The Phelan McDermid syndrome (PMS), is a neurogenetic condition linked with an alarming rate of intellectual disability (ID) and autism spectrum disorder (ASD). The research provided a more detailed and quantitative description of repetitive behaviors in the context of ID, as determined by the condition. Multicenter observational research was conducted to characterize the phenotype and natural history of PMS in children aged 3–21 with a verified PMS diagnosis. The researchers used the Repetitive Behavior Scales-Revised (RBS-R) to gather data. Among n=90 participants, 47% (n=42/90) were female, and the average age at baseline was somewhere between 8.88 and 4.72. The average best estimate IQ of the group was 26.08 to 17.67 (range 3.4–88), with n=8 people with mild ID, n=20 individuals with moderate ID, and n=62 people with severe-profound ID. The RBS-R overall score was between 16.46 and 13.9 (compared with ASD research before, which reported a range from 33.14 to 20.60) (Lam & Aman, 2007), with total items agreed on ranging from 10.40 to 6.81 (range=0–29). The RBS-R stereotypic behavior subscale score (rs=−0.33, unadjusted P=0.0014, adjusted P=0.01) and (rs=−0.32, unadjusted P=0.0019, adjusted P=0.01) . Also, the IQ is co-associated with RBS-R subscale scores. The RBS-R overall system in a PMS cohort appeared mild when compared with patients with ASD categorized in research before. Cognitive roles might get reflected by stereotypical habitation.