Objectives: The goal of this research was to get insight in the development of ToM abilities in children with PDD-NOS. This requires repeated measurements as well as a ToM instrument that can be applied in such a design. Because a new test was developed, the psychometric qualities of this test had to be dealt with and norm scores needed to be calculated. In order to compare developmental trajectory differences in children with PDD-NOS, also the ToM ability in typically developing children was researched more into detail.
Methods: A new test, the ToM Storybooks was developed. This is a comprehensive test measuring a variety of early ToM components and associated aspects children develop between their third and fifth year. Both TD children (N=324, 3-12 years) and children with PDD-NOS (N=30, 3-8 years) were administered the ToM Storybooks and language comprehension tests. The children with PDD-NOS were diagnosed as having PDD-NOS according to DSM-IV-TR criteria. Before being included in the research, they received an intelligence test and tests that could validate their diagnosis, namely the Vineland Adaptive Behavior Scales and the Children’s Social Behavior Questionnaire. The children were tested every 4 months over a period of 20 months.
Results: The ToM Storybooks were found to have good psychometric qualities: the internal consistency, test-retest reliability, inter-rater reliability, discriminate validity, divergent and convergent validity was good. The ToM development in TD children was found to differ between boys and girls. Therefore, norms for both sexes were computed separately: ToM quotients and an age equivalents can be calculated. A three-step developmental model was obvious, with the greatest increase between 3.5 and 4.7 years of age. Next to that, two statistically significant discontinuities were found: at 56 and 72-78 months. These discontinuities, which took the form of temporal decreases, were accompanied by decreases in variability, suggesting a developmental shift in ToM understanding. Similar results could be established in children with PDD-NOS. Despite their obvious ToM problems, they also displayed a spontaneous ToM development. At the first measurement, they had significant lower ToM scores than their TD peers. But, at the last measurement, they had caught up with their ToM delay. The same developmental sequence was followed, but at a slower pace. Also the temporal decrease in ToM score as known from the TD group was delayed in this group (at 85-90 months).
Conclusions: The ToM Storybooks have good psychometric properties. They can be used in both research and clinical settings. In contrast with existing models, we found evidence of a non-linear pattern of growth in both groups, comprising temporary regressions in ToM scores. The ToM development in children with PDD-NOS was delayed, not deviant.