The "Twin Bond" Study: Why Identical Twins Are Choosing Each Other Over Their Spouses

Romantic relationships are often expected to become a person’s primary emotional bond during times of stress, major decisions, and separation, however, a 2015 study determined that this pattern does not apply to everyone. The findings suggested that many identical twins continue to place their co-twin above their romantic partner in their attachment hierarchy. The research, however, did not conclude that these twins cared any less about their husbands or wives. Instead, the findings suggest that growing up alongside an identical sibling can create a uniquely strong sense of familiarity and security that remains important well into adulthood. Dylan Sprouse recently shared that he and his brother Cole talk daily, live close by, and frequently play video games together, describing it as their "love language." JEFFERSON SPYPLANE/Pexels (Not the actual image) A total of 515 participants took part in the study, conducted jointly by Bergische Universität Wuppertal in Germany and the Université du Luxembourg. The group included 114 monozygotic (MZ) twins (identical twins), 63 dizygotic (DZ) twins (fraternal twins), and 338 non-twin (NT) siblings. To make the comparison more consistent, researchers focused their analysis on participants with same-gender siblings. This left 331 participants, ranging in age from 19 to 72. Marlon Alves/Pexels (Not the actual image) Participants completed an online questionnaire about four people in their lives: their sibling, mother, father, and romantic partner. The questions about parents were included partly to prevent participants from realizing that the study was specifically comparing sibling and romantic relationships. The researchers then used three different measures to assess attachment and closeness. The first focused on four key attachment behaviors: whether participants wanted to be near someone, missed them when separated, turned to them for comfort during stressful situations, and felt confident enough to explore the world knowing that person was there for them. The second measure, called the Inclusion of Other in the Self (IOS) scale, asked participants to rate how emotionally close they felt to each person. Finally, participants completed an attachment-hierarchy task, ranking their sibling, mother, father, and romantic partner based on how emotionally connected they felt to each one. This was important because it required participants to rank these relationships rather than simply rate several people as equally close. Pascal Le Segretain/Getty Images The study’s most striking finding was that identical twins ranked their co-twin as their most important attachment figure, even when they had a romantic partner. The authors interpreted this pattern partly through the lens of genetic relatedness. Identical twins share virtually all of their genetic material, while fraternal twins and ordinary siblings share roughly half on average. Paul Archuleta/Getty Images For a real-world comparison, Dylan and Cole Sprouse and Rami and Sami Malek are examples of identical twins, while Mary-Kate and Ashley Olsen are fraternal, or non-identical, twins. However, genetics alone did not explain the finding. Identical twins also share an unusually intensive developmental history. They enter the world at the same time, often reach milestones together, and typically grow up in the same household and social environment. Lionel Hahn/Getty Images That shared history can make a co-twin a particularly familiar source of validation and emotional understanding. In simple terms, many identical twins may feel that their co-twin understands them without much explanation. A romantic partner may know an adult twin deeply, but that relationship cannot replicate the experience of having someone who has shared much of the same life from the very beginning. Matilda Jacob/Pexels (Not the actual image) The pattern was different among non-identical twins. They were found to be similarly attached to their twin and romantic partner rather than consistently preferring one over the other. Like identical twins, non-identical twins share the distinctive experience of being born and raised together. However, they are, on average, no more genetically similar than ordinary siblings. The study’s findings therefore suggest that shared upbringing alone may not produce the especially strong co-twin preference observed among identical twins. olia danilevich/Pexels (Not the actual image) Non-twin siblings showed the most conventional adult attachment pattern, with participants more attached to their romantic partner than to their sibling. This fits a common developmental expectation: as adults form long-term romantic partnerships, their partner often becomes the person they rely on most for emotional comfort, security, and support. dylansprouse/X The study did not prove that greater attachment to a twin harms a person’s relationship with their romantic partner. That distinction is important. The researchers compared the relative ranking of different attachment figures; they did not establish that identical twins have unhappy marriages or lower-quality romantic relationships. An attachment hierarchy reflects relative priority, not the absence of emotional closeness. Someone can be deeply attached to their spouse while still considering their co-twin the first person they would turn to in certain situations. For example, a twin might rely on their spouse for day-to-day intimacy and companionship but turn to their co-twin during a family crisis or when dealing with an experience rooted in their shared childhood. Still, the findings raise an interesting question about relationship expectations and boundaries. If a romantic partner expects to be the primary attachment figure but repeatedly feels secondary to a twin, that difference in expectations could potentially create tension.