MID is a neurodevelopmental disability characterized by deficits in intellectual and adaptive functioning skills (American Psychiatric Association, 2013). The term MID generally refers to people with limited intellectual capacities and adaptive skills with IQ scores in the range from 55 to 70, and may in some definitions include persons with “borderline intellectual functioning” (IQ 70–85; Kaal et al., 2015; Wieland and Zitman, 2016).

Adults with MID experience more stress in daily life than people without intellectual disabilities (Bramston and Mioche, 2001; Emerson, 2003; Hatton and Emerson, 2004; Schuengel and Janssen, 2006; World Health Organization, 2010; Casey, 2017; de Witte et al., 2020b). Daily life stressors that impact many lives of people with MID include difficulties with social interactions, experienced lack of control over minor daily and major life decisions, and occupational stress (Hartley et al., 2009b; Dulin et al., 2013; Scott and Havercamp, 2014). Next to common daily life stressors, people with MID have a higher risk of experiencing adversity in childhood, such as abuse and neglect. Mason-Roberts et al. (2018) found that 42.4% of their study participants with mild and moderate ID reported multiple traumatisation in both childhood and adulthood. In studies by Santoro et al. (2018) and Vervoort-Schel et al. (2021), over 80% of children and adults with intellectual and developmental disabilities experienced one or more adverse childhood experiences (ACE). Childhood and later life adversity have been shown to cause chronic overactivity of stress systems (e.g., Van Der Kolk, 2014).

The extent to which daily life stressors and chronic stress lead to perceived psychological stress is moderated in large part by the coping skills and resilience factors of the person facing the stressor. Psychological stress is defined by Cohen et al. (1983) as the degree to which individuals perceive that demands exceed their ability to cope. People with MID have been shown to have greater difficulty coping with stress in daily life and the effects of adversity than adults without intellectual disabilities (American Psychiatric Association, 2013). Adults with MID often seem to lack social support and self-efficacy, important factors for coping with stress (Abbaszadeh and Sardoie, 2016; Seyed et al., 2017; Everly and Lating, 2019). As in the general population, stress experienced by adults with MID is linked to many negative mental health outcomes (Hulbert-Williams and Hastings, 2008; Hartley et al., 2009a,b; Scott and Havercamp, 2014). Persistent or chronic stress in adults with MID can lead to maladaptive coping strategies and detrimental mental and physical health conditions such as depression (Hartley et al., 2009a,b), impaired cognitive functions (Heyman and Hauser-Cram, 2015), physical health problems (Lunsky, 2008), and substance abuse (Didden et al., 2009).

Both proxies and persons with MID themselves can provide information on the occurrence of stressful situations for the person with MID. There are some concerns with gaining reliable and accurate information from people with MID through self-reports, that mainly originate from the nature of their disability. However, as the perception and psychological impact of stress is a highly subjective internal (and thus not observable) state, the validity of proxy stress measures is perceived to be limited as well (Emerson et al., 2013; Scott and Havercamp, 2018).

Several studies have shown that, when compared to self-reported outcomes, proxies tend to overestimate impairment and underestimate health-related quality of life of people with (M)ID (Andresen et al., 2001; Vlot-van Anrooij et al., 2018). So because of the superior accuracy and sensitivity of self-reported information when it comes to the assessment of subjective internal states, researchers in the field of MID generally prefer self-reporting measures above proxy measures to assess the experience of stress in persons with MID (Lindsay and Skene, 2007; Scott and Havercamp, 2018). Additionally, gaining access to thoughts, attitudes and feelings about stress directly at the source, can lead to an enriched knowledge base from which opinions can be formed and interventions for stress reduction implemented (O'Keeffe et al., 2019).

