Diagnosing Autism Spectrum Disorders (ASD) is important throughout the lifespan. The objective was to investigate the transcultural diagnostic validity of the Social Communication Questionnaire (SCQ) in a clinical sample of 451 adults with Intellectual Developmental Disorder (IDD) with and without ASD in Germany, the U.S.A. and Great Britain. Variables associated with higher SCQ sum-scores were higher levels of IDD, male gender, a diagnosis of ASD and the study site (Germany > U.S.A > G.B.). An ROC analysis revealed a cut-score of 13, which resulted in a sensitivity of 0.87 and a specificity of 0.58. It is recommended to adjust the cut-score according to level of IDD and gender. Further research is needed to align diagnostic assignment of ASD across different sites and countries.