across all fields of autism research, which was attributed to challenges in recruiting and retaining participants with more severe difficulties, ethical concerns around capacity to consent, and the lack of suitable research instruments for this population. They found that 94% of participants in the reviewed studies did not have an intellectual disability, highlighting a profound gap in representation.

There has also been an increasing recognition of the value of community involvement in autism research that goes beyond research participation.^24–26 Community involvement includes engagement with community members (e.g., autistic people, their parents and family members, clinicians, and other service providers) across all stages of the research process. This can enhance research methods and contextualize findings, aiding in their practical application for autistic people^25,27 and facilitating relevant and meaningful benefits.²⁸ Although autism research is increasingly incorporating some degree of community involvement, particularly in studies focused on autistic adults and adolescents,^14,29–31 it is not clear whether this trend has filtered into the limited research that focuses on the lived experiences of autistic individuals with intellectual disability.

Several barriers contribute to the exclusion of individuals with intellectual disability from autism research, either directly or indirectly. These include difficulties in recruiting and retaining participants, an overreliance on written and verbal data collection methods with limited ability to adjust to meeting individual needs, along with time and resource constraints.^8,32,33 Researchers may also be reluctant to include autistic people with intellectual disability due to concerns around informed consent that could complicate or delay ethics committee approval. Informed consent is required for ethical research³⁴ and often necessitates a signed form, limiting participation to those who can read and write.³⁵ Researchers may perceive that ethics committees feel a heightened responsibility to protect individuals with intellectual disabilities from potential risks, especially due to concerns about unintended consequences such as increased stress, discomfort, or exploitation.³⁶ These factors may lead autism researchers to choose to focus their studies on autistic individuals who can participate in traditional data collection methods and pose fewer concerns for ethics committees. Thurm et al.³³ provide a comprehensive overview of these barriers, emphasizing that formal exclusion criteria, methodological challenges, and ethical concerns contribute to the underrepresentation of this population.

Inclusive self-report methods

Researchers in the field of intellectual disability have developed adaptations to traditional practices, including creating and sharing participant information in an easy-to-read format, using visuals during interviews, and adapting to the individual’s communication preferences.^37–40 Other methodologies have been specifically developed to capture the perspectives of individuals with intellectual disability. One such example is photovoice, a participatory research method that allows people to document and communicate their experiences, perspectives, and challenges through photography.⁴¹ Participants are given cameras to capture images that reflect their lives, environment, or specific issues they face. These photographs are then used as a basis for discussion, storytelling, and reflection. Another example is Talking Mats®, a communication tool whereby participants are encouraged to place symbols or pictures on a simple mat to reflect their opinions or choices.⁴² Broadly speaking, when seeking to gather firsthand perspectives from this population, a flexible methodological approach that involves collaborating with individuals familiar with the participant (e.g., parents) and adapting to the participant’s strengths and communication preferences has been recommended.⁴³

Autism researchers can learn valuable lessons about alternative self-report methods from those specializing in intellectual disability research.⁸ In recent years, some researchers have begun to adopt more inclusive methods to better capture the voices of autistic individuals with intellectual disability. A promising example of such inclusive research is the study by Richards and Crane,⁴⁴ which integrated methods such as photovoice and Talking Mats into a multimodal approach to data collection. This innovative combination of visual and interactive tools facilitated the expression of views, preferences, and experiences of young autistic individuals with complex needs. Similarly, Ryan et al.⁴⁵ engaged autistic adults with intellectual disability through various discussions, arts and crafts, and games to understand their perspective on autonomy.

Objectives of the current review

Despite recent efforts by researchers to better understand the lived experiences of autistic adults with intellectual disability, a comprehensive analysis of this body of research remains absent. Understanding the scope of existing research and the methodologies employed is crucial for informing and directing future studies that aim to address the significant gaps in the inclusion of autistic adults with intellectual disability. A systematic review can also provide researchers with a repository of various methods, enabling them to select and apply appropriate techniques, thereby increasing the usage and recognition of these methods as rigorous and valid approaches.

Therefore, our systematic review seeks to provide a thorough overview of the methods used to collect first-hand information about the experiences and perspectives of this underserved group. Specifically, we seek to determine the extent of the research focusing on the lived experiences of autistic adults with intellectual disability using self-report methods, including the topics covered and the nature of the information gathered. Therefore, this review addresses the following questions: 1.

What is the nature and extent of the research using self-report methods where the focus is autistic adults with intellectual disability?

