**5. Conclusions**

Findings from this study add to the list of standardized measures that may be used in clinical trials with children and adolescents with DS. The SRS-2 T-scores had normal distributions, good feasibility, moderate to excellent test-retest reliability, and no practice effects, and therefore this measure could be suitable for use in clinical trials. Although the NEPSY-II Theory of Mind raw scores were psychometrically sound, the measure was problematic overall, considering the percentile rank floor effects and lack of evidence for ecological validity. Researchers should also use caution when using NEPSY-II Affect Recognition, as feasibility was problematic in the current study. We recommend referencing the sensitivity and specificity benchmarks when using this measure to guide decisions about inclusion/exclusion criteria in future studies with this population. The psychometric evaluation of social cognition and social behavior measures supports the NIH working group initiative of determining appropriate outcome measures for individuals with DS [27] and will contribute to the success of future clinical trials in DS.

**Author Contributions:** Conceptualization, E.K.S., E.K.H. and A.J.E.; methodology, E.K.S. and E.K.H.; formal analysis, E.K.S.; investigation, E.K.S., E.K.H. and A.J.E.; data curation, E.K.S., E.K.H. and A.J.E.; writing—original draft preparation, E.K.S.; writing—review and editing, E.K.S., E.K.H. and A.J.E.; visualization, E.K.S.; supervision, A.J.E.; project administration, E.K.H.; funding acquisition, A.J.E. All authors have read and agreed to the published version of the manuscript.

**Funding:** This manuscript was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development of the National Institutes of Health, R01 HD093754.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki and was approved by the Institutional Review Board of Cincinnati Children's Hospital Medical Center (2018-0253, approved 4/23/2018).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Acknowledgments:** This research would not have been possible without the contributions of the participating families and the community's support.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
