Individuals with autism spectrum disorder and average IQ exhibit a widening discrepancy between lagging adaptive skills relative to their cognitive potential, but it is unknown when this discrepancy emerges in development. To address this important question, we measured adaptive and cognitive skills longitudinally, from 12-36 months, in 96 low-risk typically developing infants and 69 high-risk siblings of children with autism spectrum disorder who at 36 months were diagnosed with autism spectrum disorder (N = 21), the broader autism phenotype (N = 19), or showed no concerns (unaffected; N = 29). Results indicate that both cognitive and adaptive communication skills remained stable over time for all four groups, but toddlers with autism spectrum disorder and the broader autism phenotype failed to keep pace with unaffected and typically developing toddlers with regard to adaptive socialization skills and, to a lesser extent, daily living skills. The odds of having a discrepant developmental profile, with average cognitive skills and below average adaptive skills, was significantly greater for socialization and daily living skills in toddlers with autism spectrum disorder or the broader autism phenotype and increased over time from 12 to 36 months. The discrepancy between adaptive skills and cognition emerges early and widens over time for infants with autism spectrum disorder symptomology, supporting early assessment and intervention of adaptive socialization and daily living skills.