Discussion

The main aim of this study was to examine conceptual and item properties of the RSMB on a French-speaking Swiss and Belgium sample. We also studied the prevalence of the co-occurrence of psychopathology and ID, as well as differences in gender, age and etiology associated with the presence of psychopathology.

4.1. Conceptual and item equivalence of the French version of the RSMB

Confirmatory factor analysis revealed that the factor structure of the French version of the RSMB is robust across populations. The RMSEA value obtained in our sample was slightly stronger than the fit indices reported by Havercamp and Reiss (1997) and Lecavalier and Tassé (2001). The ratio of chi square to degrees of freedom was greater than 2, which is considered by many authors as a superior cut-off for good model adjustment. As mentioned by Lecavalier and Tassé (2001), the French version of the RSMB consists of a relatively large number of factors and an additional 10 individual items that load on more than one factor.

This increases the difficulty in replicating the original conceptual model. Thus, we can conclude that the Reiss (1988) model fits reasonably well to our data and that the 30 scale items measure the same eight factors as originally proposed. Furthermore, internal consistency indices were comparable or slightly better than those found in the Canadian sample (Lecavalier & Tassé, 2001). Comparing the Cronbach’s alphas originally reported by Reiss (1988), the values reported in the present study were consistent for the global scale, Aggressive Behavior subscale, and Physical Signs of Depression subscale; were lower for the

Psychosis, Dependent Personality, Avoidant Disorder, and Behavioral Signs of Depression subscales; and were higher for the Paranoia subscale. Regarding the instrument’s functional equivalence, the items seemed to estimate the same latent traits as did the original research and those traits appeared as conceptual equivalents to those reported by Reiss (1988).

4.2. Prevalence

The prevalence of psychopathology was examined in our sample of 467 adults with ID, and a global psychopathology prevalence of about 37 % was found. Rojahn and Meier (2009) reviewed the literature published between 2000 and 2009 on co-occurrence of ID and psychopathology. Regarding mental illness, the prevalence rate reported by these studies ranged from 15.7 % to 54.0 %. This wide range can be explained by, among other things, cross-study differences in sample characteristics, assessment criteria, and informant versus self-reporting tools. Cooper and colleagues (Cooper, Smiley, Morrison, Williamson, & Allan, 2007) illustrated the assessment criteria influence in examining the point prevalence of psychopathology in a population-based sample of adults with ID. These authors reported a point prevalence of 15.7 % when using the Diagnostic Criteria for Research of the International Classification of Disease (World Health Organization, 1993); 16.6 % when applying the criteria of the Diagnostic and Statistical Manual of Mental Disorders (American Psychiatric Association, 2000); 35.2 % when using the Diagnostic Criteria for Psychiatric Disorders for Use with Adults with Learning Disabilities (Royal College of Psychiatrists, 2001); and 40.9 % when a clinical diagnosis was made. Gustafsson and Sonnander (2004) analyzed the point prevalence of psychopathology among 296 Swedish adults with ID, using RSMB. The authors reported that 37 % of adults with ID scored as high as or above the clinical cut-off on the RSMB global score. The same prevalence rate was found in our French-speaking Swiss and Belgium sample. Despite this identical prevalence rate, our study highlighted different rates of clinical cases for the different psychopathological conditions assessed by the RSMB subscales compared with those found by Gustafsson and Sonnander (2004). More specifically, they reported a higher prevalence of Aggressive Behavior (13.3 %) and Autism (6.5 %) in comparison with our results; a lower prevalence of Psychosis (4.8 %), Paranoia (3.7 %), Behavioral Signs of Depression (5.4 %) and Dependent Personality (3.4 %);

and very similar levels of Physical Signs of Depression (10.9 %) and Avoidant Disorder (7.5 %).