Several studies have shown that, when compared to self-reported outcomes, proxies tend to overestimate impairment and underestimate health-related quality of life of people with (M)ID (Andresen et al., 2001; Vlot-van Anrooij et al., 2018). So because of the superior accuracy and sensitivity of self-reported information when it comes to the assessment of subjective internal states, researchers in the field of MID generally prefer self-reporting measures above proxy measures to assess the experience of stress in persons with MID (Lindsay and Skene, 2007; Scott and Havercamp, 2018). Additionally, gaining access to thoughts, attitudes and feelings about stress directly at the source, can lead to an enriched knowledge base from which opinions can be formed and interventions for stress reduction implemented (O'Keeffe et al., 2019).

High quality self-report measures on mental states, including stress, for adults with MID are few and far between (Glenn et al., 2003; Sams et al., 2006; Kooijmans et al., 2021a). There are many challenges when collecting self-reported data from people with MID that are associated with the nature of the disability, including problems with reasoning, verbal expression, reading, abstract thinking, and judgment (Schalock et al., 2010; American Psychiatric Association, 2013). To accommodate for these challenges, adaptations have to be made to “standard” instrument language, lay-out, and assessment procedures. On the basis of research into the processes underlying the construction of responses to questionnaires and interview questions, authors have put forward recommendations to tailor questionnaire formats and administration procedures to the specific needs of people with ID (e.g., Finlay and Lyons, 2001; Hartley and MacLean, 2008). A recent review by Kooijmans et al. (2021a) highlights that although many recommendations seem common-sense, most are practice-based and lack scientific validation. Examples of recommendations that are more or less evidence-based include the use of Easy Read guidelines to simplify language, the use of Likert scales with a limited number of response options and the sparing use of open-ended questions. The visualization of questions and responses merits attention, as it is generally recommended to use visual supports in the form of pictograms, drawings or photos, while it remains unclear what types of support actually promote understanding instead of causing cognitive overload or confusion (Sutherland and Isherwood, 2016). Few self-report measures are available that incorporate evidence-based adaptations to better suit individuals with intellectual disabilities (Lindsay and Skene, 2007; Scott and Havercamp, 2018). Additionally, Wieland et al. (2012) have identified a number of self-report measurement instruments developed for use in the general population which are suitable for adults with MID.

As persistent stress can lead to the development of psychopathology and severe physical conditions, it is becoming increasingly important to recognize stress-related symptoms in populations known to be more vulnerable to stress, like people with MID. It is therefore critical to gain more insight into the way stress can be assessed in this population. Although advances in the use of physiological measures to assess people's stress levels have added substantial value to stress research, it is no substitute for the use of self-report measures, since the individual's perception of stress is directly related to individuals' emotional states. As stated before, physiological and emotional stress are not necessarily directly related (e.g., Linnemann et al., 2017; Scott and Havercamp, 2018; de Witte et al., 2020b). In order to provide an overview of the existing self-report stress measures and to provide more information about their suitability for adults with MID, we conducted a scoping review. We searched the peer-reviewed literature to identify self-report stress measures. Subsequently, we searched peer-reviewed literature databases and gray literature sources to collect information on several predefined characteristics of these measures. In order to advise clinical professionals on how to correctly use the identified self-report stress measures, expert consultations were held to refine our preliminary findings. Our findings can be applied to research in which stress-related outcomes are measured in both adults with MID as well as those without intellectual disabilities. Results of this scoping review will provide guidance to clinical practitioners to assess perceived stress in adults with MID.

To provide insights into the characteristics and quality of the included self-report stress measures, criteria were formulated to describe their properties. Instrument characteristics relating to the criteria were found in the actual self-report stress measure itself, the user manual, validation studies, and other publications about the self-report measure in peer-reviewed and gray literature. The criteria applied to (1) the psychometric quality of the measure, (2) the assessment procedure of the self-report stress measure, and (3) the suitability for adults with MID. A further definition of the assessment criteria is presented below. Outcomes that relate to each criterion are presented in Table 2 in the Results section for all instruments.

The characteristics of each individual instrument are described for each of the three criteria: psychometric quality, assessment procedure, and suitability for people with MID based on the consultation of experts and the scientific literature. The findings are summarized in Table 2 and described in more detail below for each instrument (in alphabetical order).

