As hypothesized, the present study not only confirmed an earlier finding of increased endorsement of Item 110 in children with ASD (Janssen et al., 2016; May et al., 2017; Strang et al., 2014), but is also the first study to find the same results in adults with ASD. Whereas parent-reported endorsement of this item was 7.59 times more likely in the Strang et al. study and 7.76 times more likely in the Janssen et al. study, this time self-reported endorsement of Item 110 was 2.12 times more likely in adolescents and 2.46 times more likely in adults with ASD compared to non-referred standardization samples of the YSR and ASR, respectively (Achenbach & Rescorla, 2003; Verhulst et al., 1997). The findings were in line with the previously reported overrepresentation of ASD in individuals with GD (de Vries et al., 2010; Edwards-Leeper & Spack, 2012; Jones et al., 2012; Pasterski et al., 2014; Shumer et al., 2016; Skagerberg et al., 2015) and provide evidence of an increased wish to be of the opposite gender in adults with ASD. It should be realized that endorsement of this item measured with the YSR/ASR does not imply a GD diagnosis. In addition, more individuals stated that they “sometimes” rather than “often” wished to be of the opposite gender. Further, this endorsement occurred in only 6.5% of the adolescents and in 11.4% of the adults. Despite these facts, the increased prevalence of wish to be of the opposite gender in an ASD population compared to the general population deserves further study.

Contrary to our second hypothesis, adolescent girls with ASD had a significantly higher prevalence of endorsement of Item 110 compared to adolescent boys with ASD. Jones et al. (2012) found elevated symptom levels of ASD in birth-assigned females diagnosed with GD but not in birth-assigned males diagnosed with GD. They hypothesized that elevated levels of fetal testosterone may lead not only to reduced empathy, reduced social interest, reduced social skills, and more ASD symptoms (Knickmeyer, Baron-Cohen, Raggatt, & Taylor, 2005; Knickmeyer et al., 2006), but, as part of neurodevelopmental masculinization, also to have a higher chance to develop GD (Jones et al., 2012). Our finding of significantly higher prevalence of the gender item in assigned girls with ASD compared to assigned boys with ASD in adolescents but not in adults only partly supports this hypothesis. As the wish to be of the opposite gender in assigned males at birth remains unexplained by this theory, more research is necessary. A study by Bejerot and Eriksson (2014) in adults with ASD found that tomboyism and bisexuality were overrepresented in females with ASD, but certain other aspects considered as typically masculine (e.g., assertiveness and leadership) were reported to be weaker in both females and males with ASD compared to typically developing controls, suggesting that an extreme male pattern might not apply to all aspects of gender roles and sexuality. A brain MRI study in individuals with ASD also found attenuated typical gender differences in white matter tracts (Beacher et al., 2011), providing support for gender atypicality as one of the potential underlying mechanisms for co-occurring GD–ASD.

Also, contrary to earlier years, there is recent evidence suggesting that more birth-assigned adolescent girls present to specialized gender identity clinics than birth-assigned adolescent boys. This could thus explain the higher odds of the gender item in adolescent girls with ASD compared to adolescent boys with ASD as the ASD sample in the current study had been collected more recently than the sample of the general population that was used as comparison (Aitken et al., 2015; Verhulst et al., 1997). The higher prevalence of endorsement of Item 110 in adolescent girls with ASD compared to adolescent boys with ASD might thus reflect a more broader societal phenomenon instead of suggesting an underlying hypothesis involving testosterone.

Another explanation for co-occurring GD and ASD may be the tendency of individuals with ASD for an intense focus on or an obsessional interest in specific activities, as was suggested by VanderLaan et al. (2014). VanderLaan et al. suggested that when children with ASD form such intense interests in cross-gender objects or activities, a cross-gender identity might develop over time. While typically developing children become more flexible in their stereotyped ideas about gender when they get older (Ruble et al., 2007), children with ASD may be prone to develop GD because of their rigid thinking and difficulty with change (APA, 2013). However, if this were true, the stereotyped subdomain, including repetitive behavior and difficulty with change C/ASBQ subdomain should particularly be elevated in our participants, but the present data did not fully support this idea. This is in line with reports of some clinicians working with GD who do not report obsessional interests as a drive to cross-gender identification (Strang et al., 2018).

According to our third hypothesis, in adolescents there was no significant association between endorsement of Item 110 and any of the specific subdomains of ASD as measured by the CSBQ. In adults, there were only small correlations, but not specifically with regard to limited, repetitive, stereotyped patterns as measured by the ASBQ. Especially in adults, it might be that other ASD characteristics are also involved in the potential co-occurrence between ASD and GD that deserve further study. For example, their specific neuropsychological profiles with deficits in “theory of mind,” the ability to attribute mental states (beliefs, intents, desires, etc.) to oneself and others and recognize that these are different from one’s own, may hamper development of the “self” in general (Lombardo & Baron-Cohen, 2010) and gender identity development more specifically.