What self-report measures have been used in this population?

What proportion of studies reviewed include community involvement? We defined community involvement as the active participation of community members (autistic adults, their family members or support networks) in the conduct of the research beyond being study participants.

Methods

Search strategy

First, we searched PROSPERO to confirm that no other systematic reviews on this topic were underway. We then developed the search strategy. Once finalized, we registered the protocol for this systematic review on PROSPERO (CRD42023396027). We used the most current version of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines at the time of registration throughout the review process.⁴⁶ We searched the following databases from January 2013 to July 2023 for publications: PsycINFO (Psychology), Scopus (Multidisciplinary Sciences including Social Sciences), MEDLINE (Multidisciplinary Medical Sciences), ERIC (Education), CINAHL (via EBSCOhost), and Web of Science. We limited the publication period to studies published in the previous 10 years for practical purposes. The search strategy utilized a combination of searches through titles, abstracts, keywords, and full texts related to “autism” and “intellectual disability” or “cognitive impairment” or “developmental delay” or “intellectual impairment”. We limited the searches to research published in English (see Supplementary Data S1 for sample search strategy) and imported results into Covidence (www.covidence.org). We removed duplicates using Covidence’s duplication identification strategy and the first author then checked manually. To be included in the review, study samples had to consist of at least 70% autistic adults with intellectual disability and include at least one self-report measure. We chose 70% to ensure that the study had a clear focus on this particular cohort as per the aims of the review. Table 1 outlines the full selection criteria. Two research team members independently conducted abstract/title screening and full-text reviews. Inter-rater reliability was moderate to strong (abstract review, κ = 0.54; full-text review, κ = 0.63). We resolved any discrepancies through discussion.

Table 1.

Selection Criteria

	Inclusion	Exclusion
Participants	Autistic adults (including autism spectrum disorder, autistic disorder, Asperger’s disorder, pervasive developmental disorder-not otherwise specified) with co-occurring intellectual disability (IQ < 70 or reported to have a mild, moderate, severe or profound ID).	Studies only focused on adolescents or children (under 18 years of age). If studies included both adolescents and adults, studies where the mean age of respondents was <18. If studies included autistic adults with and without co-occurring intellectual disability, studies where <70% of participants had a co-occurring intellectual disability. If studies included autistic and non-autistic adults with intellectual disability, studies where <70% of participants were autistic.
Outcome	Studies where information was collected directly from the autistic adult participants (e.g., self-report questionnaires, interviews).	Studies that have only collected information via a proxy or other participants.
Study design	Quantitative and qualitative studies published in peer-reviewed journals were included.	Books, dissertations, reviews, and commentaries.

Data extraction

We developed a data extraction template in Covidence. We then extracted the following data from included articles: study data (author/s, year of publication, study setting, aims/research questions, study design, sample size), socio-demographics of participants (e.g., gender, ethnicity, age), self-report data collection methods including construct measured and any adaptations made to facilitate use for people with intellectual disability, and findings from self-report measures. Two research team members independently conducted data extraction, and any discrepancies were resolved by discussion.

Quality assessment

We assessed study quality using the Mixed Methods Appraisal Tool (MMAT).⁴⁷ MMAT is designed for mixed studies reviews and is suitable for qualitative research, randomized controlled trials, nonrandomized studies, quantitative descriptive studies, and mixed methods studies. The quality criteria that are applied to a study include five items, which vary according to the study design. Outcomes for each criterion are defined as “yes” meets criteria, “no” does not meet criteria, or “can’t tell” where sufficient relevant information was not reported. Two research team members independently completed a quality assessment for each study, and we resolved any discrepancies in ratings by consensus.

Analysis

Using the extracted data, we completed a narrative synthesis of the results. This is a systematic approach for undertaking a review where statistical methods or pooling of data is neither possible nor appropriate.⁴⁸ Narrative synthesis can highlight the quality and extent of published evidence and results in a summary of knowledge that can be used to inform policy or practice.⁴⁸ We applied a deductive approach to prepare summaries of the extracted data according to study characteristics, measures used, and study findings. The first author completed the narrative synthesis, with input from other authors before agreeing on the final categories and summaries, which we then reported in a narrative format.

