**Intrinsic functional connectivity in families genetically enriched for social anxiety disorder – an endophenotype study** Janna Marie Bas-Hoogendam 1,2,3, Henk van Steenbergen 1,3, Kathrin Cohen Kadosh 4, Nic J. A. van der Wee 2,3, P. Michiel Westenberg 1,3 1 Institute of Psychology, Leiden University, Wassenaarseweg 52, 2333 AK Leiden, The Netherlands 2 Department of Psychiatry, Leiden University Medical Center, Albinusdreef 2, 2333 ZA Leiden, The Netherlands 3 Leiden Institute for Brain and Cognition, Leiden, The Netherlands 4 School of Psychology, University of Surrey, Guildford GU2 7XH, Surrey, United Kingdom * Corresponding author Janna Marie Bas-Hoogendam, PhD Developmental and Educational Psychology, Institute of Psychology, Leiden University Wassenaarseweg 52, 2333 AK Leiden, The Netherlands Pieter de la Court Building, room 3.B47 Tel.: (+31)(0)71-5276345 j.m.hoogendam@fsw.leidenuniv.nl http://orcid.org/0000-0001-8982-1670 https://www.researchgate.net/profile/Janna_Marie_Bas_Hoogendam **Abstract** *Background* Social anxiety disorder (SAD) has a genetic background. Neurobiological changes, including changes in functional connectivity (FC), are associated with the disorder; such alterations in brain networks are, for example, reflective of atypical attentional processes. It is still unknown whether these changes are genetically linked with SAD and thus qualify as candidate endophenotypes. Using data from the Leiden Family Lab study on SAD, including two generations of families genetically enriched for SAD, we investigated whether social anxiety (SA) co-segregated with changes in intrinsic (i)FC, and examined heritability. *Methods* Functional MRI data were acquired during resting-state in 109 individuals (56 males; mean age: 31.5, range 9.2-61.5 years). FSL’s tool MELODIC was used to perform independent component analysis. Six networks of interest (default mode, dorsal attention, executive control, frontoparietal, limbic and salience) were identified at the group-level and used to generate subject-specific spatial maps. Voxelwise regression models, with SA-level as predictor and voxelwise iFC as candidate endophenotypes, were performed to investigate the association with SA, within masks of the networks of interest. Subsequently, heritability was estimated. *Findings* SA co-segregated with iFC within the dorsal attention network and frontoparietal network (positive associations). Furthermore, iFC of multiple voxels within these clusters was at least moderately heritable. *Interpretation* These findings provide initial evidence for increased iFC as candidate endophenotype of SAD, particularly within networks involved in attention; these changes might relate to the interpretation biases commonly present in SAD. *Funding* Leiden University Research Profile ‘Health, Prevention and the Human Life Cycle’.