A prospective study examined the emergence of early behavioral signs of autism in infants at high and low risk for autism spectrum disorder (ASD). The study followed 25 infants later diagnosed with ASD and 25 typically developing infants matched by gender. Participants were evaluated at 6, 12, 18, 24, and 36 months of age. Behavioral signs such as gaze to faces, social smiles, and directed vocalizations were coded from video recordings and rated by examiners. At 6 months, no significant differences were found between the groups. However, by 12 months, the ASD group showed significantly declining trajectories in these behaviors. By 18 months, group differences were significant for social smiles. The study found that behavioral signs of autism emerge over time through a process of diminishing key social communication behaviors, not at birth as previously thought. While some parents reported regression in their children's development, most did not recognize the decline in social communication skills. The study suggests that more children may present with a regressive course than previously thought, but parent report methods do not capture this phenomenon well. The findings highlight the importance of prospective studies in understanding the onset of autism. The results indicate that social communication behaviors are grossly intact in the first 6 to 9 months of life in infants who are later diagnosed with autism. The study also found that cognitive and language skills did not show the same declining trajectory as social communication skills in the ASD group, suggesting that developmental loss may occur specifically in the social communicative domain. The study's findings suggest that the traditional categories of early onset and regressive autism may not accurately portray how symptoms emerge. Instead, a dimensional approach emphasizing the timing and amount of regression may be more appropriate. The study also highlights the need for further development of onset definitions and the importance of early screening and intervention for autism. The results have significant clinical implications for early autism screening, diagnosis, and intervention. The study recommends universal screening at 18 and 24 months of age and emphasizes the importance of identifying infants at higher risk for ASD. The findings also suggest that the diagnostic process can be protracted and intervention may be delayed without early identification.A prospective study examined the emergence of early behavioral signs of autism in infants at high and low risk for autism spectrum disorder (ASD). The study followed 25 infants later diagnosed with ASD and 25 typically developing infants matched by gender. Participants were evaluated at 6, 12, 18, 24, and 36 months of age. Behavioral signs such as gaze to faces, social smiles, and directed vocalizations were coded from video recordings and rated by examiners. At 6 months, no significant differences were found between the groups. However, by 12 months, the ASD group showed significantly declining trajectories in these behaviors. By 18 months, group differences were significant for social smiles. The study found that behavioral signs of autism emerge over time through a process of diminishing key social communication behaviors, not at birth as previously thought. While some parents reported regression in their children's development, most did not recognize the decline in social communication skills. The study suggests that more children may present with a regressive course than previously thought, but parent report methods do not capture this phenomenon well. The findings highlight the importance of prospective studies in understanding the onset of autism. The results indicate that social communication behaviors are grossly intact in the first 6 to 9 months of life in infants who are later diagnosed with autism. The study also found that cognitive and language skills did not show the same declining trajectory as social communication skills in the ASD group, suggesting that developmental loss may occur specifically in the social communicative domain. The study's findings suggest that the traditional categories of early onset and regressive autism may not accurately portray how symptoms emerge. Instead, a dimensional approach emphasizing the timing and amount of regression may be more appropriate. The study also highlights the need for further development of onset definitions and the importance of early screening and intervention for autism. The results have significant clinical implications for early autism screening, diagnosis, and intervention. The study recommends universal screening at 18 and 24 months of age and emphasizes the importance of identifying infants at higher risk for ASD. The findings also suggest that the diagnostic process can be protracted and intervention may be delayed without early identification.