The search strategy identified 2618 references (Fig. 2). After removing duplicates, 1848 references remained. Two of the reviewer (altogether five) authors scrutinized all the titles and abstracts according to the inclusion criteria, and when information necessary for inclusion was lacking, they read the articles’ full texts. We obtained the full texts of 244 articles, and 38 studies met our inclusion criteria.
Characteristics of the included studies
Design and methods
Ten of the 38 included studies were quantitative (Table 1), six studies were qualitative (Table 2), one was a mixed methods study (Table 2), and 21 were case studies (Table 3, 4). The quantitative studies included one RCT , one CCT  and eight cohort studies [45,46,47,48,49,50,51,52]. The qualitative studies included three interview studies [53,54,55], one interview and document study  and two ethnographic studies [57, 58]. The mixed method study  used structured interviews, focus group interviews and register analysis. The 21 case studies included five qualitative case studies [60,61,62,63,64] and 16 quantitative experiments [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80].
The total number of participants in the 38 studies was 2,41,080. The quantitative studies included 35–1,21,335 participants (n = 2,39,506), the qualitative studies 3–27 participants (n = 58), the mixed method study included 1452 participants and the case studies 1–10 participants (n = 64).
The age of the participants varied between 16 and 51. However, two qualitative studies [53, 54] and two case studies [62, 64] did not report the age of the participants. The proportion of men and women differed in the studies. In the quantitative studies, the proportion of women varied from 29%  to 58% . In one qualitative study  all the participants were men and in another qualitative study  all the participants were women. Two studies [51, 59] did not report the gender of the participants. Seven studies reported the ethnicity of the participants [45, 47,48,49, 51, 52, 79].
Sixteen of the 38 studies reported the degree (mild, moderate, severe, profound) of ID as a background factor of the participants. People with mild ID participated in two cohort studies [48, 51], one qualitative study , and one case study . People with either mild or moderate ID participated in three case studies [71, 75, 76], and people with moderate ID participated in two case studies [68, 80]. Renzaglia et al. , Simmons and Flexer , McGlashing-Johnson et al. , and West and Patton  focused on people with moderate or severe disabilities. People with severe ID were participants in three case studies [61, 67, 69]. Two studies [45, 77] reported the participants ‘level of functional capacity.
Seven of 38 studies concerned students [46, 48,49,50,51,52, 59], one study  concerned people who worked in sheltered workshops, and one study  concerned job-seekers. Four studies reported the occupation or work tasks of the participants as the background factors [69, 72, 75, 79].
All studies contained various mixtures of intervention components. The interventions were carried out during secondary education, the transition from education to work, job-seeking and sheltered work. In general, the content of the intervention was briefly described. However, the stages of the intervention process were poorly reported, and the term rehabilitation was only seldom used.
During secondary education (Upper Secondary School for Pupils with ID) (ICF code d825), the students participated in different educational programs such as national programs, specially designed programs, vocational training and training activities or graduated with inadequate grades . National programs focused on different parts of the labor market, for example, vehicles and transportation, hotels and restaurants, or social and healthcare. Some specially designed programs or individually tailored education were also on offer. The educational programs aimed to improve vocational qualifications and work awareness. The vocational qualification courses included assisting smaller pupils during classes, helping the janitor and working in the school kitchen. Work awareness training courses included watching videos about work, talking about presenting oneself at work, and health and safety instructions at school .
Some interventions included postschool and transition services (ICF code e5900) during education to improve the transition from education to work [43, 48, 49, 51, 55, 75]. Postschool services included postsecondary education institution accommodations and services, job training services, and life skills services . Employment-related transition services were, for example, vocational assessment, career counseling, pre-vocational education, career-related technical or vocational education, pre-vocational or job readiness training, instructions for job-seeking, job shadowing, job coaching, special job skills training, placement support, internship or apprenticeship programs, work experience at school and other paid work experience [44, 48, 55, 65]. Job tasting was a short, unpaid, time-limited work experience period at the workplace which allowed people to sample a variety of different jobs and work cultures . One qualitative study  analyzed the different actors’ experiences (schools, agencies, parents, other people) of the transition process and of employment (e310), and Fasching’s  study analyzed the experiences of people with ID of the transition from school to vocational training and employment.
