The incidence of 7.8% ASD in gender identity clinic referred children and adolescents is ten times higher than the prevalence of 0.6–1% of ASD in the general population (Baird et al. 2006; Fombonne 2005). This important finding confirms the clinical impression that ASD occurs more frequently in gender dysphoric individuals than expected by chance.

Most remarkable in the clinical presentation of individuals with co-occurring GID and ASD in this study was the considerable diversity concerning: sex (both male and female), GID classification (GID, GID-NOS, transvestic fetishism), ASD classification (autistic disorder, Asperger’s disorder), age of onset of GID (before or after puberty), and the persistence of cross-sex behavior (ceasing or persisting).

The overrepresentation of males in our study is in accordance with the epidemiology of both ASD and GID (Fombonne 2005; Zucker and Lawrence 2009). Studies in girls with congenital adrenal hyperplasia, who are prenatally exposed to high levels of testosterone, showed that they had more traits of ASD than controls and some developed gender identity problems (Dessens et al. 2005; Knickmeyer et al. 2006). However, the idea that prenatal testosterone may be involved in the vulnerability to both ASD and gender dysphoria, seems not applicable in our sample. In that case, especially girls with ASD would have been susceptible to develop gender dysphoria. In addition, why gender dysphoria and ASD co-occur in males is unexplained by prenatal androgen exposure. This finding adds to the lack of experimental evidence for a link between low testosterone and GID in males.

Symptoms of GID displayed by individuals with ASD varied substantially. Individuals with an ASD frequently received a GID-NOS diagnosis. GID-NOS appeared to be given when the cross gender behavior and interests were merely subthreshold (mostly in children), or atypical or unrealistic. For example, an adolescent with ASD, who always had the feeling of being different from his peers in childhood, but had no history of childhood cross-gender behavior, became convinced that this feeling of alienation was explained by gender dysphoria. He had the hope that his communication problems would alleviate by taking estrogens. In another adolescent, the cross-gender behavior indicated transvestic fetishism rather than gender dysphoria. This corresponds with the findings of frequent non-normative sexual interests and behaviors in adolescents and adults with ASD (Hellemans et al. 2007). Feminine interests of many gender dysphoric young boys with ASD concerned soft tissues, glitter, and long hair and could be understood as a preference for specific sensory input typical for ASD.

While almost all adolescents with GID are sexually attracted to individuals of their birth sex (Smith et al. 2005; Wallien and Cohen-Kettenis 2008), the majority of the gender dysphoric adolescents with ASD was sexually attracted to partners of the other sex. This may be clinically relevant since adult transsexuals not sexually attracted to their natal sex show in some studies less satisfactory postoperative functioning compared with birth-sex attracted transsexuals (Smith et al. 2005).

Most individuals with co-occurring gender dysphoria and ASD fulfilled the strict criteria of autistic disorder. For several youth with ASD, their ASD-specific rigidity made enduring gender variant feelings extremely difficult to handle. After all, in our society a considerable amount of flexibility is needed to deal with gender variant feelings. Normally developing young children (age 3–5) display more rigidity in gender-related beliefs than older children; this rigidity decreases after the age of five (Ruble et al. 2007). Individuals with ASD may not reach this level of flexibility in their gender development.

In most gender dysphoric children, gender dysphoria will cease when they reach puberty, whereas adolescents with a GID will likely pursue their wish for sex reassignment into adulthood (Cohen-Kettenis and Pfäfflin 2003; Wallien and Cohen-Kettenis 2008; Zucker and Bradley 1995). Likewise, in children under age 12 with co-occurring ASD the gender dysphoria alleviated and in adolescents between age 12 and 18 their GID persisted.

The present findings should be considered in the light of limitations in sampling and assessment. First of all, it is apt to realize that the actual incidence of ASD in gender clinic referred children and adolescents is probably higher than the reported percentage. One of the reasons to assume an underestimation is that some children and adolescents with ASD (as diagnosed elsewhere) were unable to fulfill the assessment procedure due to serious impairment. In other cases, features of ASD were observed, but parents tended to underreport the ASD symptoms in the DISCO-10 interview, as that was not the primary concern about their child. Finally, it was not feasible to administer a DISCO-10 interview to all participants. Clinicians of the gender identity clinic identified suspected cases. Based on the epidemiology of ASD, the majority was expected to fall into the broader spectrum, whereas in this study no pervasive developmental disorder not otherwise specified was classified. This may reflect that our clinicians had a high threshold for suspecting ASD, as their primary focus was gender dysphoria.

Second, the individuals who investigated whether criteria for ASD were met, were aware that the reason for referral was a possible GID. This study was performed at a specialized gender identity clinic, and therefore blinding was not feasible.

Third, we studied a gender identity clinic referred population. Whether the same high incidence of co-occurring ASD and gender dysphoria will also be found in individuals with ASD as primary concern remains unknown and should be of interest for further studies. One might expect milder or more atypical types of gender dysphoria as part of other non-normative sexual behaviors and interests, as was found in a study of males with ASD (Hellemans et al. 2007).

Third, the categorical DSM-IV-TR classification approach is not appropriate to study more subtle expressions of both ASD and GID. Future studies should focus on dimensional measures and specific cognitive or neuropsychological profiles of individuals with co-occurring gender dysphoria and ASD. For example, the observed rigidity in gender related beliefs in young children may make children with ASD more prone to develop gender dysphoria (Ruble et al. 2007). Furthermore, it seems important to address the broader autism phenotype (clinical traits that are milder but qualitatively similar to the characteristics of autism (Losh et al. 2009) in upcoming studies in a gender identity clinic referred population. It may be that these traits occur more often in gender dysphoric individuals than a full-blown ASD as was found in, for example, children with mood and anxiety disorders and ADHD (Nijmeijer et al. 2009; Pine et al. 2008).

For clinical management, our findings on clinical characteristics of individuals with co-occurring gender dysphoria and ASD have consequences. In all cases described, the diagnostic procedure was extended to disentangle whether the gender dysphoria evolved from a general feeling of being ‘different’ or a ‘core’ cross-gender identity. Most helpful seemed an individual approach that took into consideration that rigid and concrete thinking around gender roles and difficulty developing aspects of personal identity may play a part. Concerning sex reassignment, ASD does not have to be a strict exclusion criterion. Only one case study described a woman with an Asperger’s disorder who received sex reassignment (Kraemer et al. 2005), although we know of more cases in clinical practice. In our sample, almost half of the adolescents with both ASD and GID started with gender reassignment. Worrisome are the adolescents that dropped out of care, probably finding their own ways to sex reassignment, without psychiatric treatment or medical attention. It remains a challenge to provide individuals with co-occurring gender dysphoria and ASD with proper care.