4.2.1. Age differences

Despite the absence of a linear relationship between age and psychopathology in our sample, detailed analyses revealed several differences between adults with ID of different age classes. In most cases, differences in psychopathology concerned the youngest and the oldest participants, with lower levels of psychopathology being observed in the youngest group. The latter also had fewer problems than all other groups regarding Behavioral Signs of Depression and Paranoia. Few studies have examined the relationship between age and psychopathology among adults with ID. Rojahn and Meier (2009) reported only one study exploring psychopathology and age differences in adults with ID. In that study, Moss and Patel (1993) examined mental illness in adults aged between 18 and 94 years presenting moderate to profound ID. These authors reported that 77 % of adults identified with mental disorders were aged between 51 and 94 years. This is consistent with the results reported by Cooper (1997), who analyzed the frequency of mental illness in a group of 134 adults with ID aged 65 years and above in comparison to a group of 73 younger adults with ID. The author found a significantly higher frequency of global mental illness in the older group, with higher rates of dementia, anxiety and depression. Two more recent studies failed to replicate this age effect as a predictor of mental illness (Cooper, et al., 2007; Jones et al., 2008). These results might suggest—consistent with the results found in the present study—that the influence of age on psychopathology consists of a difference in frequency among different age groups rather than being a linear predictor of mental illness severity. However, even when age classes are considered, age group differences are not systematically observed (Gustafsson & Sonnander, 2004). In their review of studies examining ageing and mental health problems in adults with ID, Torr and Davis (2007) concluded that there is no firm evidence either way that permits any type of conclusion regarding the age-associated differences in the prevalence of mental illness in the adult population with ID.

4.2.2. Gender differences

Gender differences were found in certain types of mental health problems in our sample. Males were more likely to present autism, avoidant disorder, and aggressive behavior symptoms, while females were more likely to present behavioral signs of depression. The literature on gender differences and psychopathology shows that most studies report that overall, males with ID are more frequently diagnosed with psychopathology than females are (for a review, see Lunsky, Bradley, Gracey, Durbin, & Koegl, 2009). Similarly, Tsakanikos, Bouras, Sturmey, and Holt (2006) reported that psychiatric disorders of any kind and

personality disorders were more frequently diagnosed in males, whereas dementia and adjustment disorder were more frequently diagnosed in females. This is consistent with the results of our study regarding avoidant disorder, which is a part of personality disorders. Male gender is often cited as a risk factor that predicts aggression (Gentile & Gillig, 2012). With only a few exceptions (e.g. Tsiouris, Kim, Brown, & Cohen, 2011), many studies, including ours, reported that males with ID are often more aggressive than females with ID. We also found a gender difference on the Autism subscale, with males experiencing more autistic symptoms than females, which is consistent with literature. Fombonne (2007) reviewed 32 studies with a combined 6,963 participants with autism spectrum disorder and reported an average male:female prevalence ratio of 4.3:1.

Regarding depression, several studies have reported that adult women with ID have higher rates of these symptoms than their male peers. Lunsky (2003) found significantly higher rates of self-reported depression in a group of 48 women with ID compared with a group of 51 male peers with ID. This difference, however, was not statistically significant when informant report instruments were used. However, when RSMB depression subscales were used, a higher rate of Behavioral Signs of Depression was found in the women, whereas identical rates of Physical Signs of Depression were found in the two groups. More recently, a study of gender differences in a large sample (n = 1,971) of persons with and without ID receiving psychiatric in-patient services reported that mood disorders were more than twice as common in women than in men, in samples with or without ID (Lunsky et al., 2009). Studies on the general population reported that women are twice as likely as men to be diagnosed with depression (Astbury & Cabral, 2000). Although most studies reported a higher rate of depression in females, gender differences in association with depression in adults with ID need to be further studied.

4.2.3. Down syndrome

Our results showed that adults with DS scored lower than adults with NSID on the global scale and on all RSMB subscales, with the exception of the Autism subscale. These differences were found after adjusting scores for age and gender, and the results are consistent with several studies. Mantry and colleagues (2008) examined rates of mental illness in186 adults with DS. This sample was part of a larger study analyzing mental illness in 1,023 adults with ID of mixed etiology (Cooper et al., 2007). Comparison of those two studies revealed that adults with DS were less likely to be diagnosed with mental illness of any kind, with the

adults with DS (Dykens, 2007; Määttä et al., 2011), the literature is inconsistent regarding the differences in the prevalence of depression in adults with DS versus adults with ID of mixed or non-specific etiology (for a review, see Mantry et al., 2008). In general, it seems that adults with DS present fewer psychopathological symptoms than adults with IDNS do. However, further studies are needed to clarify which specific symptoms better discriminate these populations and to reveal the specificities of psychopathological evolution during adulthood.