Studies of sibling relationships
Many researchers have observed that the sibling relationship is often the longest relationship an individual has across the lifespan. Here, I present three articles addressing the complexity of sibling relationships. First, van Berkel, Groeneveld, van der Pol, Linting & Mesman (2022) published “Growing Up Together: Differences between siblings in the development of compliance separating within-family and between-family effects” in Developmental Psychology. Here’s the edited abstract and impact statement:
This study applies a within-family, age-snapshot design to investigate differences between siblings in the development of compliance during the preschool years by disaggregating situational, within-family, and between-family effects. Using observational data of 311 Dutch families (self-identified as culturally Dutch) with 2 children when each child was 3 and 4 years old (firstborns: 36.2 months old; SD = 3.6; 48% girls, second-borns (2 years later): 36.67 months old; SD = .62; 47% girls) and both parents. Three-level cross-classified multilevel models showed main effects of observed sibling noncompliance and differential verbal discipline on noncompliance. In addition, second-born children were more compliant than their firstborn siblings, but only when the firstborn was disciplined physically more often than his/her younger sibling.
Differences in compliance between siblings are not just due to character differences, but can be partially explained by differences in how parents respond to their children. Children receiving more verbal discipline than their older or younger sibling showed more noncompliance. Moreover, when young children see that their younger sibling receives less physical discipline in response to noncompliance than they receive themselves, they tend to be less compliant. This study shows that noncompliance in siblings depends on the behavior of both parents and both children.
I like this study because it highlights both the social learning influences of siblings and changes in parenting as families grow. The next study extends and supports this one. Chi, Malmberg & Flouri (2023) published “Sibling Effects on Problem and Prosocial Behavior in Childhood: Patterns of intrafamilial ‘contagion’ by birth order” in Child Development. Here’s a highly edited summary:
We investigated longitudinal relations between siblings' problem and prosocial behavior among different sibship sizes in the UK's Millennium Cohort Study. We identified 3436 families with two children and 1188 families with three children. All children (cohort members and their older sibling [OS]) had valid data on behavior at two time points (in 2004 and 2006). Using structural equation modeling, we found that for internalizing and externalizing problems, OSs (averaging age 6-9 at T1, 49.7% boys) exerted a dominant effect on younger siblings across sibship sizes. For prosocial behavior, there was OS dominance in two-child families and youngest sibling dominance in three-child families.
Siblings can influence one another across the life course, and on various aspects such as emotion regulation, psychopathology, delinquency, prosociality, and educational attainment. Intervention programs for adolescents with a focus on siblings have also shown high efficacy. Within families, siblings' shared exposures to environmental influences, such as peer deviance, psychosocial deprivation, inadequate parenting, and family stressors, also contribute to their similarities. Siblings themselves however can also be an environmental influence as they actively provide a crucial social environment for each other, thus likely influencing each other's behavior. The much-cited sibling contagion phenomenon, for example, refers to the dynamic processes whereby behaviors get transmitted between siblings, facilitating the similarities between them. This is frequently seen as evidence supporting the argument that the skills learned in early childhood through interacting with siblings may provide the foundation for lifelong skills needed for developing and sustaining intimate relationships.
They found that: Associations between siblings' behavior are weaker in larger families – This might indicate that within larger families, with children's ages ranging from middle childhood to pre-adolescence (the age range of our sample at Time 2), socio-emotional development is also dependent on extra-familial influences, including school, society, and culture.
Problem and prosocial behavior is more stable over time for OSs – In two-child families, the OS showed greater within-person stability, as expected. In three-child families, comparatively, there were differences between siblings in the stability of problem but not prosocial behavior. These results might indicate that a larger sibship size leads to more “equality” among siblings, in the sense that OSs' prosocial behavior is not more stable than that of the youngest child.
OS dominance for problem behavior – People tend to learn from the behavior of role models around them. For young children, older sisters and brothers are important individuals to observe and imitate. For the youngest child, their OSs may be the only peers that they observe and imitate, but also interact with, in the early years of life, which means they are very likely to model their behavior. By contrast, the oldest sibling may have been used to a life without siblings, especially if the age gap between siblings is large.
Youngest sibling dominance for prosocial behavior in three-child families – With fewer children in the household, an OS would possibly spend time socializing with peers outside the family, thus limiting any influences from their younger sibling. When there are more siblings in the family, however, OSs may be more likely to find peers within the household and thus may be more likely to be influenced by their youngest siblings. In this situation, the youngest child would have more opportunities to spread their good spirit, show cooperative or sharing behavior, and affect their OSs.
OSs' negative behavior can cascade down to younger children. Interventions targeting older children with problem behavior can therefore have chain effects, protecting younger siblings. In larger families, the youngest child could play a crucial role “spreading” prosocial behavior among their siblings. Extra support and effort in guiding the youngest child to be helpful, cooperative and sharing may have an indirect effect on the OSs' prosocial tendencies too, which could potentially improve the whole family atmosphere.
This one is intriguing in highlighting the positive effects of a younger sibling on prosocial behavior when there are three siblings and in noting that, as children mature, there are more extra-familial influences. Finally, Gilligan, Diggs, Neppl, Stocker & Conger (2024) published “The Influence of Sibling Relationship Quality on Emotional Distress from Adolescence to Early Midlife” in Journal of Family Psychology. Here’s the abstract:
This study examines continuity and change in sibling relationship quality (warmth and hostility) from adolescence to adulthood, as well as how changes in sibling relationship quality across developmental stages are associated with early midlife emotional distress. Data come from the Family Transitions Project, a two-decade longitudinal study of youth and their families followed from adolescence to adulthood. The present study included target adolescent self-report data on warmth and hostility toward and received from their sibling over ten data points from ages 15 to 31. Target to sibling warmth decreased from ages 23 to 31, whereas sibling to target warmth increased in emerging adulthood and then decreased into adulthood. Both sibling to target and target to sibling hostility decreased in adolescence and emerging adulthood and then remained low and stable from emerging adulthood to adulthood. Target to sibling warmth at age 23 predicted lower levels of anxiety at age 41. Sibling to target warmth at age 23 also predicted lower levels of depressive symptoms. Target to sibling hostility at age 23 predicted anxiety and hostility in middle adulthood, whereas sibling to target hostility at age 23 predicted anxiety, depressive symptoms, and hostility. In addition, a slower decline in sibling to target hostility from ages 15 to 19 was associated with higher levels of anxiety at age 41. This study is one of the first to examine the quality of sibling relationships across developmental stages and exemplifies how relationship quality between siblings from adolescence to young adulthood can influence emotional distress into early midlife.
This study looks at longer term effects of siblings and illustrates the protective role of warm sib relationships and the damaging role of hostility. Taken together, the studies illustrate the powerful role of siblings from early childhood through the early 40s.