The experts were asked to reply to open-ended questions on the subject of how to attune self-report measures to the needs and abilities of people with MID. They unequivocally indicated that the factors that improve appropriate use by people with MID in general also apply to the self-reported measurement of stress. Thematic analysis of the answers revealed six general recommendations relevant to the measurement of stress in people with MID.

The first recommendation was to use concrete and easy-to-understand vocabulary, simple grammar, and short sentences. The next was to use relatively short time frames for the retrieval of information. Assessors should not ask to retrieve information over longer periods than 1 week, as time processing abilities are generally impaired. A third recommendation relates to the use of Likert scales. When designing self-report measures for people with MID, the number of response options in Likert scales should be limited to three for people with moderate ID to MID and five to people with MID to borderline intellectual functioning. Fourth, an “I don't know” option should be included in both forced-response and open-ended questions to prevent invalid answers from those who do not understand the question. A fifth recommendation was to use visualizations to support the meaning of questions and responses, although how exactly these should be configured was not specified. In regard to the assessment procedures, a sixth recommendation was to use pre-scripted alternative wording if the respondent seems unable to understand the question. Standardization ensures comparability of scores across assessments. The extent to which these factors were reflected in the self-report measures' design and assessment procedures differed across the included instruments. An overview of the suitability of each self-report stress measure for people with MID, according to the experts, is presented in Table 3.

The Lifestress Inventory (LI) was specifically designed for the MID population. Two others, the Glasgow Anxiety Scale for people with Intellectual Disability (GAS-ID) and the Self-rating Anxiety Scale for adults with Intellectual Disabilities (SAS-ID) reported that they were fit for use with people with ID, but the user manuals did not specify the exact intelligence range. As the items concerned mainly refer to insights, feelings and experiences from daily life, participants must be able to grasp these abstract concepts, translate them to their everyday experiences and formulate a meaningful response. This suggests that they are targeted toward adults with MID instead of the total ID population. Generally, these three self-report stress measures have in common that they use items that require a response on simple Likert scales, which could possibly be combined with visual representations of answer alternatives. This is in line with findings reported in previous studies as well as the expert consultations in our study, which show agreement that responses requiring a simple Likert rating scale or only yes/no can lead to appropriate responses from individuals with MID (Heal and Sigelman, 1995; Ramirez, 2005; Hartley and MacLean, 2006). For those individuals in the lower range of MID, pictorial representations of response alternatives could increase the likelihood of gaining appropriate responses (Hartley and MacLean, 2006), which was echoed by the experts consulted.

Despite the lack of empirical support, our findings also show that some of the other stress self-report measures seem to be more or less suitable for adults with MID. First, some evidence was provided in previous validity studies on populations in which participants with intellectual, learning, or developmental disabilities were also included. This applies to the BAI (see Lindsay and Skene, 2007), the DASS (see Psychology Foundation of Australia, 2021), and the PSS (see Janusis and Weyandt, 2010). Second, other self-report stress measures stated that they could also be used in younger aged populations, which may suggest that, at least in terms of comprehensibility, they may be suitable for people with MID. This applies to the STAI-child version (9–12 years), the ICS (from 12 years), and the PSQ (from 14 years). Hurley (2008) suggests that the use of instruments designed for children may offer a useful basis for adaptation, because the measures use concrete levels of vocabulary and simple sentence structures. This process has also been used by many other researchers (e.g., Marshall and Willoughby-Booth, 2007; Guerin et al., 2009). However, since these stress self-report measures have not been validated specifically for the adult MID population, we recommend thoroughly screening the measurement construct and assessment procedure before using them in clinical practice or in future research (Kooijmans et al., 2021a).

