Abstract
Sibling profiles, including sibling status (only-child or sibling) and sibling characteristics (sibling size, birth order, and sex), can impact on lived experiences and social interactions, and operate as protective or risk factors for a wide range of health and well-being indicators and outcomes. Using population-based data linkage to disability-specific databases, sibling profiles were compared between families of children with and without neurodevelopmental conditions. Families of children with neurodevelopmental conditions were more likely to contain siblings than only-children, and be larger families, with children more likely to have shorter gestational age, lower birthweight, younger mothers, and lower socioeconomic status than other children. The context of unique sibling experiences may be important for understanding how sibling well-being may be influenced by different family circumstances.
Introduction
Research has demonstrated differences in the overall health and well-being of children by their sibling status depending on whether they have siblings or whether they are an only-child (Baranowska-Rataj et al., 2017; Keenan et al., 2023). Children without siblings have been found to be of lower birthweight (Feng et al., 2024); and throughout life are more likely to have poorer health indicators such as being overweight/obese, a higher body mass index, and higher blood pressure; as well as higher later life mortality (Black et al., 2016; Chanfreau et al., 2022; Derraik et al., 2016; Jelenkovic et al., 2013; Keenan et al., 2023; Meller et al., 2018; Park & Cormier, 2018).
Siblings play an important role in the functioning of families, contributing to family resources, facilitating social interaction, and often providing lifelong support and companionship to each other. Sibling relationships can sometimes be disrupted by factors such as changes to a family member’s health, a new diagnosis, financial circumstances, or other challenges, which may have profound or enduring effects for children and their sibling relationships (Nygård et al., 2025; Pharoah et al., 2025). Sibling characteristics, such as the number of siblings in a family, individual birth order and sex, can also shape their experiences, relationships, and life choices, as well as influence future outcomes (Black et al., 2016; Gilligan et al., 2020; Meller et al., 2018).
In particular, birth order and/or sibling size have been associated with increased risks of poorer mental health (Carballo et al., 2013; Fukuya et al., 2021), decreased cognitive ability (Barclay, 2015), presence of attention deficit hyperactivity disorder (Marín et al., 2014), suicide and psychiatric disorder (Bjørngaard et al., 2013; Easey et al., 2019; Rostila et al., 2014), and risk of some cancers (Bevier et al., 2011; Hemminki & Mutanen, 2001; You et al., 2018). In addition, sex combinations among siblings have been associated with sibling relationship quality (Steelman et al., 2002), with sister–sister dyads found to have closer and more supportive relationships (Gilligan et al., 2020; Stocker et al., 2020), which can positively impact long-term mental health.
The associations between sibling profiles and health and well-being outcomes can be grounded by different, and sometimes conflicting, theories. The resource dilution theory, which posits the more people in a group, the less the individual share of resources becomes (Blake, 1981; Downey, 2001; Öberg, 2015; Riswick & Engelen, 2018; Stradford et al., 2017), may explain poorer outcomes observed in larger families, where reduced financial resources may impact decisions and opportunities that support optimal nutrition, health, and well-being. In contrast, social learning theory (Bandura, 1977) would suggest there are positive associations with a greater number of siblings, reasoning that more people in a family provide greater occurrences of combined benefits, social interactions, and supportive behaviour, leading to more positive opportunities and better overall health and well-being outcomes (Hayden & Hastings, 2022).
Globally, the average fertility rate is 2.3 and declining (United Nations Population Fund, 2024), but varies widely according to country, and many social, cultural, and political factors. Currently in Australia, the majority of families contain at least two children (65%) (Australian Institute of Family Studies, 2023), higher than the United Kingdom (55%) (Office for National Statistics UK, 2024) and Sweden (59%) (Statistics Sweden, 2023). Family size is on average higher for immigrant families and also for First Nation Australians (representing 3.3% of the Western Australian population) (Australian Bureau of Statistics, 2022b) who experience a greater diversity of family types across different community settings (Silburn et al., 2006). Currently, about one third of Western Australians are born outside Australia, with major immigrant populations coming from the United Kingdom, India, South Africa, and Southeast Asia (Australian Bureau of Statistics, 2022a). In recent years, the overall fertility rate across Australia is declining, falling below two children per mother since 2009, and is currently 1.5 registered births per women (Australian Bureau of Statistics, 2023). This steady decline within one generation implies changes in family demographics are rapidly changing sibling dynamics, which may have implications for relationships, social connections and family care or responsibilities over the longer term.
