Introduction

The issue of psychiatric co-morbidity in adolescents with autism and related disorders is a complex one (Volkmar and Klin, 2005). On the one hand it does appear that, as with other developmental disorders, a process of diagnostic overshadowing has tended to occur, i.e. a tendency to overlook co-morbid conditions given the significance of the individual's other developmental problems (Dykens, 2000).

On the other hand it is also quite clear that, of itself, a condition like autism will be associated with a range of additional symptoms which may not, necessarily, rise to the level of ‘disorder’. For example, problems with attention are frequently seen in autism and Asperger's syndrome, but it remains unclear whether such difficulties are sufficient to achieve an additional diagnosis of attention deficit hyperactivity disorder (ADHD). Nevertheless some researchers have proposed that such a combination may prove to be a robust subtype (Landgren et al., 1996). This problem is further compounded when the issue is co-morbid diagnosis of conditions, like anxiety or depression, in lower-functioning adolescents, i.e. in individuals with little or no expressive language. Additionally much of the data on psychiatric co-morbidity in adolescence rests largely on case reports. As noted elsewhere (Volkmar & Woolston, 1997; Kent et al., 1999; Slone et al., 1999) this literature is difficult to interpret given the bias for positive associations to be published; the critical issue is whether rates of a given disorder in larger (ideally epidemiologically representative) samples can demonstrate significant elevations in the rates of co-morbid conditions (Fombonne, 2003).