The findings from the expert consultations show the importance of adding an extra “I don't know” answer alternative to prevent people with MID who do not understand the question from filling in a random answer (Bell et al., 2018). However, none of the self-report stress measures, even those specifically developed for people with MID, included this option. The Lifestress Inventory (LI) added the answer alternative “actually not experienced,” but this refers to the fact that the participant did not experience any stress at all. In addition, none of the self-report measures included “alternative wording” to the questions and/or answer alternatives, as recommended by the expert panel. On the other hand, helping factors such as allowing assessment assistance (SAS-ID) or having someone else read the items (LI and GAS-ID) were not mentioned by any of the experts. Finally, response visualizations seem to be missing from both the GAS-ID and SAS-ID. This is remarkable, as this is considered one of the most important factors with regard to suitability for people with MID, both in the literature and by the experts consulted (e.g., Hartley and MacLean, 2006; Scott and Havercamp, 2018). Of the three self-report stress measures for people with MID, the LI appears to be most consistent with the findings of the experts. However, our findings show that in addition to consulting experts, screening the assessment procedures of existing self-report measures specifically adapted or designed for people with (M)ID is a worthwhile exercise.

Although proxy reports are commonly used in MID, self-report measures prove to be more accurate and more sensitive, even in the MID population (Moss et al., 1996; Scott and Havercamp, 2018). The importance of obtaining self-reported information on subjective stress experiences of people with MID is also reflected in the increased recognition in our society that people with (M)ID are full citizens with the same rights as non-disabled persons, meaning that participation and social inclusion should dominate all organized activities (e.g., Devi, 2014; Giesbers et al., 2019). In other words, including the opinions, feelings, and thoughts of people with MID by using self-report measures, fits the call for knowledge democratization, as citizens increasingly demand their say in policies and research affecting them (Anderson, 2017; Dedding et al., 2020). This is important, because self-determination can be seen as an essential dimension of quality of life and is linked to many positive outcomes for people with (M)ID) (Schalock et al., 2002; Wehmeyer, 2007; Frielink et al., 2018). Therefore, both the findings of this review and the empirical evidence show that increasing our knowledge of self-report stress measures for people with MID is a highly recommended addition and in line with the contemporary opinion that the voice of people with MID should be included in matters that concern them.

The way the concept of stress was operationalized by the self-report measures varied according to the theoretical underpinnings and constructs. Different paradigms or stress theories were used, such as the interactional stress model (e.g., the DSP or the PSS), theories on stress as a transitory anxiety state (e.g., the STAI and the BAI), and the tripartite model of anxiety and depression that describes stress as a common symptom for both (e.g., the DASS). Moreover, some of the self-report measures do not seem to have origins in a certain stress theory or model, but were developed empirically, involving expert consensus on the manifestation of stress in clinical practice (e.g., the GAS-ID). Others are based on the manifestation of stress symptoms described in classification systems of psychiatric disorders (e.g., the SAS-ID). In addition, a distinction can also be seen between self-report stress measures that focus mainly on stress as an experienced psychological and physiological state (e.g., the BAI, the SACL, and the STAI) and those that focus on the experience of stress in the context of situations that actually or hypothetically cause stress, such as job related stress or stressful social situations (e.g., the LI, the PSS, and the PSQ). To ensure that the concepts being studied are consistent with the design and intended use of the self-report measure, we recommend paying attention to how the concept of stress is theoretically framed when deciding to use a self-report stress measure (Cook and Beckman, 2006).

There is a strong tendency in clinical practice to move away from attributing the symptoms of psychopathology solely to the cognitive deficits of people with MID; this is known as diagnostic overshadowing (Reiss et al., 1982; Hagopian and Jennett, 2008). Clinicians are becoming increasingly sensitive to the fact that people with MID can also suffer from symptoms of psychopathology. Since the degree of stress is now recognized as a significant factor in the development of severe psychopathology, especially in people with MID, it has become more important to correctly assess stress-related states in clinical practice (Scott and Havercamp, 2014). This review therefore provides a practical basis for determining whether and which self-report stress measures are suitable for people with MID within their own clinical context.