In families of children with neurodevelopmental conditions, siblings provide integral support, advocacy, mentorship, and companionship (L. Nguyen et al., 2024). Neurodevelopmental conditions and/or disorders are collective terms used to describe conditions that affect brain development during childhood and can impact a person’s social interaction, emotional and autonomic regulation, cognition, learning, and movement (Diagnostic and Statistical Manual of Mental Disorders [DSM-5-TR]; American Psychiatric Association, 2024; World Health Organisation, 2024). These conditions include intellectual and learning disabilities, autism, attention deficit hyperactivity disorder, cerebral palsy, Down syndrome, genetic conditions as well as motor disorders. Siblings of children with neurodevelopmental conditions often provide additional support to parents via informal caring roles, or contributing more to household responsibilities (Arcous et al., 2024). Prior research has shown that the experiences for siblings in small, two-sibling families, where one child has a neurodevelopmental condition, can be quite different to larger families, relating to the reciprocity of the sibling relationship, carer burdens, and future expectations of the caring role. In contrast, having more siblings in the family allows division and dispersion of responsibilities growing up, and may lead to greater family support capability later in life. Some research has found that greater number of siblings within families of children with neurodevelopmental conditions are protective in measures of well-being, with lower risk of a mental health diagnosis (Marquis et al., 2019). Conversely, other research focusing on families of children with intellectual disability indicates that more siblings may mean less time spent with parents as children or young people, or reduced opportunities for social/recreational activities, which provide important avenues for growth and development, and peer support opportunities for siblings (Gray et al., 2024; Mulroy et al., 2008). Given the significance of the role siblings play in these families, it is important to understand more about the patterns of sibling profiles in families of children with neurodevelopmental conditions, to consider how sibling structures may contribute to elements of support and well-being (Feng et al., 2024).
Previous research has noted differences in various sibling characteristics between families of children with and without neurodevelopmental conditions, such as the proportion of only-child families or sibling families, and the number of siblings in the family (Kuja-Halkola et al., 2019; Seltzer et al., 2001). However, the contextual associations of reported risk and protective factors relevant to families of children with neurodevelopmental conditions have been limited (Gray et al., 2025). The benefits of exploring and comparing data specific to family types could result in increased awareness of circumstances that are common to family experiences, the need to offer tailored supports for families, and inform policies and support networks operating within education, health, and disability sectors.
This study aims to describe and compare the diversity of sibling profiles among families of children with and without neurodevelopmental conditions. It uses a population-level database of children diagnosed with neurodevelopmental conditions associated with intellectual disability, and links between children born to the same mother to enable utilisation of family-level data. The research objective is to explore differences in sibling profiles (sibling status, sibling size, birth order, and sex), comparing families with and without neurodevelopmental conditions, condition type, and by severity of intellectual disability. Implications are discussed for how sibling structures could influence health indicators and outcomes, be associated with different family dynamics, and the particular relevance of these findings for the support of families of children with neurodevelopmental conditions.
Method
Study Design
This retrospective cohort study used an observational design to examine data from existing records to explore the association between exposure and outcomes related to sibling structure. This study formed part of a wider study focusing on the health trajectories of children with intellectual disability that included all livebirths in Western Australia between 1 January 1983 and 31 December 2010 and used population data linkage between administrative registries and disability-specific databases. We utilised available data from this existing dataset to compare a subset of children with neurodevelopmental conditions and their siblings compared with children/siblings from families with no neurodevelopmental conditions from the total population. Disability registry information was available to the end of 2012 which allowed for a minimum two-year diagnostic follow-up of the cohort.
Population Datasets and Selected Variables
Midwives Notification System (All Registered Births)
This register contains information on all live births (≥20 weeks gestation or ≥400 g birthweight) in Western Australia since 1980 (Holman et al., 2008; N. Nguyen et al., 2008). It was also used to identify family groups by all children born to the same mother. Variables extracted: maternal age in years (presented as: mean (SD)); ethnicity (Caucasian, Aboriginal, other); marital status (married, never married, formerly married/unknown); socioeconomic indexes for areas: index of relative socioeconomic disadvantage (IRSD) (<10%, 10%–24%, 25%–49%, 50%–74%, 75%–89%, ≥90%) (based on mother’s census district at time of birth, where lower IRSD equates to greater disadvantage); accessibility/remoteness index of Australian classification (ARIA) (major cities, inner regional, outer regional, remote, very remote); sex of infant (male, female); birth year; gestational age in weeks (mean (SD)); and birthweight in grams (mean (SD)).
