This study examined several key social communication behaviors in a group of infants who later developed autism and a typically developing, low-risk comparison sample. The primary findings were that there were no group differences at 6 months of age, but that over the next 12 months, most of the infants who were later diagnosed with ASD demonstrated declining trajectories of social communication behavior and loss of skills. These results were consistent across both exact coded frequencies of behavior and examiner frequency ratings of social engagement. The second important finding was that these changes in social communication development documented through repeated evaluations provided a very different picture of symptom onset than did parents’ retrospective report. Most parents (83%) did not report any loss of skills during this period. These findings lead us to two major conclusions. First, the behavioral symptoms of ASD appear to emerge over time, beginning in the second half of the first year of life and continuing to develop for several years. Second, our most widely used and recommended practice for gathering information about symptom onset, parent-provided developmental history, does not provide a valid assessment of the slow decline in social communication that can be observed prospectively.
The comparability of social behavior at 6 months of age in the ASD and TD groups replicates findings from other prospective studies.25,26,35
The lack of group differentiation at this age does not appear to result from low power, as the group means were very similar and effect sizes were very small. Most convincingly, the effects were in the opposite direction as predicted, with the ASD outcome group demonstrating (nonsignificantly) better social communication behavior at 6 months than the TD outcome group on all variables. After 6 months, the ASD group shows a rapid decline in eye contact, social smiling, and examiner-rated social responsiveness. Group differences were significant by 12 months in gaze to faces and social smiling and by 18 months on all other variables. Similar declining trajectories in the onset of autism symptoms have also been reported by others.35
Some have suggested that the decrease in social interest in this time period is related, at least in part, to an overly focused attention to objects or difficulty with attentional switches from objects to people.36
However, our data did not support this hypothesis; we found no group differences in looking at objects at any age and duration of looks to objects did not increase over time in the ASD group. Thus, the decline in looking at faces and social smiling could not be accounted for by an increase in interest in objects. Similarly, neither group differences in cognitive functioning, slowing of cognitive growth between 12 and 36 months in the ASD group, nor less successful performance on the Mullen VR subscale (which provided the context for coding) could explain the decrease in social communication behavior over time in the ASD group.
Collectively, the present investigation and recent prospective studies suggest that signs of autism emerge over the first year or so of life in many children with ASD, rather than being present from close to birth, as once suggested by Kanner.37
Although there are likely cases in which behavioral signs are indeed evident at or before 6 months of age, this pattern may be less common than originally thought.
Interestingly, cognitive and language skills, as measured by the Mullen, did not demonstrate the same declining trajectory in the ASD group as social communication skills. Mullen raw scores increased over time in both groups, but showed significantly slower growth in the ASD group starting at 12 months. This suggests that developmental loss of skills may occur specifically in the social communicative domain, rather than generally across all developmental domains. One implication of this finding is that the Mullen and other standardized developmental tests will not be good prospective measures for tracking regression, as they appear to be less sensitive to the kinds of behaviors that decline as autism emerges.
The results of the current prospective study suggest that the traditionally defined categories of early onset and regressive autism do not portray accurately how symptoms emerge, nor does the newer-onset category involving a developmental plateau. In the present study, specific social communicative behaviors clearly decreased rather than failing to progress. Losses were particularly dramatic between 6 and 18 months. This suggests that ASD onset marked by loss of social communication behaviors occurs much more often than has been recognized using parent report methods. However, rather than the rapid and marked losses typically reported in previous studies of regression onset, the declines that could be detected through specific probes of social development every 6 months were relatively subtle and gradual, were often preceded by earlier parental concerns, and were often followed by failures to progress in other areas.38
This may have made the losses difficult for families to identify, as demonstrated by the low rate of parental reports of regression on a standardized measure, the ADI-R, administered at 36 months. It might be expected that parents who have already experienced the developmental course of autism onset in an older child would be more sensitive to these signs than the general population, underscoring how difficult it will be for most parents to report about skill loss. For this reason, it is critical to explore other methods of detecting the earliest signs of autism.
In the present study, a second method of measuring social communication development was examiner ratings, which were conducted at the end of every visit, blind to the group assignment of the participant. We found that these global ratings of the frequency of social communication behaviors were highly correlated with coded rates of the same behaviors, validating both approaches to measurement. Although it is not practical to code behavior frequencies during medical office visits, these results suggest that it may be possible to develop measures that can be rated by physicians or nursing staff during well-child visits that capture “snapshots” of an infant’s current social repertoire. If such measures are found to be valid reflections of infant sociability, then they could be added to developmental monitoring exams and serially rated over time to identify infants at higher risk for ASD and in need of further specialized evaluation.
The present findings suggest that existing definitions of onset patterns will need to undergo further development. One possibility is that we need to expand the number of categories used to describe onset. For example, perhaps there are four rather than two categories of onset, including groups characterized by developmental plateaus and by mixed features of early symptoms plus later regression, as we have suggested elsewhere.17
The prospective data acquired in the present investigation suggest another possibility, however. We hypothesize that symptom emergence may better be considered dimensionally, as a continuum characterized by the amount and timing of regression. In this conceptualization, at one end of the continuum lie children who display loss of social interest so early that the regression is difficult to see and symptoms appear to have always been present. At the other end of the continuum lie children who experience losses of social interest and communication skills so late that the regression appears quite dramatic. Supporting the idea that there may not be two different processes of symptom onset, but rather one dimension, several recent studies have found few differences in later developmental, functional, and adaptive outcomes between children with traditionally defined early onset and regressive autism.18,39
Resolution of opposing viewpoints regarding onset (e.g., multiple categories v. a dimensional view that emphasizes timing) is urgently needed for etiologic studies, which have been hindered already by the tremendous heterogeneity of the autism phenotype.
Limitations of the present investigation include the relatively small sample size and the coding of behavior during only one segment of a longer visit. Future studies with more participants, longer sampling of behavior, and examination of cross-task consistency in behavior are necessary. Nonetheless, since the present results are consistent with other prospective infant studies,25,26,35
we believe that they have significant clinical implications for early autism screening, diagnosis, and intervention. Universal screening has been recommended by the American Academy of Pediatrics (AAP) twice by the second birthday, and many are hoping that identification may be possible even earlier.40
This study suggests that identification of autism by the first birthday may not be possible in the majority of affected children. Therefore, the AAP recommendation for screening at both 18 and 24 months is essential and may even need to be supplemented by screens after age 2 years, as many children will be missed at earlier time points. Finally, we urge professionals to refer to intervention any infant or toddler who displays a sustained reduction in social responsivity over time. Given the gradual course of symptom emergence and the paucity of diagnostic tools for infants and toddlers with suspected autism, the diagnostic process can be quite protracted and intervention may be needlessly delayed.