3.1. Qualitative Synthesis
The 30 studies included in the review that explicitly addressed the study objectives are contained in Table 1
. The United Kingdom (n
= 9) conducted the most studies. Next was the United States of America (USA) (n
= 7), followed by the Republic of Ireland (n
= 2), South Africa (n
= 2), Spain (n
= 2), Sweden (n
= 2), and Taiwan (n
= 2). Each of the following countries had one study: Canada, Norway, Sri Lanka, and the Netherlands. The studies included sample sizes of sexually abused adults with intellectual disability ranging from 8 to 1071 (M = 114.5). The whole sample was comprised of 3434 abused adults with intellectual disability. Comparison samples ranged from 13 to 11,878 participants (M = 1076.6; N = 32,298). A total of 23 studies (76.6%) were performed in a clinical context, meaning that all participants were identified as victims with intellectual disability or alleged victims of sexual abuse, or the participants came from reported incidents of sexual abuse, or were attending an intervention program. These are convenience samples rather than samples drawn from the general population. In some cases, the absence of comparison data prevents the obtaining of global or sex-disaggregated prevalence data, which explains why some studies were removed from the meta-analysis. It can be noted that the studies offer prevalence data from information gathered over several years (M = 4.5; SD = 3.3). All the studies were retrospective.
Although four studies claim that they are longitudinal in nature, the data reported has a cross-sectional nature. The study from Beadle-Brown at al. [30
] reported findings from one of the largest databases in the UK, collected between 1998 and 2005. However, data on people with intellectual disability were available for only eight months of 2005, so the authors utilized estimations and reported a rate of 17.3% of referrals for sexual abuse for this group. Likewise, Brown et al. [32
] were carrying out an ongoing project, but the study focuses on the data from reported incidents of sexual abuse during a year. Similarly, Cambridge at al. [34
] started from a larger study that examined 6148 adult protection referrals between 1998 and 2005, of which 397 were referrals for alleged sexual abuse. Global estimations and percentages of sexual abuse in men and women were utilized for prevalence. Finally, Lin et al. [45
] analyze nationwide data from 2002–2007 on sexual assault and report prevalence and trends, although data on intellectual disability are not disaggregated by gender.
3.3. Prevalence of Sexual Abuse in Adults with Intellectual Disability
A total of 23 studies were included for the overall meta-analyses and the pooled prevalence of sexual abuse in adults with intellectual disability was 32.9% (95% CI: 22.7–43.0) (see Figure 2
). The Q
analysis showed significant results (Chi square = 5024.72, p
< 0.001), pointing to a high heterogeneity in the included studies (I2
= 99.8%). We further investigated the source of heterogeneity by doing a leave-one-out sensitivity analysis to identify whether individual studies outweighed the average prevalence of sexual abuse. Our result revealed that the average obtained when each study was omitted one at a time from the analysis ranged between 32.2% (95% CI: 22.2–42.2) and 35.5% (95% CI: 25.4–45.6). This implied that the average prevalence of sexual abused among participants with intellectual disability in the different studies was not outweighed by a single study.
Since significant heterogeneity was found, age was assessed as moderator and this was not found (β = 0.001, SE = 0.01, 95% CI: −0.013–0.012, p = 0.982) significantly correlated to effect sizes. Results were compared according to several factors. First, more general features such as (1) country and (2) period when the study was published were analyzed.
The overall prevalence, where more than one study was carried out in the same country, for the UK studies (n = 8) was 34.1% (95% CI: 15.1–53.0, p
< 0.001). Prevalence in the USA studies (n
= 5) was 15.2% (−1.6–32.0, p
= 0.077). Prevalence in the Spain studies (n
= 2) was 20.3% (95% CI: −7.7–48.8; p
= 0.155) and prevalence in the Taiwan studies (n
= 2) was 29.4% (95% CI: −17.7–76.5; p
= 0.221) (see Figure 3
Concerning the period when the studies were published, three groups (quartiles 25, 50, and 75, respectively) were identified: (1) up to 1994 (n = 9); (2) from 1995 to 2007 (n = 7); (3) from 2008 or later (n = 7). The prevalence for older studies was 48.4% (95% CI: 29.6–67.3; p = 0.005); for intermediate studies 26.9% (95% CI: 8.0–45.7; p < 0.001); and for more recent studies 25.3% (95% CI: 11.9–38.6; p < 0.001).
Next, more specific features of sexual abuse were analyzed: (1) setting or place(s) where the abuse happened; (2) profile of the abuser, (3) severity of the intellectual disability; (4) informant of the abuse (self-report, third parties, or both). Table 2
summarizes the results.