The Intellectual Disability Exploring Answers (IDEA) Database
This database contains information on children born in Western Australia since 1983 and diagnosed with neurodevelopmental conditions associated with intellectual disability through the WA Department of Communities (previously Disability Services) and the Department of Education (Balogh et al., 2019; Petterson et al., 2005). Variables extracted: diagnosed condition type (intellectual disability, autism, cerebral palsy, Down syndrome); severity of intellectual disability (mild-moderate, severe); and diagnostic age.
The WA Register of Developmental Anomalies – Cerebral Palsy Register
This register contains information on children born in Western Australia since 1956 and diagnosed with cerebral palsy (Australian Cerebral Palsy Register, 2023; Garne et al., 2023). Variables extracted: severity of intellectual disability (mild-moderate, severe) and diagnostic age.
Statistical Analysis
The following sibling profile variables were created: sibling status (only-child, sibling) and sibling characteristics: sibling size (the number of children in the family born to the same mother) (count variable: 2, 3, 4, 5+, and categorical variable: smaller n = 2, larger n > 2); birth order (categorical variable: first born, later born); and sex ratio (male: male, male: female, female: female). Comparisons between families with and without neurodevelopmental conditions were made across a number of sibling, birth, and maternal characteristics: Based on variable type, descriptive statistics, performed using Stata 18, were presented at the individual level, that is, maternal age in years (mean (SD)), birthweight in grams (mean (SD)), gestational age in weeks (mean (SD)), gender, or the family level, that is, maternal ethnicity, marital status, ARIA, IRSD.
Results
Births
There were 720,901 livebirths in Western Australia between 1983 and 2010, including 42,001 (5.8%) born to Aboriginal mothers; 644,657 (89.4%) born to married mothers; 321,228 (49.8%) with middle range IRSD (25–74%); 467,735 (72.4%) from major cities; 20,040 (2.8%) multiple births; and 369,626 (51.3%) male (Table 1). Among all livebirths, 146,781 (20.4%) births were the only child in the family at the time of data extraction, and the remainder 574,120 (79.6%) births constituted 231,466 families with more than one child, of which there were 37,279 births (6.5%) from Aboriginal families.
Births: Selected Maternal and Birth Characteristics by Neurodevelopmental Condition Status, Condition Types, and Aboriginal Families.
There were 12,205 children registered on the disability-specific databases with neurodevelopmental conditions relating to intellectual disability (1.7% total births) (Table 1). Of these, 1417 children (11.6%) were over-represented among Aboriginal families (compared to the proportion of total Aboriginal births, 3.4%). Children with neurodevelopmental conditions were more likely than children without these conditions to have younger mothers (mean age in years) (28.0 (5.9), 28.9 (5.5)); be from lower range IRSD (<25%) (40.7%, 27.6%); be a multiple birth (4.4%, 2.7%); be male (65.2%, 51.0%); have shorter gestational age (mean age in weeks) (37.9 (3.2), 38.8 (2.0)); lower birthweight (mean weight in grams) (3087 (765), 3358 (566)); and be from families with at least two children (85.1%, 79.5%) (Table 1). They were less likely to be born to married mothers (82.0%, 89.5%); and to be from major cities (71.3%, 72.4%).
Diagnosis age for children with neurodevelopmental conditions was on average 6.2 years, across the period 1983–2010 (Table 1). This reduced over the three decades from 8.8 (5.9) during 1983–1989 to 6.6 (4.1) during 1990–1999 and to 3.0 (2.2) during 2000–2010. Aboriginal children were older than other children at age of diagnosis (8.5 (4.6), 6.2 (4.7)) (Table 1). The difference in age of diagnosis was observed across the three decades from 10.8 (4.9) during 1983–1989 to 8.8 (3.7) during 1990–1999 and to 3.3 (2.7)) during 2000–2010.
