Having siblings in early childhood contributes to the development of social competence, including the development of peer relationships, but the following questions must be asked: How does the development of the social competence of only children occur with age, and how does this differ from that of children with siblings? Can having peers replace the effects of having siblings on young children? In Taiwan, few studies exist on the influence of having peers on the social competence of only children and children with siblings, and even fewer exist regarding these influences among preschool children. This study investigated the effect of “years with peers” on the social development trajectory of preschool only children and children with siblings. The research questions were as follows: (1) What was the trajectory of children’s social competence development with age? (2) What was the impact of having or not having siblings on the development trajectory of social competence? (3) What was the impact of “years with peers” on the development trajectory of social competence of only children and children with siblings?

This study used the Kids in Taiwan: National Longitudinal Study of Child Development and Care (KIT) database, a longitudinal study of 36-month-old (T1), 48-month-old (T2), and 60-month-old (T3) children from April 2016 to April 2017. The T1 sample included 2,164 children. Among the enrollees, 660 were only children and 1,497 had siblings. To clarify the influence of having or not having siblings on the development of social competence, this study extracted data on T1, T2, and T3 children, a total of 1,590 only children or children with siblings. Among them, 1215 were children with siblings at T1; 375 remained only children at T3.

This study used the data collected from the KIT panel study sample to analyze and explore the impact of changes in sibling and peer factors on changes in children’s social development abilities. The KIT database divides the social competence scale for children aged 2–6 years into four dimensions: Independence, assertiveness, sociability, and compliance. The reliability and

validity of the scale are high. This study used Mplus version 8.6 to conduct the following three analyses: (1) The unconditional latent growth model (LGM) was used to understand the changes in preschool children’s social competence between T1, T2, and T3. (2) If the initial level or growth rate of individual children’s social competence differed significantly in the unconditional LGM, this study applied the conditional LGM to further clarify the influence of having siblings and “years with peers” on the changes in the children’s social competence. (3) In the conditional LGM, if the changes in children’s social competence of “only children or not” or of “years with peers” significantly differed, multiple group analysis was applied to understand the similarities and differences between the only children and children with siblings groups in the conditional LGM to clarify the influence of “years with peers” on the development of social competence among only children and children with siblings.

The following three main results were observed: (1) The four social competence dimensions, namely independence, assertiveness, sociability, and compliance, increased with age. At T1, the initial level of each child’s social competence differed significantly. Both T2 and T3 surveys of each child’s social competence demonstrated significant growth, and growth rates also differed significantly by individual. (2) In the conditional LGM analysis with the covariate of having siblings or not, only the growth rate of compliance of children with siblings was significantly higher than that of only children. The initial values of the four dimensions of social competence did not differ significantly, and the growth rate of the children’s independence, assertiveness, and sociability did not differ significantly. In addition, in the conditional LGM analysis with “years with peers” as a covariate, “years with peers” at T1, T2, or T3 exerted significant positive effects on the children’s independence, assertiveness, and sociability, but it did not influence compliance. Under the influence of covariates, the initial values and growth rates of the four dimensions also differed significantly. (3) This study further compared the children with siblings and only children in groups to demonstrate the influence of “years with peers” on the growth of social competence in these groups of children. The initial levels of the four dimensions of social competence of only children were higher than those of children with siblings, but the growth rates of the four dimensions of social competence of children with siblings were higher than those of only children. Peers positively affected the development of children’s independence, assertiveness, and sociability, but as children grow older, the influence of “years with peers” on their social competence in these three dimensions decreased. Moreover, social competence in these three dimensions was more positively affected by “years with peers” in only children than in children with siblings.

The study provides the following recommendations: (1) Siblings and other peers contribute to the development of children’s independence, assertiveness, and sociability. Under the joint influence of siblings and other peers, children with siblings progressed from an initial developmental level lower than that of only children to a level higher than or equal to that of only children within 2 years. This type of results are difficult to observe using cross-sectional research of a single point in time. Therefore, tracking the social competence development of only children and children with siblings using panel study databases such as KIT is necessary. (2) This study proposes research questions that can only be answered using panel studies. Examples of such questions are as follows: Does the impact of having siblings and peers on children increase gradually as they grow older, and does the gap between them and only children with peers also grow? As the literature indicates, from toddler to teenage years, do only children and children with siblings exhibit no significant differences in either positive social skills or negative aggression? Therefore, this study suggests that panel study databases of the social competence of only children and children with siblings should be maintained with information that extends to adolescence. (3) Siblings are important for the development of children’s social competence. For only children without siblings, peers become important. The effect of peers on the development of the social competence of young children can be observed in the only children who had peers before T1. Having peers in this study referred to children having attended kindergarten (nursery) on average every day at the time of the survey, regardless of the average attendance duration. Thus, if only children are sent to kindergarten or nursery school, the development of their social competence is enhanced, regardless of the duration of their attendance. Therefore, this study suggests that only children should attend school early in their lives, regardless of the duration of their attendance.