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Autism spectrum disorder (ASD) frequently co-occurs with intellectual disability (ID) and often remains undiagnosed until adulthood. The Social Communication Questionnaire (SCQ) is a widely used measure to screen for ASD. To improve the utility of the SCQ for adults with ID, the aim of this study was to develop an ID-specific and adult appropriate algorithm for the SCQ using a core set of valid items. These items were identified in one sample (N = 226) and further cross-validated in a second, independent sample (N = 225) from Germany, England and the U.S. The newly developed algorithm has 24 items compared with the 40 items in the original instrument. The reduced item core set yielded similar diagnostic validity as the original algorithm with good sensitivity values (0.81–0.89) and low specificity values (0.62–0.72). Overall, these results suggest that the removed items may not carry diagnostically relevant information in adults with ID; thus, excluding these items may result in a more efficient and age-appropriate screening measure for this population. However, due to the low specificity values, a comprehensive assessment is essential for a final diagnostic assignment. Autism Res 2017, 10: 1481–1490.