Where relevant, children were sub-grouped based on the following condition types: intellectual disability only (7,306) (i.e. no co-occurring diagnosis for autism/cerebral palsy/Down syndrome); and separately for autism (2,299); cerebral palsy (1,730); and Down syndrome (653) (Table 1). There were fewer only-child families found in each condition group when compared to families of children without neurodevelopmental conditions. The intellectual disability group had the fewest only-child families (13.7%), and the Down syndrome group had the most only-child families (19.3%). The average age of diagnosis, across the period 1983–2010, was the highest for the intellectual disability only group (8.0 (4.5)) and lowest for the Down syndrome group (0.3 (0.9)) (Table 1). In addition, children diagnosed with intellectual disability (with or without co-occurring conditions) (10,039) were sub-grouped based on severity levels of ‘mild/moderate’ (91.2%) and ‘severe’ (8.8%), with the highest age of diagnosis across the period 1983–2010 for the ‘mild/moderate’ group (7.0 (4.7)) (Table 1).
Siblings
Compared to children from only-child families, children from families with two or more children (sibling families) were more likely to have younger mothers (28.7 (5.4), 29.6 (5.9)); be from lower range IRSD (27.3%, 25.4%); slightly more likely to have married mothers (86.7%, 86.2%); be male (51.3%, 51.0%); have shorter gestational age (38.8 (2.0), 38.9 (2.0)); and higher birthweight (3355 (574), 3350 (560)) (Table 2). They were less likely to be from major cities (71.3%, 76.5%).
Births: Selected Maternal and Birth Characteristics by Sibling Status and Sibling Cohorts.
General Cohort (Children From Families With At Least Two Children Without Neurodevelopmental Conditions)
There were 545,910 births from families of two or more children without any diagnosed neurodevelopmental conditions, forming 222,009 families (Figure 1; Table 3). Of these, 32,857 (6.0%) births were from Aboriginal families.
Families: Sibling Status by Neurodevelopmental Condition Status.
Sibling Profiles by Neurodevelopmental and General Cohorts, Condition Type and Aboriginal Families.
Neurodevelopmental Cohort (Children With Neurodevelopmental Conditions and Their Siblings)
There were 10,382 children with neurodevelopmental conditions in families of at least two children (Table 1). Of these, 1,295 children (12.5%) were from Aboriginal families. There were 17,828 siblings who did not have a neurodevelopmental condition (Table 3). Together, children with neurodevelopmental conditions and their siblings (total of 28,210 children) formed the neurodevelopmental cohort. All children in the neurodevelopmental cohort were connected to 9,457 families (Figure 1). Most families (88.8%) had only one child in the family with a diagnosed condition plus their sibling(s).
Compared to the general cohort, children in the neurodevelopmental cohort were more likely to have younger mothers (27.7 (5.8), 28.7 (5.4)); be from lower range IRSD (39.3%, 26.8%); be a multiple birth (4.6%, 3.4%); be male (56.1%, 51.1%); have shorter gestational age (38.3 (2.7), 38.8 (2.0)); and lower birthweight (3194 (691), 3364 (566)) (Table 2). They were less likely to have married mothers (75.8%, 87.1%) and be from major cities (72.2%, 74.4%). A higher proportion of births from Aboriginal families were found in the neurodevelopmental cohort compared to the general cohort (10.9%, 4.4%).
Sibling Characteristics
Sibling Size
Sibling size (the number of children in the family) was higher among families in the neurodevelopmental cohort than the general cohort (3.1 (1.3), 2.5 (0.8)) (Table 3). A similar pattern was observed among Aboriginal families in the neurodevelopmental cohort when compared to Aboriginal families in the general cohort (4.3 (1.8), 3.4 (1.5)). For condition type, sibling size was the highest for intellectual disability (3.2 (1.4)) and the lowest for autism (2.7 (1.0)).
In small families (sibling size n = 2), children in the neurodevelopmental cohort compared to the general cohort were more likely to have lower gestational age (38.2 (2.9), 38.8 (1.9)), lower birthweight (3190 (715), 3371 (550)), be male (58.5%, 50.7%), and have lower range IRSD (32.6%, 24.0%) (Table 4). In larger families (sibling size n>2), children in the neurodevelopmental cohort compared to the general cohort were also more likely to have lower gestational age (38.3 (2.7), 38.7 (2.0)), lower birthweight (3195 (682), 3356 (581)), be male (55.1%, 51.5%), and have lower range IRSD (44.2%, 31.8%).