The interventions also included supported work (supported employment, SE) (d855) , the use of a job coach or other support person (e340, e360) [45, 63], and designing personal solutions (e5900) . One cohort study  analyzed whether attending a sheltered workshop (e5900) improved the employment outcomes of supported employees with ID.
Some interventions developed the independence of people with ID, applying the ICF model to the areas of body structures/functions, activities and participation. The interventions included support in psychological and behavioral issues (behavior) (b122) [56, 64] and communication abilities (d3) ; and a self-regulated problem-solving process in which students set their goals, developed and implemented their action plans (b164, d175) , made career-related decisions (d177) , were able to perform daily activities (d620, d630) , learned navigation skills such as finding the workplace and using public transportation (d4602) , and gradually reduced help in carrying out work tasks (d850) [72, 75, 77].
The use of digital solutions (e135) was intended to improve the daily work performance of people with ID in the open labor market. They could use these tools for receiving digital instructions for work tasks, work processes, work techniques or schedules. These solutions consisted of video modeling and audio coaching [72, 76], photo activity schedule books , smart phones , palmtop-based job aids , and self-operated auditory prompting systems [68, 80]. In the case studies, the interventions were carried out at workplaces such as restaurants [60, 66], hospitals , factories and warehouses , at markets , at schools [70, 72], and in conference rooms .
The outcomes were employment in the open labor market [43, 45, 47, 51, 52], transition from school to the open labor market [44, 46,47,48, 54, 55], and work performance [43, 44]. The outcomes in all 16 quantitative experiments were job skills and work performance [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80].
The RCT study  was considered to be of high methodological quality, with scores of 11 out of 22, and the CCT study of Kilsby and Beyer  was of low methodological quality, with scores of 9 out of 22 according to van Tulder et al. . The methodological quality of four cohort studies [47, 49, 50, 52] out of six were high, with the maximum of nine ‘stars’ according to Wells et al. . Four cohort studies [45, 46, 48, 51] and one multimethod study  were considered to be of intermediate quality. All six qualitative studies [53,54,55,56,57,58] were considered to be of high quality, with scores ranging from 12 to 20 out of 20, according to the modified CASP method. (Table 6–9, Supplementary Files).
The quantitative studies showed that supported work increases the employment of people with ID in the open labor market (Table 5). This result was based on one high-quality RCT study , one high-quality cohort study , and one moderate-quality cohort study  of altogether 16,947 people with ID. The quantitative studies also showed that both secondary and postsecondary education, including support services and work training, increased the transition of people with ID from school to the open labor market. The result was based on two high-quality cohort studies [50, 52] and three moderate-quality cohort studies [46, 48, 51] with 2,07,484 participants altogether.
However, on the basis of one high-quality RCT study  and one high-quality cohort study  covering a total of 15,089 participants, sheltered work did not increase the employment of people with ID in the open labor market (Table 5).
Barriers to and facilitators of employment
The qualitative studies concerned both the barriers to and the facilitators of employment in the open labor market (Table 2). The main barriers to employment were that the school and service system tried to guide people with ID towards traditional services such as sheltered work, sometimes against their own needs and interests . However, these people’s work skills were not developed or their needs for support were not noticed in sheltered work . Further barriers were discrimination in vocational experience after leaving school , poor experiences of class teaching and lack of work experience [58, 59].
The main facilitators of employment were people’s own activity and support from their families (e310) , effective job coaching (e360) , a well-designed work environment (e2) , appreciation of their work and support from their employer and work organization . Other facilitators were the knowledge and experience of work during education , and for entrepreneurs, the use of a support person .
The case studies showed that the use of digital solutions (e135) improved the work performance of people with ID in the open labor market. Work performance was measured as the percentage and rate of the tasks completed correctly [69, 72], the number of movements measured by a costume , the number of independent task changes , task steps , and percentage of intervals with task engagement and interactions .