Results

As shown in Figure 1, after removing duplicates, we screened 4451 unique articles for eligibility, before conducting a full-text review of 277 full-text articles. We excluded 263 articles for the following reasons: <70% of the sample with intellectual disability (n = 171), no self-report measures (n = 80), no adult data or the participant mean age <17 years (n = 6), not peer-reviewed (n = 4), or the article was not original research (n = 2). Thus, we identified 14 studies as eligible for inclusion in the systematic review (see Supplementary Data S1 for a reference list of included studies).

FIG. 1.

PRISMA flow diagram showing article selection process. PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analyses.

Quality assessment

Results of the quality assessment using MMAT are provided in Supplementary Data S1. Where adequate information was provided, the overall quality of the included studies appeared sound. However, one methodological issue noted across several studies was the lack of adequate detail, including unclear research questions (n = 2), omitting information about sampling procedures and inclusion and exclusion criteria (n = 3), and limited details about the sample population such as ethnicity of participants or presence of any co-occurring conditions. It was challenging, therefore, to judge aspects of the studies, such as the appropriateness of measures, statistical analysis, and participant representativeness. In addition, only two quantitative studies used measures that had been validated for use with individuals with intellectual disability.

Nature and extent of research

Table 2 summarizes the study and participant characteristics for each of the 14 included studies. Studies were conducted in the United Kingdom (n = 6), United States (n = 4), Ireland (n = 2), Germany (n = 1), and Hong Kong (n = 1) and published between 2013 and 2022. The majority (n = 11) were investigating or evaluating interventions using single-subject designs (n = 5), case studies (n = 3), and randomized or quasi-randomized controlled trials (n = 2), with one study using a qualitative approach. Interventions were aimed at improving social-emotional competence (n = 4), health-related behaviors (n = 2), offending behaviors (n = 3), employment skills (n = 2), and reducing social anxiety (n = 1). The remaining three studies explored how to increase patient voice in an inpatient setting for adults with severe behaviors of concern, the association between physical activity level and quality of life, and the relationships of adult siblings when one sibling is autistic and has an intellectual disability.

Table 2.

