Rett syndrome (OMIM #312750) is a progressive neurodevelopmental disease with clinical manifestations including loss of spoken language and apraxia. We summarized per PRISMA guidelines findings on their non-verbal social skills. Twelve studies (n = 479 females, 1.6-52 years) were sorted into a (non-)interventional design including video-coding, eye-tracking, and questionnaire studies. Across methodology, poor non-verbal social functioning was found, with eye gaze function being the most examined modality. Findings showed similar responsiveness to social stimuli as typically developing subjects but with difficulties in sustained attention and in recognizing novel social stimuli. Particularly, the music treatment showed to improve their skills. Concluding, due to severe movement disorder and apraxia, the brief gaze intent might be misconstrued beyond the caregiver's perception or familiar situations.