Sibling interaction is one of the first and longest-lasting relationships an individual experience. Sibling interactions provide children with opportunities to share experiences and emotions, as well as to practice various communication skills. Through these interactions, children often acquire a wide range of social skills that are crucial for their development. In fact, children are more likely to engage with their siblings than with their parents in certain types of interactions, such as nonverbal communication [1]. Studies have shown that children with siblings have higher social competence compared to those without siblings [2]. Moreover, it has been observed that children with siblings have a better understanding of others and tend to learn social skills by imitating their siblings [3,4]. These sibling interactions during early childhood have a lasting impact on social skill development, extending into early adolescence, making siblings an important source of social skill development [5]. Autism is characterized by difficulties in social communication and interaction, along with the presence of restricted or repetitive behaviors and interests (RRBs). These difficulties can hinder the autistic child’s ability to interact with peers, which is an important factor in maximizing their potential to adjust to their surrounding environment. Similar to typically developing children, siblings can provide opportunities for autistic children to interact with individuals of similar age and developmental stage at home, allowing them to practice and apply the skills they have learned in a comfortable setting. Studies have demonstrated that older siblings can positively influence the cognitive, academic, and communication skills of autistic children [6], as well as their social playing skills [7-9]. Additionally, autistic children have shown more initiation responses from their siblings compared to their parents [10], indicating that siblings can serve as a promising source of social interaction improvement. Moreover, specific characteristics of siblings have been linked to distinct social skill development in autistic children and other psychiatric disorders. According to Ben-Itzchak et al. [10], individuals with older siblings exhibited better social communication skills and adaptive behaviors compared to those without siblings. Similarly, a study done by Montes [11] demonstrated that those with older siblings were less likely to experience mental health or behavioral problems. Having multiple siblings was associated with better social skills due to the increased opportunities for imitation and social interaction, yet the sex of the sibling appeared to have no significant impact on the social functioning of the autistic child [12]. Furthermore, a meta-analysis by Cuskelly et al. [13] delved into the influence of siblings on autistic children. Interestingly, while some studies, like Matthews et al. [14], revealed the benefits of having an older sibling in terms of tasks involving false beliefs or nonverbal tasks, others, such as one by Brewton et al. [15] demonstrated that the adaptive skills were associated with the social skills of the autistic child. In summary, existing research has explored how siblings affect social skills in autistic children, although it has been somewhat limited in the range of variables examined. More notably, inconsistencies in the results have emerged. Some potential explanations for these discrepancies include, include the utilization of different outcome measures evaluated by different diagnostic tools, uncertainties surrounding the diagnosis of certain participants, and the relatively small size of study population [13]. To address these limitations, we designed this study as a single-center investigation involving over 400 participants who were assessed using consistent diagnostic criteria. Our aim was to explore various sibling variables and their effects on the social functioning of autistic children.

This current study aims to examine the manifestation of symptoms in autistic children based on sibling status, birth order, sex, sibling’s diagnosis of autism, and age gap. The study hypothesized that autism-related symptoms, measured by various clinical assessment tools, will differ according to these sibling characteristics. First, given that sibling interaction provides opportunities for social imitation and the practice of play skills [7], we anticipate that the group with siblings will show less difficulties in social skills compared to autistic children without siblings. Second, children with older siblings, who are expected to show proactive behavior to younger siblings, are likely to present better social communication and interaction skills [16]. Third, similar to previous studies [11], the sex of the siblings will not significantly affect the social skill development of the autistic child. Fourth, being a “concordant pair,” the case in which the sibling is also diagnosed with autism, may limit active sibling interactions involving identifying social cues, responses, imitation, and/or joint attention. Lastly, sibling pairs with smaller age gaps will be associated with better social interaction skills, as closer age proximity would mean that the sibling may share similar levels of maturity with common interests or day-to-day activities.

Our study included 895 children ranging from 2 to 13 years old, and the majority of participants were male (81.3%). Among the children, 51.0% had siblings, with 37.2% having older siblings, 39.9% having younger siblings, 8.1% being twins, and 14.7% having multiple siblings. Autistic children who had siblings diagnosed with autism were analyzed as a separate subgroup. The participant demographics are presented in Table 1, providing further details.

