Unique socio-behavioural phenotypes are reported for individuals with different neurodevelopmental disorders. Here, the effects of adult familiarity and nature of interaction on social anxiety and social motivation were investigated in individuals with fragile X (FXS; n = 20), Cornelia de Lange (CdLS; n = 20) and Rubinstein-Taybi (RTS; n = 20) syndromes, compared to individuals with Down syndrome (DS; n = 20). The Social Anxiety and Motivation Rating Scale was employed whilst participants completed four social tasks, each administered separately by a familiar adult, and also by an unfamiliar adult. Compared to participants with DS, those with FXS and RTS exhibited high levels of social anxiety but similar levels of social motivation. Participants with CdLS showed heightened social anxiety and reduced social motivation only during interactions with an unfamiliar adult when active participation was voluntary.