To provide some guidance for clinical practice, we have formulated several recommendations based on earlier studies on stress in people with MID and on our own findings from the current review. First, with this review, we want to draw attention to the concept of stress and the importance for clinical practice to consider the degree of (daily or present) stress as a crucial factor in the quality of life and course of further psychological treatment in people with MID. In our view, stress assessment should be included as a regular part of the diagnostic phase of clients with MID when consulting clinical practice. Second, as mentioned, we strongly advise clinical practice to always strive to obtain self-reported information in addition to proxy-reports when it comes to medical, psychological, and service decisions involving people with MID. The third recommendation is based on the results of the current review. We particularly recommend using the three self-report stress measures specifically designed for adults with (M)ID. These self-report measures are characterized by simple Likert rating scales and/or items requiring yes/no responses. Specifically, the use of simpler wording, fewer response options, and the ability to provide supportive visualization are the main differences with the self-report stress measures developed for the non-ID population. Another significant difference is that self-report measures developed for (M)ID often allow the respondent to be assisted during the assessment (SAS-ID) and that the items can be read aloud by someone else (LI and GAS-ID). Although our assessment of the suitability for MID populations show that, even for MID-specific instruments, there is ample room for improvement, these measures remain a clinician's primary choice.

While there is general consensus that it is necessary to timely assess stress in people with MID, we are also aware that this requires experiential knowledge of clinical professionals working with the MID population. The challenge for clinical practice is to prevent that difficult-to-understand behavior of people with MID too quickly leads to a psychiatric classification, which often has far-reaching consequences (Didden et al., 2016). On the other hand, psychological problems still have to be recognized timely. This requires continuous in-depth behavioral observations and careful consideration by clinical professionals, as people with MID, certainly in combination with additional behavioral/psychological problems, often are unable to clearly request help (ten Wolde et al., 2006). Decisions made should therefore be adequately aligned with personal and environmental circumstances, as well as with the level of cognitive functioning (Nouwens et al., 2020). Determining and applying suitable self-report measures for clients with MID could contribute to this purpose. Moreover, as indicated earlier, the use of self-report measures is also a way of letting the client's voice speak, and thereby enhances feelings of autonomy, initiative and freedom of choice. In this study, we have attempted to provide a first guide with regard to the use of self-report stress measures.

There are some limitations of the present study that should be noted. First, because we strictly followed our inclusion criteria, we may have excluded some self-report measures which could be also suitable for assessing stress in people with MID (see Appendix B). For example, they may have not yet been applied in (clinical) outcome studies published in peer-reviewed scientific journals. Searching grey literature sources in addition to the peer-reviewed literature, would probably have resulted in many more measures to include. Another reason for exclusion was that measures were unavailable in the English language; it is possible that suitable measures exist in other languages.

Second, for the appraisal of the psychometric properties of each measure, we had to rely on the parameters reported by authors in their publications. Often, the developers of a measure conducted their own validation research, which may have willingly or unwillingly introduced bias. Nearly all studies report Cronbach's alpha as the main indicator of reliability. Recent advances in psychometric research suggest that this may be a flawed indicator of the internal stability or reliability of a measure. It is stated that other indicators, such as omega, are more robust, and that reliability research should be preceded by Factor Analysis (Crutzen and Peters, 2017).

Third, we would have liked to share more specific information from the expert consultations. However, due to the use of an online survey, there was no opportunity to ask further questions. Therefore, for future research, we recommend adding a more interactive form of data collection when consulting experts on similar questions, such as a multidisciplinary focus group method. Another limitation concerning the expert consultations is that the results reflect the participating experts' professional opinion. Although their clinical and research expertise are highly valued, the experts were not asked to substantiate their statements with references to empirical literature. Therefore, the suggestions by the experts must be valued as tentative and supplementary to the evidence from empirical studies. Finally, the experts' findings were only compared with the published information in the user manuals of the self-report measures, i.e., only with the information already described. An option for follow-up research would be to use a more detailed screening list and to screen the individual instruments with different researchers in the field of MID blinded from each other. This would ensure more accurate statements about the use of existing self-report stress measures in people with MID.

