Purpose: In this article, the authors present a tutorial on the use of developmental trajectories for studying language and cognitive impairments in developmental disorders and compare this method with the use of matching. Method: The authors assess the strengths, limitations, and practical implications of each method. The contrast between the methodologies is highlighted using the example of developmental delay and the criteria used to distinguish delay from atypical development. Results: The authors argue for the utility of the trajectory approach, using illustrations from studies investigating language and cognitive impairments in individuals with Williams syndrome, Down syndrome, and autism spectrum disorder. Conclusion: Two conclusions were reached: (a) An understanding of the underlying mechanism will be furthered by the richer descriptive vocabulary provided by the trajectories approach (e.g., in distinguishing different types of delay) and (b) an optimal design for studying developmental disorders is to combine initial cross-sectional designs with longitudinal follow-up.

Recent studies have shown that sleep is influenced and shaped by cultural factors, including cultural values, beliefs and practices. However, a systematic understanding of how cultural factors in countries may influence sleep duration and sleep disturbances is still lacking. Therefore, we focused on a comparison of sleep duration and disturbances in young populations between countries. We report cross-cultural differences between the child, parent and environmental factors, and their association with sleep duration and disturbances. The review is based on literature searches of seven databases published until December 2020. Studies were included if they investigated sleep duration and disturbances of individuals up to 18 years across at least two or more countries. The results of this review have shown that sleep duration and disturbances vary between countries and regions and certain factors (e.g., bedtime routines, sleeping arrangement, physical activity and psychological functioning) have been associated with sleep duration or disturbances. This review also demonstrates that certain factors which were associated with sleep duration or disturbances in one country, were not shown in other countries, suggesting a need for recommendations for age-related sleep duration and sleep interventions to consider cultural differences that influence sleep duration or disturbances in individual countries or regions.