Summary of Study Characteristics

Reference and geographic location	N of autistic adults	% with co-occurring ID	Gender	Age	Ethnicity	IQ	Other co-occurring conditions	Other informants	Design	Aim of study
Ashworth et al., 2020; UK⁴⁹	1	100%	Female, n = 1, (100%)	23	NR	Mild ID (FSIQ = 60)	Paranoia, depression, and anxiety	Clinician	Pre-post intervention case study	To summarize psychological intervention
Bergmann et al., 2021; Germany⁵⁰	12	100%	Females, n = 4, (33.3%); Males, n = 8, (66.7%)	M = 35.69 (SD = 12.49); range, 20–55	NR	Mild ID (FSIQ <7) = 7 (53.8%) Moderate ID (FSIQ <50) = 5 (35.8%)	NR	Parent/caregiver	Quasi-randomized partial controlled waitlist design	To investigate the acceptability and effectiveness of AutCom (a psycho-educative approach with music and movement interventions)
Borghino et al., 2022; UK⁵¹	1	100%	Male, n = 1 (100%)	35	NR	Mild ID (FSIQ = 69)	Pedophilia	None	Single case study	To explore the use of neurofeedback as an approach to treatment with an individual residing in a secure hospital setting
Horn et al., 2019; USA⁵²	4	100%	Males, n = 4 (100%)	M = 18 (SD = 1.41); range, 17–20	African American, n = 2 (50%); White, n = 2 (50%)	Mild to moderate ID	NR	Clinician	Multiple probe research design	To measure the effectiveness and perceptions of an intervention package incorporating constant time delay and eCoaching to promote employment skill acquisition
Melvin et al., 2019; UK⁵³	13	92.3%	Males, n = 13 (100%)	M = 38 (SD = 11 years 1 month); range = 21–57	NR	FSIQ, M = 71, SD = 9.5, range = 57–85	“Various psychiatric comorbidities”	None	Qualitative	To gather the views and experiences of autistic men who have completed an adapted CBT sex offender treatment program
Nabors et al., 2021; USA⁵⁴	17	100%	Females, n = 7 (41%); Males, n = 10 (59%)	Young adults	NR	NR	NR	Parent/carers College staff	Single subject design	To examine program structure, implementation, and outcomes of a healthy eating and exercise program for young autistic adults with ID
Rowe & Nevin, 2013; UK⁵⁵	4	100%	Males, n = 4 (100%)	20s to 40s	NR	Mild ID, n = 1; moderate ID, n = 2, severe ID, n = 1	Paranoid schizophrenia (n = 1); Psychotic illness (n = 1); Bipolar disorder (n = 1)	Clinician	Case studies	To assess the feasibility of developing patient voice at an inpatient autism service for adults with ID and severe challenging behaviors
Savage et al., 2022; USA⁵⁶	34, Treatment group, n = 18, Control group n = 14	100%	Females, n = 10 (29.5%); Males, n = 24 (70.5%)	Treatment group, M = 23.75 (SD = 5.67); Control, M = 29.72 (SD = 10.85)	Treatment group, White, n = 11 (61.1%), Mixed, n = 4 (22.22%), Black, n = 3 (16.67%); Control, White, n = 13 (81.25%), Mixed, n = 1 (6.25%), Black, n = 1 (6.25%), Latino, n = 1 (6.25%)	Treatment group, M NVIQ = 60.65 (SD = 10.23); Control group, M NVIQ = 62.69 (SD = 7.61)	NR	Researcher Parent/carer	Randomized controlled trial	To determine whether a supported self-managed exercise program was an effective and feasible program to increase engagement in physical activity for autistic adults with ID
Tomaszewski et al., 2022; USA⁵⁷	38	100%	Females, n = 11 (28.9%); Males, n = 27 (71.1%)	M = 25.79 (SD = 8.6)	White, n = 26 (68.4%), African American, n = 6 (15.8%), Mixed, n = 5, (13.2%), Arabic, n = 1 (2.6%)	M NVIQ = 60.2 (SD = 10.3)	ADHD, n = 9 (23.7%); anxiety, n = 8 (21%); depression, n = 3 (8.6%)	Parent/carers Researchers	Randomized controlled trial (baseline)	To describe average physical activity levels for autistic adults with intellectual disability using wearable activity trackers for 1 week and describe quality of life and examine the relationship between step counts and quality of life
Tozer et al., 2013; UK⁵⁸	12	100%	NR	NR	NR	NR	NR	Sibling Clinician	Qualitative	To examine the relationship between adult siblings, where one has autism and a severe learning disability
Tsang et al., 2022; Hong Kong⁵⁹	33	100%	Females, n = 3 (9.1%); Males, n = 30 (90.9%)	M = 28.03 (SD = 10.14)	Asian, n = 33 (100%)	FSIQ, range = 55–70	Language disorder, n = 8 (24%); ADHD, n = 7 (21%); bipolar disorder, n = 1 (3%); psychosis, n = 2 (6%)	Parent or support worker	Pre-post intervention	To report on applicability and effectiveness of a social competence intervention for autistic adolescents and adults with ID
Walsh et al., 2019; Ireland⁶⁰	6	100%	Females, n = 3 (50%); Males, n = 3 (50%)	21–27 years	White, n = 6 (100%)	Mild ID, n = 3; Moderate ID, n = 3	Anxiety, n = 1 (16.66%)	Staff member	Pre-post intervention design	To investigate the role of social skills curricula and the use of behavior technologies on improving social communication skills for autistic adults with intellectual disabilities
Walsh et al., 2020; Ireland⁶¹	3	100%	Female, n = 1 (33.3%); Males, n = 2 (66.6%)	20–21 years	White, n = 3 (100%)	Mild ID, n = 3	Language disorder plus ADHD, n = 1	Researcher Job coach	Alternating treatments design	To compare the combined effects of the myPref app and the video models as a prework assessment in determining work preferences for autistic adults with ID
Wright, 2013 UK⁶²	1	100%	Male, n = 1 (100%)	19 years	NR	NR	Social anxiety	None	A–B single case study design	To investigate the effectiveness of CBT for social anxiety in a 19-year-old autistic man with ID

ADHD, attention-deficit/hyperactivity disorder; ID, intellectual disability; M, mean; SD, standard deviation.

Regarding participant characteristics, 13 of the 14 studies provided at least some demographic details related to a total of 157 autistic participants. There was a higher representation of males than females (males, n = 127, 67%; females, n = 40, 23%). Participants ranged in age from 17 to 57. Where mean age was reported (n = 6 studies), mean age ranged from 18 to 38 years. Eleven studies provided some information about the level of intellectual disability of participants, with all falling within the mild to moderate range. Nine studies provided some detail about co-occurring conditions, which included attention-deficit/hyperactivity disorder (ADHD) (n = 17), anxiety (n = 11), language disorder (n = 9), depression (n = 4), psychotic illness (n = 3), paranoia/paranoid schizophrenia (n = 2), bipolar disorder (n = 2), and paraphilia (n = 1). Only six studies (comprising 113 participants in total) reported on ethnicity. Just over half were White (n = 61, 54%), 29.2% were Asian (n = 33), 10.6% were African American/Black (n = 12), 4.4% were mixed race (n = 5), with one each of Latino and Arabic background. All but three of the studies included informant measures from parents/family members (n = 6), clinicians, staff or support workers (n = 7), and/or researchers (n = 3) in addition to self-report measures.

