Selective mutism (SM) is an anxiety disorder that usually onsets during childhood. Its main symptom is inability to speak in some situations despite normal speech in others. Sociodemographic factors are known risk factors for psychiatric disorders, but research on their association with SM is limited. Psychiatric disorders tend to cluster in families, but findings on psychiatric disorders among family members of subjects with SM are not uniform. The aim of the current thesis was to validate the use of the SM diagnosis and to investigate family psychopathology and demographic risk factors associated with SM, along with its long-term outcomes.

The study used a nested register-based case-control setting. Information on subjects, controls and their parents and siblings were obtained from Finnish health registers. Each subject was matched by gender and age with four control subjects. Study I included 860 subjects with SM along with 3,250 controls, and Study II included 658 subjects with 1,661 siblings and 2,029 controls with 4,120 siblings. The validity of the SM diagnosis in use was evaluated by assessing the patient records of a subsample of 53 subjects in Study I. Study I examined parental psychopathology, parental age, maternal socioeconomic status, urbanicity, maternal marital status and parental immigration status. Study II explored sibling psychopathology. A systematic literature review was conducted to investigate the long-term psychiatric outcomes of SM.

The studies found that both parental and sibling psychopathology were associated with SM among the subjects and showed non-specific pattern of clustering of disorders. The validity of SM diagnosis was found to be good. Advanced paternal age, low maternal socioeconomic status and single motherhood at the time of the child’s birth were also associated with SM. Among studies included in the review, most children with SM recovered during the follow-up period, but anxiety disorders were common later in life.

Current findings point towards a shared etiology between SM and other psychiatric and neurodevelopmental disorders. In particular, the relationship between the etiology of SM and autism spectrum disorders requires further study. High-quality studies with follow-up times extending to adulthood are warranted.