In terms of the ADOS-2, there were no significant differences in ADOS-2 total CSS, social affect CSS or RRB CSS scores. However, in terms of ADI-R scores, autistic children with siblings scored higher in social interaction domain (F=3.013, p=0.038), while they scored lower in RRB domain (F=4.079, p=0.044). Further subdomain analysis of the RRB domain revealed significant differences in preoccupation and unique interests (F=5.370, p=0.021), as well as routine following (F=4.937, p=0.027) (Table 2) with children with siblings scoring lower than children without siblings.

When analyzing ADI-R subdomains under the social interaction domain according to sibling birth order, it was found that those with multiple siblings performed better in using nonverbal behaviors compared to those with twin siblings (F=3.791, p=0.005) and when analyzed by items, those with younger siblings performed better in eye contact (F=2.683, p=0.031) and social smiling (F=3.948, p=0.003) compared to twins. Sharing enjoyment abilities were better in those with younger siblings (F=3.473, p=0.008) compared to those with twin siblings. In terms of items, those with younger siblings were best at showing and directing attention (F=5.131, p<0.001). Socioemotional reciprocity was also best seen in those with younger siblings (F=2.458, p=0.044) compared to twin siblings. When analyzing socioemotional reciprocity by breaking it down into items, children with younger siblings were best at using other’s body to communicate (F=4.132, p=0.003) and quality of social overtures (F=4.196, p=0.002). When analyzing communication skills of autistic children, instrumental gesture (F=2.452, p=0.045) were also found to be better in autistic children with younger siblings than those with older siblings (Table 3).

When analyzing the data based on the autism spectrum disorder (ASD) diagnostic status of the sibling, it was found that autistic children with siblings who were also autistic scored higher in the social interaction domain (F=8.541, p=0.004). This difference was seen in subdomains such as peer relationships (F=7.458, p=0.006), sharing enjoyment (F=8.625, p=0.003), and socioemotional reciprocity (F=5.458, p=0.020). In the communication domain, probands with autistic siblings showed greater deficits in social imitative play (F=13.463, p<0.001) (Table 4).

Additional analysis was conducted based on sibling sex and age gap. The scores on the ADI-R, ADOS-2, VABS-II, and SCQ did not differ according to the sex homogeneity of the sibling (Supplementary Table 1 in the online-only Data Supplement). However, a greater age gap with the sibling was negatively correlated with higher scores in the ADI-R social interaction (r=-0.264, p<0.001) and ADI-R verbal communication (r=-0.293, p<0.001) domains (Supplementary Table 2 in the online-only Data Supplement).

This retrospective cohort study aimed to explore the social skills of autistic children according to sibling status and characteristics such as birth order or the affected status of the sibling.

One of the primary findings of this study was that autistic children with siblings showed fewer difficulties RRBs. These findings are consistent with previous studies that have highlighted the positive influence of typically developing siblings on the social communication and social responsiveness of autistic children in various environments, including intervention and natural settings such as home or school [21-23]. Autistic children often lack opportunities to interact with peers, making siblings crucial partners for practicing social skills.

In terms of RRB, RRBs are considered an inherent diagnostic feature of autism, and preoccupations or unique interests are thought to be less influenced by social or environmental factors. However, our study found that autistic children with siblings exhibited fewer RRBs on both evaluation scales of ADI-R and ADOS-2. While the interaction with siblings itself may contribute to reducing these behaviors, it is also possible that a change in parenting style associated with having multiple children could influence how children adhere to routines and schedules. A previous study by Gass et al. [24] revealed that parents who had raised an older typically developing child exhibited lower levels of anxiety in their parenting styles. Stressful or authoritative parenting style have been associated with increased child anxiety, and some studies have suggested a link between higher anxiety levels and more pronounced RRBs [24]. Therefore, considering that parenting style can affect the autistic child’s stress levels, there may be a correlation between parenting style and the severity of RRBs. Furthermore, RRBs not only impact the autistic individual but also require family accommodation behaviors (FAB) [25]. FABs encompass the actions taken by the family to help the autistic child adjust to their environment by implementing routines that assist the child in coping with different situations. FAB often necessitate modifying family plans and interests to support the affected child and require substantial effort from all family members. Such accommodations may pose challenges in families with multiple children, potentially requiring the autistic child to adapt to situations where RRBs cannot be carried out.