Neurodevelopmental/General Cohort Comparisons: Selected Maternal and Birth Characteristics by Sibling Characteristics.
Within the neurodevelopmental cohort, children in small families compared to larger families had slightly lower gestational age (38.2 (2.9), 38.3 (2.7)) and birthweight (3190 (715), 3195 (682)), different to observations in the general cohort (Table 4). Children in small families also had the greatest proportion of males compared to larger families (58.5%, 55.1%). Children in larger families in the neurodevelopmental cohort compared to small families were more likely to have lower range IRSD (44.2%, 32.6%).
Birth Order
When comparisons were made between the neurodevelopmental and general cohorts, greatest differences were observed between later-born groups relating to gestational age (38.2 (2.6), 38.7 (1.8)), birthweight (3224 (676), 3407 (554)), and lower range IRSD (44.8%, 28.3%) (Table 4). Similar patterns were observed in first-born groups between the cohorts for gestational age (38.3 (3.0), 38.9 (2.1)), birthweight (3134 (716), 3300 (577)), and lower range IRSD (39.3%, 26.8%).
Within the neurodevelopmental cohort, first-born children were more likely than later-born children to have lower birthweight (3134 (716), 3224 (676)), longer gestational age (38.3 (3.0), 38.2 (2.6)), and to be male (58.0%, 55.1%) (Table 4), while later-born children in this cohort were more likely than first-born children to have lower range IRSD (44.8%, 39.3%).
Sibling Size by Birth Order
In the neurodevelopmental cohort, across all sibling sizes, later-born children were more likely than first-born children to have a neurodevelopmental condition (60.5%, 39.5%). Where there were two children in the family, first-born children were more likely than later-born children to have a neurodevelopmental condition (53.8%, 46.2%) (Table 5). Where there were more than two children in the family, later-born children were more likely than first-born children to have a neurodevelopmental condition (70.0%, 30.0%). Sibling size was smaller among families of first-born children with neurodevelopmental conditions than among families of later-born children with neurodevelopmental conditions (2.7 (1.1), 3.4 (1.5)) (Table 5), and this observation remained when condition-type autism was removed (3.1 (1.3), 4.0 (1.7)).
Neurodevelopmental Cohort: Sibling Size Associations by Birth Order.
Sex
The sex ratio in the neurodevelopmental cohort had a similar pattern to the general cohort; however, different proportions were observed between the cohorts (50.9% male:female; 33.1% male:male; 16.0% female:female) (52.4%; 24.5%; 23.1%) (Table 3). For male children, differences were observed between the neurodevelopmental and general cohorts for gestational age (38.3 (2.8), 38.8 (2.0)) and birthweight (3248 (704), 3426 (575)). A similar pattern was observed for female children between the neurodevelopmental and general cohorts, for gestational age (38.3 (2.7), 38.8 (1.9)) and birthweight (3124 (668), 3298 (548)) (Table 4). Within the neurodevelopmental cohort, males compared to females had similar gestational age (38.3 (2.8), 38.3 (2.7)) and higher birthweight (3248 (704), 3124 (668)). There were more males than females diagnosed across all condition types (65.2%, 34.8%) (Table 1), highest for autism (82.4%, 17.6%), and overall, males tended to have a slightly younger age at diagnosis than females (6.2 (4.5), 6.3 (4.9)).
Discussion
Children’s experiences are shaped by their family circumstances, their status as an only-child or a sibling, and other sibling characteristics, with the sibling profile being an important influence on the quality of sibling relationships, and opportunities for family support (Holmes et al., 2024). This study used linked population data to examine the patterns of sibling profiles among families of children with and without neurodevelopmental conditions, across a variety of maternal and birth characteristics. Differences in sibling profiles between family types suggests that diverse family dynamics are operating, with implications for the lived experiences, protective factors, and influence on future health and well-being outcomes for children in these families (Hayden & Hastings, 2022; Roy & Balaratnasingam, 2014; Shenoy et al., 2024; Strobel et al., 2020).