Self-report measures

Table 3 summarizes the 24 self-report measures used across studies, with 1 measure (Quality of Life Questionnaire⁶³) used in two studies.^56,57 Fourteen of the measures were novel and included (a) questionnaires that asked participants about experiences of an intervention (n = 3), changes in target behaviors or symptoms as a result of an intervention (n = 3), and preferences for care in an inpatient setting for adults with severe behaviors of concern (n = 1); (b) interviews to gather information about experiences and/or perceived benefits of an intervention (n = 3) and sibling relationships (n = 1); (c) tracking forms to measure progress (n = 2); and (d) images to assess participant preferences for different jobs (n = 1). Half of the novel measures were described as including some accommodations for participants, such as the use of visuals (e.g., “smiley” faces, yes/no answer cards) to support response options,^50,52 questionnaire items read aloud and/or supported with verbal assistance/explanation of items,^51,52,59 and the use of photos or images to aid comprehension.^55,56,58,61 In one study, the authors conducted pre-interviews with the siblings of autistic adults to gain a better understanding of communication abilities and preferences.⁵⁸

Table 3.

Summary of Self-Report Measures

Reference	Self-report measure	Constructs measured	Novel or existing measure	Reported psychometrics for quant measures	Accessible features
Ashworth et al., 2020⁴⁹	Clinical outcomes in routine evaluation—learning disability (CORE-LD, Brooks et al., 2013⁶⁴)	Psychological distress	Existing	“Validated self-report outcome measure” for people with ID in therapy with “good psychometric properties”	Simpler language, use of visuals to aid questioning^a
Ashworth et al., 2020⁴⁹	The Glasgow Depression Scale for people with a learning disability (GDS-LD, Cuthill et al., 2003⁶⁵)	Depression	Existing	None reported	Simpler language^a
Bergmann et al., 2021⁵⁰	Client Satisfaction Questionnaire (CSQ-8, Schmidt et al., 1989⁶⁶)	Satisfaction with treatment	Existing	Reliability is “considered high α = 0.91”; testing of factorial validity revealed a strong principal factors”	Use of simpler language and adding colored “smileys” to 4-point scale^b
Borghino et al., 2022⁵¹	The Symptom Checklist 90 Revised (SCL-90-R, Derogatis, 1994⁶⁷)	Psychopathology and psychological distress	Existing	“A reliable and valid inventory. Internal consistency ranges from 0.77 to 0.90 for the subscales. Test-retest coefficients at a 1 week interval range from 0.80 to 0.90 for the subscales”	None reported
	The Compulsive Sexual Behavior Inventory (CSBI-13, Miner et al., 2017⁶⁸)	Functional impairment/distress associated with difficulty controlling sexual feelings, urges, behaviors	Existing	“Adequate reliability, reliable criterion validity, and discriminant and convergent validity”	None reported
	Multidimensional Inventory of development, Sex and Aggression (MIDSA, 2011)	Child sexual arousal; sexual compulsivity; sexual preoccupation; hypersexuality	Existing	“Good test-retest reliabilities and high internal consistencies”	None reported
	The Warwick Edinburgh Mental wellbeing scale (WEMWBS, Tennant et al., 2017⁶⁹)	Psychological well-being	Existing	“Good face validity, content validity, internal consistency (Cronbach’s α of 0.89) and good test-retest reliability (0.83)”	None reported
	Semi-structured interview	Feedback about experiences of neurofeedback	Novel	N/A	None reported
	Self-evaluation form	Benefits of neurofeedback	Novel	N/A	None reported
	Tracking form	Self-reported duration, intensity, frequency, shifts related to focus areas	Novel	N/A	Supported by clinicians to complete^b
Horn et al., 2019⁵²	Survey	Perceptions of the learning process	Novel	N/A	Questions were read aloud to each participant, and students were instructed to circle the face (happy, sad, neutral) that best described their response^b
Melvin et al., 2019⁵³	Interview	Perceptions of the group program	Novel	N/A	None reported
Nabors et al., 2021⁵⁴	Goal attainment sheet	Whether goals were achieved	Novel	N/A	None reported
Nabors et al., 2021⁵⁴	Group interviews	Feedback about any changes in healthy eating, exercise and knowledge	Novel	N/A	None reported
Rowe & Nevin, 2013⁵⁵	Oral questionnaire	Preferences for care at an inpatient unit	Novel	N/A	Questions involved the use of a topic symbol with an oral question, and three answer cards with “yes,” “no,” and “don’t know” set out in a line^b
Savage et al., 2022⁵⁶	Quality of Life Questionnaire (QOL.Q, Schalock & Keith, 1993⁶³)	Quality of life	Existing	“Adequate internal (coefficient α = 0.90), interrater (r = 0.83), and test-retest (r = 0.87) reliability as well as evidence of construct and concurrent validity”	Simpler language, administered in an interview format^a
	Intervention feasibility questionnaire	Perceived benefit related to health, information provided, feature usage, and plans to continue using the app	Novel	N/A	Visual were presented with all potential responses^b
	Adapted Childhood Usage Rating Profile (CURP)—Actual (Briesch & Chafouleas, 2009⁷⁰)	Opinions about the feasibility and acceptability of the intervention	Existing	None reported	Visual were presented with all potential responses^b
Tomaszewski et al., 2022⁵⁷	QOL.Q (Schalock & Keith, 1993⁶³)	Quality of life	Existing	“Adequate internal (coefficient α = 0.90), interrater (r = 0.83), and test-retest (r = 0.87) reliability as well as evidence of construct and concurrent validity”	Simpler language, administered in an interview format^a
Tozer et al., 2013⁵⁸	Interviews	Feelings about their relationships with siblings	Novel	N/A	Pre-interview information gathering about communication/likes/dislikes; use of pictures/family photos^b
Tsang et al., 2022⁵⁹	Self-evaluation form	Knowledge acquired and confidence in applying skills in after each session	Novel	N/A	Verbal assistance was provided to complete self-evaluation^b
Walsh et al., 2019⁶⁰	Social validity questionnaire	Participant satisfaction	Novel	N/A	None reported
Walsh et al., 2020⁶¹	Images of three jobs	Preferred job	Novel	N/A	Use of photos to depict jobs^b
Wright, 2013⁶²	Brief Symptom Inventory (BSI, Derogatis, 1975⁷¹)	Symptoms across nine psychological dimensions, including anxiety and depression	Existing	None reported	None reported
Wright, 2013⁶²	Rating scale	Anxiety symptoms	Novel	N/A	None reported