According to our fourth hypothesis, of clinical relevance is the finding of our study that emotional problems were more prevalent in adolescents and adults with ASD and co-occurring endorsement of Item 110 compared to those without, especially internalizing problems (depression, anxiety, and somatic complaints). We also found increased problems on all other subscales of the YSR/ASR among those who endorsed sometimes or often wanting to be the opposite gender, but only the internalizing problems were in the clinical range. This is in contrast with the findings of Strang et al. (2014) who did not find increased emotional problems and argued that due to reduced awareness of gender and gender nonconformity among individuals with ASD with increased endorsement of Item 110, they might experience less stigma. Still, the current literature on gender nonconformity gives ample evidence for reduced psychological well-being mainly mediated by experienced stigma (e.g., Baams, Beek, Hille, Zevenbergen, & Bos, 2013), experienced victimization, and ostracism (e.g., Toomey, Ryan, Diaz, Card, & Russell, 2010). Children and adolescents with GD show more self-destructive behavior, including self-harm and suicide attempts (Scourfield, Roen, & McDermott, 2008). As we do know that individuals with ASD have social deficits which put them at risk for a sense that they are “different” from others (de Vries et al., 2010), and with the addition of GD-related stigma, they might be extra prone to experience stigma and show more emotional problems. Further, it might be even more difficult for them than for people without ASD to be open about their gender-variant feelings and “come out,” a step that is thought to be helpful in improving mental health of gender-variant individuals (e.g., Kuyper & Fokkema, 2011). Therefore, clinicians caring for individuals with ASD should realize that their clients might struggle with gender-variant feelings and, if they exist, pay attention to them in an open and nonjudgmental way.

The current findings should be interpreted in light of various limitations. First, the use of only one item of the YSR/ASR (the wish to be of the opposite gender) is a very restricted measure for one aspect of GD. This measure does not acknowledge the non-binary gender identities that are increasingly recognized (Richards et al., 2016). The possibility of responding to this item with “sometimes” or “often” also does not represent the wide variation that is found when these feelings occur. This wish is often not an easily classified “yes” or “no” answer, but a gradual continuum on several aspects of oneself; psychological and physical, ambivalent or incongruent, and with or without a wish for medical gender confirming interventions (see Kuyper & Wijsen, 2014). While the question “wishes to be of the opposite gender” might be sufficient to explore this wish in some individuals with ASD, the rigidity, problems with abstract thinking, and “black and white thinking” might further bias the answering of these questions (e.g., because certain privileges are seen as part of the opposite gender). Therefore, in future studies, it is advisable to use another measure and also qualitative study techniques to examine symptoms of GD.

Second, this study did not make use of a control group other than the YSR/ASR standardization samples. For the YSR, this was a Dutch sample (Verhulst et al., 1997), making comparisons possible, but for the adult sample this was a U.S. sample (Achenbach & Rescorla, 2003). At present, there are, to our knowledge, no U.S. studies on the wish to be of the opposite gender that suggest the prevalence in North America is very different compared to the Netherlands. At the moment, there is some limited evidence that children and adolescents with GD and gender incongruence experience less social ostracism in the Netherlands compared to Canada (Steensma et al., 2014). If this is also true for adults (with or without the wish to be of the opposite gender), it might be easier to report these feelings. The prevalence of reported endorsement of the gender item in our adults with ASD could therefore be somewhat lower if compared with a Dutch adult standardization sample. If increased social ostracism occurs in the U.S., it is expected that expressing feelings of GD is more demanding compared to the Netherlands. Therefore, our finding that individuals with ASD more often experience the wish to be of the opposite gender compared to the standardization sample might be a “sociological” finding of the Dutch population in general experiencing more of those feelings. However, two recent studies on transgender prevalence estimates, with 0.2–0.6% reporting gender dysphoric feelings in a Dutch population study (Kuyper & Wijsen, 2014) and 0.6% in a recent U.S. population study (Flores, Herman, Gates, & Brown, 2016), are making a social explanation only for our results less likely.

In addition, unlike the Strang et al. (2014) study, we did not use other referred control groups. While in their study an overrepresentation of endorsement of Item 110 was found not only in children with ASD, but also in those with ADHD, it would be important to include other referred controls in future studies on the potential occurrence of feelings of GD. It might be that the wish to be of the opposite gender, compared to the general population, is not only overrepresented in individuals with ASD, but also in people with other conditions as well, which might call for different explanations.

Finally, although the use of self-report is a common methodology, the reliability of this method might be different in individuals with ASD compared to typically developing individuals. Individuals with ASD might have problems with introspective ability because of problems with theory of mind, and preliminary evidence suggests caution in the use of self-report measures in adolescents with ASD (Mazefsky, Kao, & Oswald, 2011).

In sum, the present study further confirmed a possible association between ASD and the wish to be of the opposite gender by establishing increased endorsement of this wish in adolescents and adults with ASD compared to the general population controls. Strang et al. (2018) provided a first clinical guideline for adolescents with feelings of GD and ASD based on expert opinions and mention that clinicians working with individuals referred for GD as well as for ASD should pay attention to this possible co-occurrence. Although the wish to be of the opposite gender is not the same as a clinical diagnosis of GD, this is relevant for clinical practice because clinicians working with clients with ASD should be aware of the possible co-occurrence when a wish to be of the opposite gender is expressed or endorsed in self-report measures. Future studies should focus on the underlying mechanisms of this potential co-occurrence and thereby take into account the neuropsychological profiles of individuals with ASD.