**Background** Social anxiety disorder (SAD) is a disabling psychiatric condition with a genetic background. Brain alterations in cortical and subcortical gray matter (GM) related to SAD have been previously reported, but it remains to be elucidated whether GM measures are candidate endophenotypes of SAD. Endophenotypes are measurable characteristics situated on the causal pathway from genotype to phenotype, providing insight in genetically-based disease mechanisms.
**Present study** Here, we examined whether GM characteristics meet two endophenotype criteria, using data from a unique sample of SAD-patients and their family-members of two generations. Firstly, we investigated whether GM characteristics co-segregate with social anxiety within families genetically enriched for SAD. Secondly, the heritability of the GM characteristics was estimated.
**Methods** Nine families with a genetic predisposition for SAD participated in the Leiden Family Lab study on Social Anxiety Disorder (n = 110; see Bas-Hoogendam et al., 2018 for a detailed description of the study). Structural T1-weighted magnetic resonance imaging brain scans were acquired. Subcortical volumes, cortical thickness and cortical surface area were determined for a-priori determined regions of interest (ROIs). Next, associations with social anxiety and heritability estimates were determined.
**Findings** Several cortical and subcortical GM characteristics co-segregated with social anxiety within families (uncorrected p-level) and showed moderate to high heritability.
**Data** Raw data are available in excel-format and are described in detail in Bas-Hoogendam et al., 2018 (EBioMedicine, in press).
Furthermore, supplementary results are attached.