Accessible features already in existing measure.

Accessible features developed for the study.

The remaining 10 measures were validated questionnaires assessing self-reported psychological symptoms and/or distress (n = 4), well-being and quality of life (n = 2), sexual behavior (n = 2), and acceptability/feasibility of treatment (n = 2). All but three of these measures were reported to have adequate to good psychometric properties by the study authors based on previous studies; however, only two of these measures had been developed specifically for individuals with intellectual disability (clinical outcomes in routine evaluation—learning disability⁶⁴ and Glasgow Depression Scale for people with a learning disability⁶⁵). None of the studies reported on the psychometric data for their samples. Only two of the remaining measures (Client Satisfaction Questionnaire⁶⁶ and Childhood Usage Rating Profile—Actual⁷⁰) were adapted with the use of simpler language and/or visual supports for response options in two studies.^50,56

Community involvement

None of the studies included any co-production elements with autistic people in the design and conduct of their studies. One study sought input from siblings of autistic adults to develop the interview content and structure.⁵⁸ Another study included community involvement with employers providing some input into the task analysis methods used throughout the project.⁶¹

Discussion

This review aimed to synthesize existing research that incorporates self-report methods with autistic adults who have an intellectual disability. The goals were to assess the scope and nature of such research, evaluate the level of community involvement, and identify the measures used with this population. Our most striking finding was the limited number of studies that met the inclusion criteria. Only 14 studies conducted in the 10-year period between 2013 and 2023 where the sample was predominantly autistic adults with intellectual disability (>70% of the sample) have collected self-reports directly from participants. This is consistent with the findings of Russell and colleagues,³⁵ who found that of the 100,245 participants across 301 studies published in autism-specific journals in 2016 covering all fields of autism research, only 6% had an intellectual disability. Furthermore, there were no participants in the included studies with levels of functioning below the moderate range of intellectual disability. This has also been the case in the research literature more broadly where few studies utilizing self-report measures have included people with severe or profound levels of intellectual disability.⁴⁰