In this study, overall findings of fewer only-child families among those with neurodevelopmental conditions indicate that the presence of a diagnosis may influence decisions to have additional children (siblings) for some families, which may provide a protective and supportive element for this group (Seltzer et al., 2001). Families of children with cerebral palsy or Down syndrome had the highest proportion of only-child families, which has also been observed in other population studies that reported parents of first-born children with cerebral palsy or Down syndrome were less likely to have further children (Kimura & Yamazaki, 2019; Michelsen et al., 2015). These studies suggest specific differences may occur by neurodevelopmental condition in the family, possibly influenced by earlier diagnoses in children, older/younger maternal ages, religion, or culture (Fairthorne et al., 2016; Michelsen et al., 2015; Müller et al., 2022; Pillay et al., 2012; Povee et al., 2012). Research suggests that an only-child is more likely to experience health and mortality disadvantages across several health indicators such as higher bodyweight and blood pressure and lower fitness, with differences sometimes being observed after birth (Baranowska-Rataj et al., 2017; Feng et al., 2024; Keenan et al., 2023). In this study, despite low proportions of single-children with neurodevelopmental conditions, the finding of lower birthweight for single-children, in particular for this group, may indicate increased risk for future health outcomes. An only-child with a neurodevelopmental condition, especially those children with severe disability, may be at further disadvantage based on their existing diagnosis, as well as the increased likelihood of comorbidities (John Cherian et al., 2023; Reddihough et al., 2021), and higher rates of early mortality (Bourke et al., 2017; Sun et al., 2023). Alternatively, single-children could have health advantages if families are able to direct more resources towards the needs of their only child (Jensen et al., 2022; Tøssebro & Wendelborg, 2017; Traustadóttir et al., 2015). However, we also found lower socioeconomic status among families of single-children with neurodevelopmental conditions compared to other single-child families, suggesting higher risk of continued disparity between family types. Further research in this area using record linkage to population health data, such as hospitalisations, healthcare visits, and mortality records, would facilitate a better understanding of the age-related patterns of health outcomes and risk/protective factors occurring for single-children with neurodevelopmental conditions.
Information on sibling size in similar families at the population level is relatively sparse (Gray et al., 2025), but the findings of greater sibling size among families in the neurodevelopmental cohort in this study are supported by other large sample size studies that have shown greater numbers of siblings in families of children with intellectual disability and Down syndrome (Langley et al., 2020; Liu et al., 2021) compared to other families of children without neurodevelopmental conditions. Greater sibling size is often associated with poorer health indicators, health outcomes, and higher mortality risk (Baranowska-Rataj et al., 2017; Keenan et al., 2023), where less resources may be available per individual (resource dilution theory) (Blake, 1981; Downey, 2001; Öberg, 2015; Riswick & Engelen, 2018; Stradford et al., 2017). Although slight, this study found birthweight reductions with increasing sibling size only among families of children without neurodevelopmental conditions, suggesting protective factors may be occurring for children in larger families in the neurodevelopmental cohort. The observation of higher risk of disparity for larger families in the neurodevelopmental cohort relating to lower socioeconomic status suggests a difference in the way this risk factor operates between families of children with and without neurodevelopmental conditions. Other research indicates that sibling risk related to greater sibling sizes for families of children with neurodevelopmental conditions may become compounded if resources are stretched, indicating increased levels of required support (John Cherian et al., 2023). Despite this, benefits of having more siblings could result in better health outcomes for children in the neurodevelopmental cohort, due to shared carer roles and dispersion of responsibility (Bandura, 1977; Hayden & Hastings, 2022). Future work measuring the association between number of siblings at the family level, and health outcomes for different family groups, will help to understand family support needs.
