Sex chromosome aneuploidies (SCA) are the most common chromosome irregularities, but phenotypes vary greatly, and SCA are underdiagnosed. Boys with SCA represent an understudied group. The temperament of boys with SCA has often been described with terms like shyness, withdrawal, low self-esteem, social isolation. Behavioral inhibition (BI) is a temperamental trait that reflects a relatively consistent pattern of behavioral and emotional responses to unfamiliar or novel situations. A temperament characterized by high BI involves a tendency to react with fear, withdrawal, and cautiousness in the face of unfamiliar settings or people. BI is suggested to be a significant predictor of later onset psychopathology, particularly anxiety disorders. In the current thesis, BI is examined in a group of boys with SCA. A sample of 25 Norwegian boys with SCA (mean age = 11.7 years, SD = 4.5) were examined with the Behavioral Inhibition Questionnaire (BIQ). Parents rated their children on the BIQ, which yield total scores and scores on six subscales. The scores of the SCA sample were compared to the scores of a clinical sample comprised of boys referred to mental health clinics for anxiety related problems, as well as to norms. The boys with SCA were scored as high as the clinical sample on most aspects of BI, and were reported to be significantly more inhibited in physical settings. Both the SCA and the clinical sample were overall scored significantly higher than norms. Given that BI is considered an important predictor of later onset anxiety, boys with SCA might have the same need for psychological assessment as clinic-referred boys. This is currently not part of standard care.