Another notable finding was that most studies (11 of the 14) employed self-report measures to evaluate discrete interventions rather than to explore people’s lived experiences. It appears, therefore, that although there has been an encouraging increase in studies focused on autistic adults,⁷ with many seeking to better understand the perspectives and lived experiences of autistic adults across a range of life domains, such as education and higher education,^72,73 employment,^74,75 health care,⁷⁶ and relationships,^77,78 these studies have largely excluded those with intellectual disability. This is despite recent calls for increased attention³⁵ and more accessible self-report instruments¹⁷ for this cohort. Their exclusion is likely attributable to several factors, including researcher concerns about informed consent and securing ethical approval and the language and attentional demands of participating in research as well as the lack of reliable and valid measures and methodologies for this population.^26,79

Our review found a total of 24 measures used to gather self-report data from autistic adults across the included studies. Most of these measures (n = 14) were novel questionnaires or interviews designed for each study, with only half of these described as including accommodations such as visual aids, options for items to be read aloud, or the provision of additional explanation of items/questions if required. The remaining 10 measures were established questionnaires; however, only 2 had been validated for use with adults with intellectual disability, and only 2 of the other studies reported incorporating adaptations, that is, simplified language or visual supports. Given the participant sample, it is possible that accommodations were used in all or most studies but not necessarily explicitly reported; however, without explicit descriptions, replicability is challenging. This is also concerning given the critical importance of accessibility of materials to the validity of data collected and, ultimately, the understanding of the experiences and needs of individuals with intellectual disability.⁸⁰ A range of adaptations for this cohort, such as clarifying question content, simplifying response options, reducing reading level, shortening measures, incorporating more visual aids, and considering accessibility in administration procedures, have been recommended^40,81and have been shown to improve usability such as increased response rates, item comprehension, and independent completion.^82,83 In fact, Kooijmans and colleagues⁴⁰ identified 161 recommendations to guide clinicians and researchers in the development of “ID inclusive” self-report measures based on the findings of their systematic review of 49 studies.

It is also noteworthy that all but three studies included informant measures in addition to self-report. Although gathering information from a range of key stakeholders can provide a comprehensive picture of a particular individual’s circumstances and support needs, it may be problematic if greater emphasis is placed on informant ratings or perspectives, given that self-reports and informant reports have been shown to vary across a range of areas,⁸⁴ particularly for more subjective constructs such as quality of life, mood, and well-being.^85,86 This was apparent in our review where, in some cases, the self-report data collected represented a relatively small proportion of the overall data collected, for example, two out of six in the Nabors et al. study⁵⁴ and one out of eight in the Tomaszewski et al. study.⁵⁷

Our final research question related to the proportion of studies that included community involvement. Disappointingly, although two studies documented some input in the design stage from siblings⁵⁸ and employers of autistic adults,⁶¹ none reported any co-production elements that included autistic adults. This is despite the growth in community involvement in autism research more generally, where increasingly research is being co-produced with autistic adults^87–89 or at least actively involved in the design and conduct of research as well as the interpretation and dissemination of findings.^17,28 Although there are undoubtedly practical and ethical considerations in conducting co-research with adults with intellectual disability, co-research can bring added value and critical insights by including the unique perspectives of those with lived experience. A number of good practice models for co-production have been developed specifically for intellectual disability research,^90,91 which describe elements of inclusive design, measure development, and data interpretation that could assist autism researchers to expand co-production efforts to this particular subgroup of autistic people.

Limitations

As with other literature reviews, the current review has several limitations. We only included peer-reviewed studies in English, which means we may have missed research conducted by non-English speaking researchers. Similarly, for practical reasons we limited our initial searches to publications between 2013 and 2023. These restrictions on our search terms mean we may have missed studies that would have yielded more information that was pertinent to our research questions. In addition, we excluded studies where <70% of the sample were autistic adults with intellectual disability. This decision was a deliberate one in order to identify only those studies that had a clear focus on this particular cohort. However, there may be other studies with smaller proportions of our target group excluded from our review that have utilized, developed, or adapted self-report measures or elements of community involvement that would be informative for autism researchers. One such example is Kent et al.,⁹² which described the adaptation of a common autism screening measure (short version of the Autism Spectrum Quotient; AQ-10⁹³); however, this study was excluded as <70% of the participant sample reported having a mild intellectual disability. Finally, the use of consensus to resolve disagreements rather than a third reviewer may have introduced bias or compromise, potentially reducing the robustness of the review process.