When comparing the results by most frequent settings where the abuse took place, four subgroups were made: (1) several places, (2) home (family home, own home, or group home with supervision or supported home, which typically consists of living in a home with two other people, or where any other small number of unrelated members live together), (3) institution (large facilities, institutionalized), and (4) service (educational, training or social services). As Table 2
summarizes, the prevalence of sexual abuse was significantly different among the four subgroups (Q = 4767.23, p
< 0.001). Prevalence in studies where abuse took place in several places (n = 19) was 39.1% (95% CI: 21.1–57.1; p
< 0.001); (2) prevalence in institutionalized individuals (n = 8) was 29.1% (95% CI = 15.1–43.2). In subgroup comparisons, the several places subgroup showed the highest prevalence (39.1, 95% CI: 21.1–57.1) of sexual abuse, followed, in descending order, by sexual abuse experienced in services (34.3, 95% CI: 2.3–66.3), institutions (28.1, 95% CI: 12.0–44.1), and at home (13.1%, 95% CI: −10.6–36.9).
For analyzing results by the most frequent profile of the abuser, five subgroups were made: (1) several, (2) peers (another user, roommate, partners, and friends), (3) professionals, (4) relatives, and (5) non-specified. The prevalence of sexual abuse was significantly different among the five subgroups (Q = 4767.23, p
< 0.001) (see Table 2
). The most prevalent abusers were peers (42.7%, 95% CI: 19.7–65.7), followed by relatives (36.2%, 95% CI: −30.5–100.0), several abusers (25.4%, 95% CI: 10.0–40.8), and professionals (17.6%, 95% CI: −1.9–37). Non-specified abusers obtained a prevalence of 39.2% (95% CI: 13.2–65.2).
According to the severity of the intellectual disability, four groups were made: (1) mild, (2) moderate, (3) severe, and (4) profound. As Table 2
shows, the prevalence of sexual abuse was significantly different among the four subgroups (Q = 2545.31, p
< 0.001). The highest prevalence of sexual abuse corresponded to severe levels (67%, 95% CI: 59.5–74.4) of intellectual disability; followed by moderate (34.3%, 95%, CI: 17.2–51.4), mild (24.3%, 95% CI: 10.2–38.4); and profound (18.6%, 95%: CI: −15.7–53.0) levels. Rates of sexual abuse were significantly different among the subgroups of informants (Table 2
) (Q = 4767.23, p
< 0.001). Prevalence for self-reports was (38.0%, 95% CI: 20.1–55.8), and from someone else was 26.8% (95% CI: 9.0–44.6%). When the informant was both, prevalence was 37.4% (95% CI: 17.9–56.9).
Separate meta-analyses were conducted with male and female population (see Supplemental Material, Tables S1–S15, and Figures S1–S8
). A total of 22 studies were considered for females with intellectual disability, and the prevalence of sexual abuse was 31.3% (95% CI: 8.7–43.8; Q = 2200.98, p
< 0.001; I2
= 100%, T = 0.30). In clinical studies, prevalence was higher (58.4%, 95% CI: −3.9–>100) than in general or non-clinical studies (28.4%; 95% CI: 16.1–40.8). Concerning the place where the sexual abuse happened, studies with occurrence at home had the highest prevalence (45.2%, 95% CI: −42.9–>100), followed by several places (38.5%, 95% CI: 19.1–57.8), institutions (27.5%, 95% CI = 8.1–46.8), and lastly, in educational, training or social services (14%, 95% CI: −4.8–3.2). Regarding the profile of the abuser, in descending order, this consisted in peers (44.1%, 95% CI: 21.8–66.5), relatives (28.7%, 95% CI: −2.72–84.7), by several abusers (11.1%, 95% CI: 2.0–20.2), and by professionals (6.4%, 95% CI: −6.1–19.0). It should be noted that non-specified abusers had a prevalence of 62.1% (95% CI: −39.6–84.6).
A total of 16 studies were included for males with intellectual disability (see Supplemental Material
), where the prevalence of sexual abuse was 39.9% (95% CI: 21.5–58.3; Q = 5620.28, p
< 0.001; I2
= 99.84%, T = 0.37). In non-clinical studies, prevalence was 37.7% (95% CI: 19.0–56.4). In clinical studies, prevalence was 54.4% (95% CI: −33.0–>100.0). Grouping the studies according to place, studies where abuse occurred in institutions have the highest prevalence (50.8%, 95% CI: 15.8–85.8), followed by different places (41.8%, 95% CI: 14.1–69.4), educational, training or social services (15.9%, 95% CI: −10.2–42.1), and at home (9.8%, 95% CI: 3.3–16.2). Regarding the profile of the abuser, prevalence of sexual abuse by peers was the highest (56.1%, 95% CI: 26.3–85.9), followed by the abuse caused by several abusers (34.9%, 95% CI: 6.3–63.5), non-specified abusers (30.7%, 95% CI: −22.4–83.9) and by professionals (2.0%, 95% CI: 0.4–3.6). No studies on abuse by family members were available.