Our study provides an overview of existing self-report stress measures, but can only offer limited guidance on the suitability of the self-report measures for people with MID. Despite many relevant arguments for the use of self-report measures in intellectual disability research, there are few validated self-report measures available, with even fewer for sensitive topics like stressful experiences (Ruddick and Oliver, 2005; Ali et al., 2008). Information on the suitability of a self-report measure for certain subgroups within the general population such as persons with cognitive impairments, limited verbal abilities, or clinical populations, is generally found in the manual or published peer-reviewed validation research. However, in many cases, self-report measures do not have detailed manuals, the manuals are unavailable, or they do not even exist. We therefore strongly advise future researchers to publish clear user manuals and/or assessment procedures of self-report measures, even if they seem to be simple and easy to use. In addition, for those self-report measures not specifically designed for people with (M)ID, there is no published research on the use in the MID population. Norm data from validity studies are often based on research that excluded people with MID a priori based on their level of IQ. The relevance and suitability of many of the self-report stress measures found for people with MID therefore still remains unclear. More research is needed on the “performance” of a measurement instrument in populations including people with MID. Therefore, we recommend that future validation studies of self-report measures always include a subpopulation composed of respondents with MID.

As noted earlier, stress is operationalized by many different theoretical constructs in the self-report stress measures analyzed. This raises the question of whether this could affect the measured results. On the other hand, research also shows that the operationalization of apparently different concepts, such as “stress” and “state-anxiety,” essentially measure the same items and therefore can be regarded as the same type of outcome (Hook et al., 2008; de Witte et al., 2020a,b). This has led to these concepts being used interchangeably in literature when it comes to outcome studies (Wetsch et al., 2009; Bradt and Dileo, 2014; de Witte et al., 2020b). Nevertheless, we think it essential to provide a theoretical framework underpinning the measurement concepts involved. Not only will this offer the necessary background information for future users, like clinicians, but it also increases the content validity of the self-report measure (Lynn, 1986; Higgins and Straub, 2006).

In order to validly and reliably assess stress-related outcomes in people with MID, attempts should be made to make the self-report stress measures more “MID-inclusive.” However, it is still not entirely clear which specific instrument components or adaptations are required for this purpose. The recent study by Kooijmans et al. (2021a) shows that there are still many gaps to fill on this topic. Findings show, for example, that researchers and clinicians assume questions should be read aloud by the assessor in order to assist people with MID. However, there is reason to believe that this may introduce various forms of bias in the results, arising from complex interviewer-interviewee dynamics (Finlay and Antaki, 2012). More research on the impact of assistance on the outcome of self-report measures is needed to decide whether this is an acceptable practice.

Lastly, the literature shows that Likert scales with three to five answer alternatives can be reliably used in research with people with MID (Fang et al., 2011). However, in the field of stress research, more nuanced response formats may be needed to capture the subtle differences in perceived stress over time. Visual Analogue Scales (VAS), for example, may offer an interesting alternative for this and have potential for assessing stress levels in people with MID. The Subjective Units of Distress Scales (SUDS) developed by Wolpe (1969) is an example of this. Notably, Mevissen et al. (2016) show promising results when using the SUDS in the treatment of trauma-related symptoms of people with MID. As many VAS scales differ in form, more research is advised on how to optimally attune these VAS scale formats to the needs of people with MID.

In conclusion, many adults with MID frequently experience stress in daily life and this has a major impact on their well-being. This emphasizes the importance of assessing stress levels as part of their support needs assessment. Research suggests that self-report measures are more accurate and sensitive compared to proxy measures. However, this scoping review found that there are few self-report stress measures suitable for this purpose.

This underlines the need for continuing efforts to develop high quality and “MID-sensitive” self-report stress measures.