Current findings of birth order effects for birthweight are supported by other research (Feng et al., 2024; Hinkle et al., 2014; Wells et al., 2011). Some studies reporting standardised outcomes recorded in population databases suggest lower risk of adverse health outcomes, such as cardiovascular disease, and some forms of cancer for first-born children compared to later-born children (Hemminki & Mutanen, 2001; Jelenkovic et al., 2013), with increased risk also observed for mental health conditions and emotional disorders (Carballo et al., 2013). For the neurodevelopmental cohort, birth order effects are further influenced by the birth order of the diagnosed child (Currie et al., 2024). Higher proportions of later-born children with neurodevelopmental conditions in this study were similar to proportions found in other studies on families of children with physical, intellectual, and multiple disabilities (Leonard et al., 2005; Lundeby & Tøssebro, 2008). These findings suggest children with neurodevelopmental conditions have increased opportunities to receive care and social support from older siblings, which can be a protective factor for their own mental health outcomes, and have significant benefits for their development (Ben-Itzchak et al., 2019; Braconnier et al., 2018; Hayden et al., 2023). However, the birth of a later-born child with a neurodevelopmental condition may present significant change for older siblings, as the family adapts to new experiences caring for a child with a neurodevelopmental condition (Plessis et al., 2020). The birth order findings in this study indicate that many siblings may experience this change within the family environment during their early childhood years (Currie et al., 2024; Lummer-Aikey & Goldstein, 2020), and may be influential in earlier initiation of the sibling caring role. Later-born siblings may have different experiences of learning from and modelling the behaviour of an older sibling with a neurodevelopmental condition (Bandura, 1977; Vogt Yuan, 2009). Therefore, while risk or protective factors of birth order can operate in general, sibling dynamics at the family level may influence sibling health and well-being differently across family types, and is an important area for future research.
In the current study, the influence of birth order of the child with a neurodevelopmental condition on a reduction in sibling size, particularly where the diagnosed child is first born, and/or the diagnostic age is earlier, may indicate the possibility of ‘reproductive stoppage’ decision making operating in some families (Jones & Szatmari, 1988; Kimura & Yamazaki, 2019; Lundeby & Tøssebro, 2008; Seltzer et al., 2004). However, other influences on sibling size may occur, particularly for families who always plan to have larger families. For condition types with older diagnostic ages (e.g., autism), there may be other considerations occurring for families, more aligned with average family size, supported by evidence from other large data linkage research that has not found evidence for stoppage in families of children with autism (Kuja-Halkola et al., 2019; Ugur et al., 2019). Information collected directly from families about their decisions would help to understand the decision processes related to the observed patterns. Further, in the neurodevelopmental cohort where smaller families were observed, different sibling dynamics could be occurring, impacting sibling social support and reciprocity of the sibling relationship (Dirks et al., 2015; Hayden & Hastings, 2022; Hughes et al., 2018; Shenoy et al., 2024; Walton & Ingersoll, 2015). For some families with only two children, one of whom has a neurodevelopmental condition, the sibling experience may be different. There may be additional pressure on the sibling to provide support as they age, feelings of greater isolation, psychosocial impacts, and resource dilution (Borchet et al., 2020; Cheung et al., 2020; Şimşek et al., 2015). Considerations for extra support mechanisms for siblings in smaller families or related to birth order may be warranted, and could include opportunities for peer or older sibling mentors.
Among children in the neurodevelopmental cohort in this study, there were a higher proportion of males diagnosed with a neurodevelopmental condition, and they were more likely to have female siblings. Generally, gender is an important factor influencing sibling relationships, for example, opposite sex sibships are considered to be less close in the long-term (Padilla et al., 2023; Stocker et al., 2020), albeit with less conflict (Plessis et al., 2020). Supporting studies also found a higher proportion of males with autism (Ugur et al., 2019) and a greater proportion with female siblings (Walton & Ingersoll, 2015). The presence of sisters has general protective effects for sibling relationship quality, and relationship closeness is often considered to positively influence long-term mental health (Vogt Yuan, 2009; Waid et al., 2020; Walton & Ingersoll, 2015). In families where one child has a neurodevelopmental condition, female siblings may provide support advantages for families, given findings of increased likelihood of undertaking caring roles (Australian Bureau of Statistics, 2018; Untas et al., 2022) and greater communication skills (Padilla et al., 2023; White & Hughes, 2017). Some studies have found that male siblings were less likely to take on a carer role, with differences noticed between males and females relating to the type of caring activities performed, such as domestic, emotional, and personal care (Leu et al., 2019; Robison et al., 2020). For male siblings, previous findings suggest a higher risk of psychosocial concerns (Walton & Ingersoll, 2015), in particular older brothers of children with autism. Further qualitative research in this area would help to better understand sibling gender dynamics, as well as parental expectations of sibling roles. This could be beneficial for parents to understand how different expectations may unconsciously be placed upon different siblings according to their gender, and also for organisations providing carer support skill building to tailor their resources for siblings undertaking carer roles.
