Consistent with the results of previous longitudinal studies of autism, we observed a high degree of preservation of inter-individual differences in autistic social impairment (ASI) over time, but also subtle improvements over the course of the study period in both clinically-affected and “unaffected” children. In this first wave analysis of our longitudinal study of quantitative
ASI, inter-individual variation, as reported by mothers using the Social Responsiveness Scale (SRS), exhibited a baseline to follow-up correlation exceeding 0.70 for both twins from the general population and for a clinically-ascertained sample of boys with PDD. Congruence in the longitudinal course of autistic traits across
the range of severity observed in nature extends findings from our previous family/genetic studies supporting a biological link between clinical and subthreshold levels of symptomatology (Constantino et al., 2006
, Duvall et al., 2007
). Furthermore, the level of stability in inter-individual differences over time indicates that quantitative measurements of ASI can serve as extremely reliable markers of symptomatology, which might relate to neurobiological and genetic determinants of autism, and which might be useful for ascertaining the response of core symptoms to successful intervention.
The changes that we observed over time were very gradual and, on average, would only be construed as clinically significant when allowed to accumulate over years of time. There was a clear (statistically significant) tendency in both the clinical and general population samples for more severely affected children to exhibit greater reduction in impairment scores over time. Although the prediction of improvement by baseline severity is consistent with some influence of regression toward the mean on our results, baseline severity accounted for only 11 percent of the variance in change in our clinical sample, and the overarching trend for improvement extended to even the most socially-competent group of children at baseline. Moreover, the nature of that improvement in the clinical group encompassed the entire constellation of symptoms observed in autism. As we have observed for the factor structure of autism itself (Constantino et al., 2004
), the factor structure of improvement over time is consistent with a singular underlying component, best characterized quantitatively by a total impairment score.
If it is true (as our data suggests) that general improvements are occurring in the absence of age (developmental) effects on cross-sectional measurements of ASI—in essence that all children, irrespective of age, are getting better (albeit slowly)—this would raise the possibility of subtle period effects on autistic social impairment. Period effects are influences on an entire population (in this case, children across the entire range of ages represented by the sample) for a specific period of time (in this case, the years over which the study was conducted). The identification of true period effects—possible examples would be beneficial effects of new treatments across the age distribution, or recent improvements in the capacity of the educational system to assist children affected by a wide range of autistic traits—would have potentially important implications for public health strategies for improving the outcomes of children with pervasive developmental disorders.
One would expect that period effects, if present, would result from environmental (not genetic) influences on the course of ASI over time. Our analysis of the genetic and environmental structure of change over time among twins revealed strong evidence for the importance of genetic factors in determining the longitudinal course of symptoms and the interesting possibility that different sets of genetic factors might account for some of the heritable influences on ASI in childhood versus adolescence. The caveat, however, is that in the analysis of twin data, any interaction (or correlation) between genetic factors and unmeasured environmental factors is subsumed under the parameter for genetic influence. Thus, if there is a socio-environmental change that is resulting in the observed improvements in symptomatology over time, it is likely interacting with genetic factors to exert such an influence on outcome. A possible candidate for such an interaction with the environment is inherited deficiency in social behavior itself—in this study, we directly observed that the children who experienced the most improvement over time were the ones who were most affected by ASI at baseline.
Finally, we observed that among higher-functioning adolescents with PDD, a high proportion (50%) meet or approach DSM-IV diagnostic criteria for Schizoid Personality Disorder. It is rare for non-retarded adults to carry PDD diagnoses, in spite of the fact that these are common childhood conditions with enduring symptomatology over time; this suggests a possible lack of systematic recognition of the impairments that are carried forward into adulthood by individuals who were diagnosed with PDD-NOS, Asperger’s Disorder or “High-Functioning Autism” in childhood. One hypothesis supported by our exploratory accrual of data on schizoid personality disorder symptoms is that a sizeable portion of higher-functioning PDD subjects become diagnosable with Schizoid Personality Disorder in adulthood. Elucidating the continuity and discontinuity between these two conditions warrants further study and remains a goal of our ongoing longitudinal research. Since the subjects themselves had minimal insight into the presence of schizoid personality disorder symptoms (that their parents identified in them), it will be important for future research efforts to incorporate observations of parents or other informants, rather than relying exclusively on reports by the subjects themselves.
There are some limitations of this analysis of first-wave data from our ongoing longitudinal study; the sample size, though the largest to date for tracking the longitudinal course of quantitative ASI, warrants efforts at replication in still larger samples. In addition, our clinical subjects (for whom there has been minimal attrition in the sample to date) have been studied at only two time points thus far. Nevertheless, the convergence of findings across two disparate study groups—with respect to both
stability of inter-individual differences and
change over time—supports the reliability of the findings in each respective sample. Reliance on maternal report SRS scores is potentially vulnerable to rater bias; however the absence of age effects at either baseline or follow-up, make it less likely that the findings regarding change are substantively influenced by systematic maternal reporting bias. We have previously reported an absence of evidence for maternal rating bias on SRS scores obtained in considerably larger samples of twins (Constantino & Todd, 2003
). Continued follow-up of this sample, which will include yearly reassessments of the clinical subjects described herein, and every-other-year assessments of the clinical subjects’ male siblings (see Constantino et al., 2006
), by both parent- and teacher-report, will help elucidate any ongoing effects of reporting bias. An additional limitation was that current IQ data was available for only a minority of clinical subjects and none of the twins. Furthermore, this report is limited to an analysis of male sibling pairs; parallel data collections involving female subjects are in progress.
In this study we showed that the average SRS score recorded by parent and teacher exhibits very high levels of consistency (r=0.63) over time even when two different teachers provide ratings at respective baseline and follow-up assessments. Use of multi-informant data from parent- and teacher-report on a given child may more comprehensively represent the social functioning of children across home and school settings than when relying on information from a single source. This issue is being addressed in an ongoing psychometric study of the SRS.