A new study reveals that typically developing twins consistently take on the dominant role over their nontypically developing siblings, even after developmental gaps are resolved. By highlighting how parental perceptions shape these dynamics, the research offers fresh insights into the delicate balance of twin relationships and the importance of fostering equality to support both children’s growth.

[Hebrew University of Jerusalem]– A study conducted by Prof. Ariel Knafo-Noam and Dr. Yonat Rum from the Hebrew University of Jerusalem, alongside Dr. Hila Segal and Adi Barkan from The Academic College of Tel-Aviv–Jaffa, sheds light on how developmental differences influence the dynamics of dominance in twin relationships throughout childhood. Twins, unlike singletons, share the same age and often a similar environment, creating unique relational dynamics where traditional factors like birth order play no role. This study, published in Family Relations, explored how differences in developmental conditions between twins, particularly when one child has a developmental condition and the other does not, affect their dominance dynamics over time.

The research surveyed over 1,500 parents of monozygotic and dizygotic twins aged 3 to 9. It found that when one twin was typically developing and the other had a developmental condition, parents consistently perceived the typically developing twin as the dominant sibling. This dominance imbalance was stable throughout childhood, even in cases where the developmental challenges of the nontypically developing twin were resolved. In twin pairs where both siblings shared the same developmental condition, whether typical or nontypical, no significant differences in dominance were reported. These findings suggest that it is not the developmental condition itself, but the asymmetry in abilities between the twins, that drives dominance imbalances.

Parental perceptions played a significant role in shaping these dynamics. The study relied on previous work showing that parents often emphasize differences between twins through what is termed the “contrast effect,” a bias that exaggerates perceived differences. This parental tendency can reinforce dominance roles, affecting how twins interact with one another and shaping their long-term relational dynamics. The findings align with family systems theory, which posits that families function as interconnected units where the behavior and roles of one member influence the others. In this context, parental perceptions and behaviors may inadvertently solidify certain roles, such as “dominant” or “submissive,” in twins’ relationships.

Interestingly, genetic similarity also influenced dominance patterns. Monozygotic twins, who share nearly identical genetic material, were more likely to display symmetrical dominance relationships compared to dizygotic twins, even in cases where developmental conditions differed. This suggests that genetic factors and personality traits may play a role in shaping dominance behaviors and creating more balanced relationships in identical twin pairs.
The study also revealed that dominance roles established early in life tend to persist, even when the developmental condition of a nontypically developing twin improves. This persistence can be understood through role theory, which explains how family roles, once established, become ingrained through both behavior and parental reinforcement. To mitigate the effects of entrenched dominance roles, parents are encouraged to create opportunities for more balanced dynamics. Encouraging twins to engage in relationships outside their twinship, fostering individuality, and providing avenues for the less dominant twin to assert themselves in specific domains can help reduce the long-term impact of these dynamics.

This research offers a nuanced understanding of how developmental differences shape twin relationships and underscores the importance of parental awareness in navigating these dynamics. By addressing these patterns early, parents and caregivers can foster healthier, more balanced sibling relationships that benefit both twins’ social and emotional development.