The present study aimed to examine psychopathological symptoms beyond failure to speak in children and adolescents with SM. To the best of our knowledge, we provided the first empirical outline of possible symptoms of SM beyond failure to speak without using a predefined list or prior theoretical restrictions (e.g. anxiety). Our results replicated findings from previous studies and add symptoms not systematically described in SM before.

Do children and adolescents with SM experience additional symptoms beyond failure to speak?

Our results suggest that children and adolescents with SM experience multiple symptoms, with an average of M = 4.74 symptoms from different categories. This aligns with previous studies [2, 8, 12], indicating additional symptoms beyond failure to speak in SM. As no parent reported more than six different symptom categories, symptoms of SM, at least those symptoms observable by parents, seem to be limited to a maximum of six different symptoms per individual. Therefore, it is unlikely for any child or adolescent with SM to exhibit all potential symptoms. This aligns with findings from other mental disorders, in which individuals often share core symptoms but vary in additional symptoms [16, 19]. The symptoms' heterogeneity may be based on various subtypes with distinct pathomechanisms [19]. While specific symptom profiles couldn't be drawn from our results, the study may have outlined potential symptoms that potentially occur in symptom profiles of SM and thus might provide a good starting point for identification of subtypes and pathomechanims.

Which psychopathological symptoms are present in SM?

As expected, our findings suggest that children and adolescents with SM show predominantly anxiety-related symptoms such as fear, freezing or avoidance, but also symptoms, albeit less prominently, that may not be primarily driven by anxiety, such as impaired self-esteem and externalising behaviors. This essentially reflects that anxiety is the central phenomenon of most children with SM as indicated by the higher order category of anxiety disorders in DSM-5 and epidemiological research [3, 20]. Furthermore, the findings suggest that additional clinical features beyond anxiety are relevant in SM, supporting the view of SM as a heterogeneous disorder. [4].

Anxiety-related symptoms

As expected, we found that anxiety-related symptoms, such as fear, freezing, and avoidance/safety behavior, were the most commonly reported among individuals with SM. This supports the classification of SM as an anxiety disorder [1] and is consistent with prior research on symptoms of SM [2, 8, 12]. However, this current study builds upon prior research while addressing their methodological limitations in identifying unknown symptoms and their relevance. Previous studies relied on a predetermined list of symptoms [8, 12], failed to define reported symptom categories [8], included potentially normative behaviors as symptoms [12] or limited reported symptoms to the domain of anxiety [2]. For example, previous studies may have included symptom categories that obscure anxiety-related symptoms. Reduced eye contact, which is a safety behavior, may have been classified as 'lack of contact' (e.g. 76% of individuals behavior [8]), and motor inhibition as part of a freezing response may have been assigned to 'psychomotor disturbances' (e.g. 76% of individuals [8]). Therefore, the current study provides a more precise representation of these vague symptom categories from prior research. Additionally, current findings indicate that marked fear is a central phenomenon among most individuals with SM, making it a potential core symptom of the disorder. It is worth noting that we did not specifically inquire about fear in our study, unlike the previous study that employed an open-response format [2]. Nevertheless, our findings demonstrate that typical fear responses outlined in DSM-5 for childhood (such as avoidance, freezing, clinging, whining, and tantrums) [1] are also present in individuals with SM. While freezing may be a relevant pathomechanism in SM [13][13], and avoidance/safety behaviors are transdiagnostically relevant to anxiety disorders [21], the fear responses of clinging, crying, tantrum have been less studied in children with anxiety disorders (e.g., [22]. Therefore, the conclusion that the behavior described here occurs due to a fear response is not well-founded and requires further research. Moreover, the observed symptoms of the regressive behavior/displacement activity category may also be partially interpreted as a fear response. According to biological models of fear, displacement activity is an inappropriate action resulting from a conflict between two incompatible actions, such as avoidance and approach [23, 24]. Regressive behaviors may also be explained in the context of developmental delays, which occur in children with SM [3]. However, this is a new hypothesis in the SM literature that situationally inappropriate behavior in SM may also result from an fear response, which may have important implications for making diagnoses of SM in distinction to developmental delay.

The symptom category of reduced communication, specifically speaking at a lower volume or quantity, showed a rather low prevalence of 10.5%. It is unclear whether this is an independent symptom or a weaker manifestation of the symptom failure to speak. Based on clinical experience, many individuals with SM speak at a lower frequency or in a whisper, suggesting a high prevalence of the symptom in numerous situations. The low prevalence suggests that parents in this study did not distinguish between failure to speak (which was already assumed and should not be reported) and reduced communication, and therefore did not report the latter. For future studies, it would be beneficial to differentiate explicitly between failure to speak and reduced communication and investigate their mutually explanatory variance to determine whether they are distinct or part of the same dimension.

