This prospective study examines the emergence of behavioral signs of autism in infants at low and high risk for autism. The study compares 25 infants later diagnosed with an autism spectrum disorder (ASD) to 25 gender-matched low-risk children with typical development, evaluating them at 6, 12, 18, 24, and 36 months of age. The results show that at 6 months, there were no significant differences between the two groups in gaze to faces, social smiles, and directed vocalizations. However, by 12 months, the ASD group demonstrated a significant decline in these behaviors, while the typical development group showed an increase. By 18 months, the decline was more pronounced. These findings suggest that behavioral signs of autism emerge over time, starting in the second half of the first year of life, and that parent report methods may not accurately capture the slow decline in social communication skills observed prospectively. The study also highlights the need for alternative methods to detect early signs of autism and the importance of early screening and intervention.This prospective study examines the emergence of behavioral signs of autism in infants at low and high risk for autism. The study compares 25 infants later diagnosed with an autism spectrum disorder (ASD) to 25 gender-matched low-risk children with typical development, evaluating them at 6, 12, 18, 24, and 36 months of age. The results show that at 6 months, there were no significant differences between the two groups in gaze to faces, social smiles, and directed vocalizations. However, by 12 months, the ASD group demonstrated a significant decline in these behaviors, while the typical development group showed an increase. By 18 months, the decline was more pronounced. These findings suggest that behavioral signs of autism emerge over time, starting in the second half of the first year of life, and that parent report methods may not accurately capture the slow decline in social communication skills observed prospectively. The study also highlights the need for alternative methods to detect early signs of autism and the importance of early screening and intervention.