When exploring the relationship between specific social skills and sibling birth order, we found that difficulties in nonverbal behaviors were most prevalent in children with twin siblings and least prevalent in children with multiple siblings. For those with younger siblings, it is possible that the close proximity between the social age of autistic child and the chronological age of the younger sibling provided an opportunity for the autistic child to learn from a similar level of social maturity. While for children with older siblings, as one by Ben-Itzchak et al. [10] have also emphasized, older siblings may help in terms of communication and adjustment to socially stressful settings. While having a twin sibling, the unaffected sibling may be much more mature socially, but at the same time may not be mature enough to provide a positive influence as older siblings do. Furthermore, through item analysis it was found that children with younger unaffected siblings showed less deficits in enjoyment sharing and socioemotional reciprocity compared to those with older siblings. However, interestingly, when including the group with no siblings, those with no siblings performed better in nonverbal behavior, enjoyment sharing and socioemotional reciprocity than children with older siblings. This may perhaps be due to greater investment and emotional support given to the only child compared to those with older siblings, indicating that there are multiple children. Further research is suggested in child investment patterns of single versus multiple children families.

The comparison of concordant to non-concordant pairs revealed differences in social interactions and communication, such as sharing enjoyment and social imitation. Sharing enjoyment and imitation require mutual interest, and having a less social sibling may hinder the development of mutual interest by providing fewer opportunities for imitation and practicing sharing enjoyment. These results align with a previous study that highlighted the lack of imitation in pairs of autistic children due to deficits in joint attention [26]. Furthermore, concordant pairs are more likely to be syndromic, where the autistic presentation is caused by a chromosomal abnormality. Autistic children with syndromic autism, such as Fragile X syndrome or tuberous sclerosis, are likely to have other conditions, such as intellectual disability, which further present challenges in developing social skills [27].

In alignment with the findings of the study conducted by Ben-Itzchak et al. [10], the sex of the sibling did not influence the social functioning of the autistic child. A larger age gap was correlated with lower ADI-R scores. However, interpretation of these results should be done with caution as we did not analyze the age gap according to the birth order of the sibling. For instance, when the age gap of the autistic child and sibling is smaller, both participants are expected to be at similar level of development and so when the autistic child shows delay in development, this difference could be exaggerated due to direct comparisons in parental reports. On the other hand, when the age gap between the autistic child and sibling is larger, the comparison between the two participants may be less pronounced. For future studies, it may be more helpful to analyze the age gap based on to the birth order of siblings.

This study is novel in its approach to exploring multiple sibling characteristics and their effects on social functioning of autistic children, which may pave the way for more individualized sibling-mediated interventions. However, it also has several limitations. First, the study heavily relied on caregiver-reported results, which are susceptible to overestimation or underestimation of their children’s abilities and are prone to recall bias. Secondly, as mentioned earlier, the heterogeneity of the study population necessitates further analysis after classifying them into subgroups based on other traits, such as language ability or comorbidities. Thirdly, a larger sample population would be beneficial to accurately examine the results of concordant sibling pairs. Finally, a longitudinal analysis of the sibling pairs could provide valuable insight into how sibling relationships evolve over time and how these changes influence both the autistic child and their siblings.

This study explored how siblings and four different characteristics of siblings including birth order, affected status, sex, and age gap, are associated with the social functioning and communication skills of autistic children. As hypothesized social skills differed according to sibling status and ASD diagnosis status of the sibling. In addition, as expected social skills did not differ according to the sex of siblings. However, while the study hypothesized children with older siblings to show less deficits in social skills similar to previous studies, in our study, children with younger, older, and multiple siblings displayed better outcomes compared to twin siblings, especially nonverbal behaviors. In terms of age gap, unlike the hypothesis, larger age gap was associated with better results in social skills. This may be due to several reasons, one of which is that the optimal age gap may be different for children of different age ranges. The optimal age gap may be different for children of different age ranges. Nonetheless, the findings suggest that siblings may offer opportunities to enhance social and communication skills in autistic children. Based on these results, we urge clinicians to consider providing more individualized sibling interventions for autistic children, involving their siblings, and encourage researchers to investigate the longterm influences of such interventions.