Symptoms beyond anxiety

Beyond symptoms that appear to be primarily related to anxiety, parents in this study also reported symptoms that may not be driven by anxiety. Self-esteem impairment/negative affect refers to negative feelings other than anxiety (e.g., helplessness) or reduced self-worth. This symptom is not included in diagnostic criteria for anxiety disorders, but it has been found to be associated with almost all anxiety disorders [25]. In the context of SM, there is evidence that SM is associated with reduced self-esteem. This is supported not only by the current study, but also by the study of Remschmidt et al. [8], in which individuals reported a decrease in their self-esteem. A crucial question in this context is whether reduced self-esteem is a result of not speaking in certain situations or whether it leads to a failure to speak. It should be noted that this category may not necessarily be an inherent symptom of SM, but rather a result of SM that has persisted for some time. A recent study has shown that individuals with (former) SM have lower self-esteem in adulthood than individuals without SM [26]. This may indicate that impaired self-esteem might be a consequence or at least still be present during adulthood. On the other hand, SM typically has a pre-school onset [7], earlier than most other anxiety disorders. Longitudinal studies in children with selective mutism are needed to disentangle the causal relation between selective mutism and reduced self-esteem. Externalizing behaviors may also occur as part of a fear response (e.g. temper tantrums) in children. However, it is possible that the reported symptoms from the externalizing behavior category are not solely related to fear-related situations. This is reasonable because some children with SM do not appear to have elevated levels of anxiety [20] and may present with a diagnosis of oppositional defiant disorder [4]. The reports assigned to this category suggest that a subset of individuals exhibit aggressive behavior that goes beyond tantrums. This subset does not only report anger or temper tantrums but also instances of aggressive behavior (“boxes or beats us parents”). It is important to note that subjective evaluations have been excluded from this analysis. However, due to the method of the study, it is not possible to differentiate which reported symptoms in this category are fear responses and which are classified as fear-independent oppositional or aggressive behaviors. To our knowledge, reduced body tension and slackness has not been previously reported in systematic research on SM yet. It is interesting to note that this symptom appears to be opposing to the symptom of freezing, which is associated with increased body tension. Although reduced body tension was present in only a small subset of children and adolescents with SM in the study at hand, this could indicate a subgroup of individuals with SM characterized by a specific pathomechanism. This highlights the fact that certain phenomena in children and adolescents with SM remain unidentified. It seems necessary to conduct basic psychophysiological research as well as qualitative research to gain a better understanding of this phenomenon in the affected individuals.

In this study, it was found that symptoms, excluding fear, did not show any correlation with SM severity. One possible interpretation is that, apart from the core symptom of failure to speak, only fear is an integral symptom of SM. However, this seems unlikely given the high prevalence of symptoms such as freezing or avoidance. It is more probable that the low correlations are due to the categorical data structure and the associated low variance.

Clinical and research implications

Considering SM as a mental disorder with a number of additional symptoms beyond failure to speak may have important implications for a better understanding of SM and thus for improving detection and treatment for affected children. Differentiating diagnostic criteria based on empirically identifiable symptoms seems promising in distinguishing SM from normative periods of silence [49] or from social anxiety disorder. Given the heterogeneity of symptoms indicated by the present study, individuals with SM might present with different symptoms and symptom profiles. Modularizing therapy for SM could be a promising approach, as different individuals displaying different symptom profiles may benefit from different interventions. Future studies should focus on subgroups based on empirically identified symptom profiles as these may be related to certain pathomechanisms that require different interventions in the treatment of SM. As we have solely focused on the frequencies of symptoms in individuals with SM, it is crucial to investigate the burden and impairment that these symptoms cause in affected children and adolescents. Moreover, interactions between symptoms should also be addressed as they may offer insights into the maintenance of the disorder (e.g. relation between avoidance and fear).

Strength and limitations

The current study has both strengths and limitations. One strength is the replication of findings on symptoms of SM from previous studies and the addition of symptoms that have not yet been systematically described in the literature. The study employed a qualitative approach with an open response format, resulting in the first comprehensive overview of possible symptoms of SM without any prior theoretical constraints or specified symptoms. In contrast to most previous studies, we provided a comprehensive description of the possible symptom categories with prototypical examples. The high interrater reliability indicates that symptoms were assigned to intersubjectively reasonable and discriminative symptom categories. However, it should be noted that the survey was conducted online and individuals were included based on a screening instrument rather than a clinical interview. While we were able to conduct a nationwide study and collect a representative sample using a screening instrument that distinguishes well between children with and without SM, we were unable to make a comprehensive diagnosis of SM or identify any comorbidities in our sample. Therefore, it is possible that the symptoms reported in this study may be related to other syndromes besides SM. However, it may not be possible to provide an overview of 'pure' SM symptoms, given the significant symptom overlap typically observed in related syndromes, the prevalence of comorbidities in clinical practice, and the presence of various subclinical symptoms in any individual. Additionally, it is important to note that we only asked about situations in which there was an expectation to speak, so symptoms may be different in other situations (e.g. social situations in which there is no expectation to speak). It is important to note that the survey only included parents' reports of their children's symptoms, rather than the children or adolescents themselves. This may explain why the symptom categories mainly describe observable behavior. The study design does not allow for the assessment of concrete internalizing symptoms from the children's perspective, such as cognitions or fear content. Finally, the study did not take into account factors such as socio-economic status or parental mental disorders. Therefore, it is questionable to what extent the results can be generalised to the entire patient population.