To the best of our knowledge, this is the first study to examine whether having siblings is associated with the language development in young children with DD in the long term. The longitudinal design provides a thorough investigation of the association between the familial-ecological factors, presence of siblings, and language developmental trajectories from a young age and across a critical period of development. Another strength of our study was the high validity of the medical diagnosis and language evaluation, which were all made by board-certified clinical child experts with the standard clinical diagnostic procedure in the developmental evaluation center designated by Taiwan’s Ministry of Health and Welfare for assessing and identifying young children with DD, as compared to other community studies collecting data from parent-report questionnaire only.1,2,4
Are siblings associated with language development among young children with DD? We found that the probability of language delay in young children with DD who had siblings became gradually higher, exceeding that of children who did not have siblings. Furthermore, the magnitude of the association remained significant even after adjusting for well-measured potential confounders. Our study supported that having siblings was associated with an increasing probability of language delay before school age. This finding obviously contradicted the findings of previous studies showing that siblings facilitate language development in children with TLD.7,8 The results can be explained by two reasons.
One reason is that children with DD may not be able to benefit from natural social learning environments provided by their siblings. To become an effective language facilitator to children with DD, siblings may need to learn and receive training. Efficacy studies on sibling-mediated interventions that include siblings as active treatment implementers, rather than as passive bystanders, have reported positive outcomes.25,26 Training programs might be a prerequisite for siblings to be successful in helping children with DD.
The other reason is the theory of intra-household resource allocation. Some studies reported that parents may invest more on children with DD to equalize the ability gap among their children (“compensating strategies”).27,28 However, in a household with finite resources, parents may invest more on the higher-achieving child, thus increasing the ability gap among children (“reinforcing strategies”),9,10 and these parents would not adopt compensating strategies to equalize the ability gap among their children. Our finding supports that parents of children with DD might have an infinitesimally small chance to adopt compensating strategies to reduce the ability gap between children with DD and their siblings. In other words, siblings are likely to compete for the parents’ attention and resources; therefore, children with DD who had siblings would receive less resources than those without siblings, placing those with siblings at a relative disadvantage.
Our study attempted to consider having siblings only at the first evaluation because we wanted to identify the red flag for language development for pediatricians and other clinical child experts at the first encounter with children with DD. However, children with DD might have upcoming siblings after the first evaluation. The information on sibling presence, regardless which time points, will be collected and considered again at each evaluation, with the estimated probabilities of language delay re-calculated and modified accordingly. Hence, our models can still explain the trajectories of language development in children with DD even under the condition of time-varying changes in sibling presence.
Microlevel sibling experiences and their meaning in children’s development are embedded in macrocontexts6; that is, having a sibling has an impact on the ability development of children with TLD, which is intricately linked to family context, such as birth order of the children, sex composition in siblingships, and family size; however, this was not reported in our study. Based on the findings that sibling presence was associated with language development among young children with DD, future studies to determine the association between the siblingship characteristics in macrocontexts and language development in children with DD are warranted. Next, to consider alternative associations for language development among children with DD, most notably, family history of speech and/or language disorders, impact of intervention, and language development cascaded by other domains of functioning should be integrated to be examined. By focusing on longitudinal transactional models29 to measure brain maturation in the context of endogenous characteristics (i.e., language competence associated with hereditary or cascaded by other domains of functioning30) and exogenous characteristics (i.e., environmental exposures), we can understand the mechanisms by which these constructs act together in shaping brain plasticity in children with DD from early ages. Additionally, only the impairment level (i.e., language delay) was examined in this study. To further comprehend the role of siblings in the trajectories of language development in children with DD, these trajectories could be traced using the International Classification of Functioning, Disability and Health for Children and Youth model as a framework, specifically by using indexes, such as activity, participation, quality of life, and psychosocial health as outcome variables.