Introduction and Purpose
It is widely assumed that twins share a very special and unique relationship, a relationship that is so close that words are often unnecessary when communicating with one another¹. While it has been hypothesized that this type of close relationship between twins may actually restrict twins’ development of close relationships with others, a direct examination of the twin relationship has not yet been done. Therefore, the purpose of the current study was to examine the different aspects of intimacy between twins (i.e. openness, honesty, trust, loyalty, sharing, etc.) to see if levels of closeness vary according to twin type (identical or fraternal), sex or age.  We were also interested in studying twins' relationships with their friends, especially their closest friend of the same sex and their closest friend of the other sex.

Each twin was sent a questionnaire package that included questions regarding their relationship with their twin, their closest same-sex friend, and their closest other-sex friend.  In addition, twins completed a questionnaire to determine their twin type. Overall, 214 individual twins (158 females, 56 males) between the ages of 16 to 73 participated in this study.  There were 128 monozygotic (identical) twins, 60 dizygotic (fraternal) twins of the same sex (i.e. boy-boy, or girl-girl twins), and 24 dizygotic mixed-sex twins (i.e. boy-girl twins). 

Key Findings
The Importance of Intimacy in Twins’ Relationships     The importance of twins’ relationships with others was measured on a scale that included eight different components of intimacy. These components measured the extent of frankness/spontaneity, sensitivity/knowing, attachment, giving/sharing, imposing/taking, exclusiveness, common activities and trust/loyalty that twins felt in their relationships with their twin, their closest same-sex friend, and their closest other-sex friend. Overall, the twins in the current study reported that frankness/spontaneity was the most important component of their close relationships. Thus, self disclosure and openness about one’s self (including pleasant and unpleasant information, thoughts and feelings) were reported to be the most important aspects of close relationships. When relationships were examined individually, trust/loyalty (the ability to trust another with one’s thoughts and feelings and to count on one another for loyalty) was perceived as the most important component of twins’ relationships with their closest same-sex friend.  Twins reported that the most important aspect of their relationship with their closest other-sex friend was a strong sense of attachment: Liking them and feeling close to them, missing them and wondering what they are doing when they are not around. Finally, imposing/taking (the ability to be open with one another and a readiness to be vulnerable when being helped) was perceived to be the most important component of twins’ relationships with each other.

 On the other hand, twins indicated that participating in activities with others (common activities) was one of the least important aspects of their close relationships. Indeed, researchers have found that shared activities are more important in the early phases of friendship, and that as a closer relationship develops, more emphasis is placed on affection rather than on frequent contact².  Therefore, it appears as though twins place a high value on a sense of closeness and sharing in their close relationships, and that these relationships are maintained even when frequent contact with one another may not be possible (i.e. when separated by distance).

Intimacy in the Twin Relationship     As expected, the twins in this study reported closer, more intimate relationships with their twin than with their closest same-sex friend, or their closest other-sex friend. This heightened level of closeness was reported by all twins regardless of their age. Indeed, the majority of twins (64%) indicated that their closest friend overall was their twin. Although we expected that monozygotic twins would report a closer, more intimate twin relationship, this was not found; monozygotic (MZ) and dizygotic (DZ) twins reported similar levels of intimacy in their twin relationships. Nevertheless, members of a MZ twin pair were more likely to nominate each other as their closest friend when compared to DZ twins, and those twins who chose each other as their closest friend reported higher levels of intimacy with each other (these twins were more likely to be MZs). Finally, while males and females reported similar levels of intimacy in their relationships with their co-twin and closest other-sex friend, females reported significantly higher levels of intimacy with their closest same-sex friend.

Twinship and Peer Relationships     Contrary to expectation, twins who reported higher levels of intimacy with their twin, did not report lower levels of intimacy with their closest friends. Thus, while researchers have concluded that some of the isolating behaviors observed in twins during childhood are linked to the twin relationship (i.e. twins are less likely to get involved in social activities with others, or they prefer to play alone), this study does not support the conclusion that having an intimate twin relationship interferes with the formation of close relationships with others during late adolescence or throughout adulthood. These findings are in line with the belief that the existence of one satisfactory and highly intimate relationship does not necessarily occur at the expense of other close relationships³.  Furthermore, while it was anticipated that twins who reported an exclusive twin relationship (i.e. prefer to be alone together) would report less intimate relationships with their closest same-sex friend and closest other-sex friend, the opposite trend was found.  Twins who prefer to be together without anyone else around reported closer relationships with their close friends.  Perhaps then, substantial amounts of one-on-one time with another individual over extended periods of time allows twins to gain a better understanding of how to relate to close others.  Moreover, it was found that high levels of empathy, understanding, helping and shared activities with one’s twin benefitted twins’ relationships with their closest same-sex friend. Therefore it appears as though some of the skills learned in the twin relationship may be transferred to twins’ relationships with close others.

Implications and Conclusions
 By using a direct approach in the examination of twins’ relationships with their co-twins and close others, it has been found that the twin relationship may actually benefit twins rather than harm them. While previous researchers have focused on the disadvantages of being a twin and the hazards that the intimate twin bond may bear, few have ventured to examine the positive implications of such a close and enduring relationship. Indeed, twins may have a lot to teach us about seemingly “exclusive” relationships and the development of future social functioning. While twins encounter their fair share of the trials and tribulations involved with close relationships, the end result appears to be positive.  All considered, the importance of intimate relationships with others should not be undermined by the fear that they will restrict us; intimacy is an important component in all relationships.

References
1.  Adelman, M.B., & Siemon, M. (1986).  Communicating the relational shift:  Separation among adult twins.  American Journal of Psychotherapy, XL(1), 96-109.

2.  Adams, R.G., & Blieszner, R. (1994).  An integrative conceptual framework for friendship research.  Journal of Social and Personal Relationships, 11, 163-184.

3.  Collins, W.A., & Repinski, D.J. (1994).  Relationships during adolescence: Continuity and change in interpersonal perspective.  In R. Montemayor, G.R. Adams, & T.P. Gullotta (Eds.), Personal Relationships During Adolescence.  In Advances in Adolescent Development, Annual Book Series No. 6. London: Sage.