A strength of this study was access to dedicated data containing information on children with neurodevelopmental conditions and the ability to examine sibling profiles retrospectively at a population level without the need to collect data at the family level. It also offered a longitudinal perspective to enable sufficient follow-up and ensure representation of diagnostic subgroups that may be otherwise excluded using other study designs. It is recommended that future research conducts further exploration of the relationship between sibling profile differences and specific health and well-being outcomes for families of children with different types of neurodevelopmental conditions and in conjunction with family-level data to understand family experiences in more detail. This will help understand how well-being outcomes are associated with sibling characteristics and dynamics operating in families, and whether sibling or family supports could be tailored accordingly.
A limitation of this study was not being able to include a wider range of neurodevelopmental conditions in the analysis, including those not associated with intellectual disability, such as attention deficit hyperactivity disorder. A recommendation for future research is to widen the scope of included conditions studied, which would give broader perspectives on any diagnosis-dependent family support needs. Limitations also exist relating to gender role discussion and how this is shaped by culture and social fabric of Western Australian families. Further, this study observed unique circumstances for siblings from Aboriginal families compared to non-Aboriginal families, for which dedicated research towards understanding the cultural influences of sibling structures and family characteristics for the health and well-being of Aboriginal children is an important area of future research. Another limitation was that population demographics change over time, and the data used in this study may have reduced applicability to that of future cohorts, particularly in terms of changing sibling sizes and the types of identified diagnoses for which diagnostic trends may have changed. In addition, advances in prenatal diagnoses and assisted reproductive technology continue to occur, with possible changing relevance for the observed family demographics in more recent cohorts. There may also be some misclassifications for disability in children not yet born into families or diagnosed, for children diagnosed in earlier years using older classification systems or genetic knowledge, or with respect to final sibling size that occurred after the time of data extraction. Given reduced fertility rates over time towards less than two children per mother, the relevance of these data to future families regarding family size may lessen. It is recommended that future research regularly updates information regarding contemporary sibling profiles at the population level when opportunities for linkage with more recent data becomes available. This will maintain data relevance and enable analysis of time trends.
Implications for Practice
This study found that in families of children with neurodevelopmental conditions, there has been a decline in sibling size over time. This trend, combined with limitations to the availability of appropriate sibling supports, could lead to reduced availability of informal sibling carers, and may result in greater demand for more formal community-based services in the future. This would have important implications for future disability policies, especially when parent carers age and can no longer provide a supportive role. However, ensuring appropriate sibling supports are available which would ensue many benefits for people with disability, siblings, families, and the community, including improved sibling and family well-being, and long-term sibling mental health, longevity in a carer role, and stronger sibling relationships.
Conclusion
Recognising differences in sibling characteristics can help understand associations with risk and protective factors for children with or without neurodevelopmental conditions, applicable from early ages and relevant through to adulthood. Linked population data allow exploration of sibling profiles at the macro-level, from which community-level implications as well as family-level support considerations can be observed and pursued further using other methods. Future research using qualitative data could strengthen the data from this study, to understand how families can be best supported. Supports tailored to family circumstances could be developed to effectively support siblings and their families in their relationships and caring roles throughout different life stages, benefitting all family members and the community in general.
Footnotes
Acknowledgements
The authors wish to thank the staff at the Western Australian Data Linkage Services, the Department of Health WA, and custodians of the Midwives Notifications System, WA Register of Developmental Anomalies–Cerebral Palsy, Birth Registrations and the IDEA Database. We also acknowledge the Department of Communities and the Department of Education WA for provision of data for the IDEA Database.
Disposition editor: Cristina Mogro-Wilson
Ethical Approval
Ethical approval was provided by the Western Australian Department of Health’s Human Ethics Review Committee (RGS0000002731), the Western Australian Aboriginal Health Ethics Committee (613), and The University of Western Australia (ET000798).
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research is funded by the Australian National Health and Medical Research Council (NHRMC)(GNT1184770), and a Senior Research Fellowship (1117105); and a UTP/RTA Scholarship from The University of Western Australia. The contents of the published material are solely the responsibility of the individual authors and do not reflect the views of NHMRC.
Declaration of Conflicting Interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.