Implications

Our findings have implications for future autism research. As has increasingly been the case in autism research among autistic children and adults without intellectual disability, there needs to be an increased focus on the lived experience and perspectives of autistic adults with intellectual disability in future research. Conducting research that identifies the priorities and support needs of this specific subgroup of autistic adults using inclusive methods would be a crucial first step. Furthermore, there needs to be a prioritization of self-report measures and a reduction of reliance on informant or proxy responses as the only method of seeking this group’s perspectives, particularly when assessing outcomes or constructs related to quality of life or well-being where the information gathered must reflect the point of view of that person. This will require a more widespread uptake of self-report measures than has been identified in this review, which may be related to the fear of autism researchers (or perhaps the reality!) that the creative methodologies that are needed in this space will be rejected at the ethics approval or peer-review stages. One way to advance this area of autism research is to be open-minded as peer reviewers or editorial teams, allowing some leverage for researchers to engage in trial and error of these methodologies in order to gather evidence of their utility over time. To circumvent unnecessary roadblocks from ethics committees, consideration should be given to including individuals with intellectual disability and/or knowledgeable advocates on these committees. To allay the concerns of ethics committees regarding informed consent and possible harms, researchers should employ rigorous co-production approaches^33,94 in the design stage and clearly articulate in their applications how they have proactively considered and addressed potential ethical “sticking points.” Feedback loops whereby researchers work collaboratively with committees by sharing participant and researcher experiences, particularly for novel or creative methods, would also be beneficial in building trust and shared understanding.

There are several barriers to conducting research that involves gathering lived experiences from autistic adults with intellectual disability, including resource limitations, recruitment challenges, and knowledge gaps in self-reported measures. To address the latter, autism researchers can look to the intellectual disability field for strategies to develop and adapt self-report measures across content, layout, and administration procedures such as simplifying content and phrasing, using situational anchors when asking about specific time periods, avoiding abstract concepts or generalizations, regularly recapping and summarizing during interviews, and avoiding forced-choice questions.^{40,80,81,83,95} It is important to note that most of these accommodations have been based on studies involving individuals with mild to moderate intellectual disability. There are certainly additional practical and ethical barriers to conducting research with individuals with severe or profound intellectual disability⁹⁶; however, it is important that they are not omitted from future research. Inclusive research and methods that “give voice” to this particular cohort may require a shift in thinking about inclusion and participation, relying less on words and more on “working alongside” them to understand the world from their perspective.^97,98

Research efforts also need to expand beyond the evaluation of interventions to explore how to improve quality of life in a way that is informed by a clearer understanding of what is meaningful and important from their perspective. Finally, future research in this area must seek to incorporate community involvement, particularly from autistic adults themselves. While full participation in the research process may not be necessary or practical in every research project, meaningful inclusion and opportunities for contribution and collaboration should be genuinely explored.

Conclusions

In conclusion, this review highlights the persistent and significant gaps in autism research, particularly concerning the inclusion of autistic adults with intellectual disability. Despite a growing recognition of the need to center the voices of autistic individuals in research, the exclusion of those with intellectual disability remains a significant oversight. The scarcity of self-report methods tailored to this population and the minimal involvement of community stakeholders underscore the urgent need for a more inclusive research agenda. Future studies must prioritize the development and validation of accessible self-report tools and actively involve autistic adults with intellectual disability in the research process. By doing so, we can ensure that research not only reflects the diverse experiences within the autistic community but also generates outcomes that are truly meaningful and impactful for those most marginalized. This shift is not merely a methodological challenge but also a moral imperative to advance autism research in a way that is equitable and inclusive of all autistic individuals.

Footnotes

Authorship Confirmation Statement

V.G.: Conceptualization, methodology, formal analysis, writing—original draft, and writing—review and editing. R.Y.C.: Conceptualization, methodology, formal analysis, and writing—review and editing. A.L.: Conceptualization, methodology, formal analysis, and writing—review and editing. C.E.: Conceptualization, methodology, formal analysis, and writing—review and editing. M.A.A.: Conceptualization, methodology, and writing—review and editing.

Author Disclosure Statement

The authors have no competing interests of which they are aware.

Funding Information

The research was supported by Autism Spectrum Australia.

Supplementary Material

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