Nancy L. Segal Ph.D.


Twin Typing by Self-Report

Twins may not be who they think they are

Posted Dec 11, 2010

  • A number of identical twins fail to see the physical similarities between themselves and their co-twin, leading these twins to believe they are fraternal. I have observed this phenomenon on many occasions, as have my colleagues. This situation was brought to my attention once again by recent correspondence from a twenty-year-old female twin, Susan G., who believed that because she and her twin sister, Sarah, did not look exactly alike in every way they were most likely fraternal twins.

Susan’s twin sister Sarah disagreed, perhaps the only point of contention between these close sisters. Sarah believed that she and Susan were identical twins because of their strong physical and behavioral similarities. She spoke only of “small differences” between them, such as one twin being slightly tougher and the other twin being slightly stricter in personality.

Interestingly, when the twins were born their parents had been told that they were fraternal twins because of their separate placentas, amnions (inner fetal membrances) and chorions (outer fetal membranes). However, approximately one-third of identical twins have this placental arrangement so the family was misinformed.

I examined a photograph of the twins and was struck by their extraordinary physical resemblance. I was certain that they were identical, recalling Race and Sanger’s (1975) statement that, “For many years, Mr. James Shields of the Genetics Unit at the Maudsley Hospital has been sending us samples of blood from the twins. We find that the blood groups practically never contradict the opinion of such a skilled observer of twins.”  

Susan and Sarah were interested in undergoing DNA testing to learn the truth about their zygosity and I encouraged them to do so. Like many twins they lacked scientific proof of their twin type and wanted to respond with certainty to questions from family and friends.

Several weeks later I received the following message from Susan: “We are both speechless to discover that we are in fact identical--honestly, it's like a NEW world. Now it's clear to us that the only reason we can combine pictures is because we are identical and not fraternal.” (Combining pictures meant cutting a photograph of each twin’s face at the midline and joining one twin’s left half with the other twin’s right half.)  By “new world,” Susan explained that she now realized that she and her sister had originated from a single zygote—and had it not divided one of them would not have existed. She seemed awed by the immense chance associated with that event. The twins’ similarities, social closeness and confusion by others became fully understandable to her in light of the DNA report. In contrast, Sarah who always believed they were identical was not surprised by the findings.


The scientific literature on twins lacks a satisfactory explanation for why some identical twins are either confident that they are fraternal or are uncertain as to whether or not they are identical. It has been generally assumed by the scientific community that identical twins focus on the slight differences between themselves, leading them to believe they are not identical, but this explanation may be one of several. The popular literature does hold some clues that may explain the self-misclassification of some identical twin pairs. For example, celebrity twins and actresses Mary-Kate and Ashley Olsen who look very much alike have been described as fraternal twins because they claim to be.  Twins who look alike may refer to this well-known pair as an example of how similar twins can look and still be fraternal; however, I am certain that the Olsen twins are identical based on their physical resemblance. Of course, there are similar looking fraternal twins just as there are similar looking non-twin siblings, but such sets are relatively rare. A photograph of a very similar looking fraternal female twin pair is included in my 2000 book Entwined Lives. When I studied these twins my first impression was that they were identical, but blood-testing proved otherwise. Nevertheless, such sets are not common and deserve comment.

I explored the situation of how and why IDENTICAL co-twins misperceive their resemblance with Susan and Sarah. It turned out that the assumption that some IDENTICAL twins focus on their differences, even magnifying them, was true in their case. Susan detected small variations in the shape of the nose and mouth which to her were evidence of dizygosity. Both twins draw, but their approach to producing art is different—Sarah observed that Susan draws from imagination, while she sketches objects that are in front of her. Susan has explained what she considered her lack of artistic talent by referring to her “different genes,” but admits she can no longer use that excuse.

Most researchers rely on DNA analysis to document the zygosity of the twin pairs they study. However, some investigators outside twin research who encounter interesting twin cases often rely on the twins’ or families’ self-report for assigning zygosity.  Some attorneys managing twins’ wrongful death, injury and custody cases do the same unless advised by expert witnesses that DNA testing is the most reliable and accurate methodology for classifying twins. Interpretations of case study findings and life history information can change significantly depending on the twin type of the twin pair in question. Thus, DNA testing which is now a simple and relatively inexpensive procedure should be considered mandatory in case study reports. 

It is striking that twins’ and parents’ misclassifications of twin type tend to be in the direction of labeling true identical twins as fraternal, rather than the reverse.  This may have two underlying causes.  First, people may assume (incorrectly) that identical twins must be identical in every way and if not they must be fraternal.  Such reasoning would reflect misinformation on the part of the public, as well as provide support for abandoning the term identical in favor of monozygotic. It is well known among scientists that monozygotic twins can differ across virtually every behavioral and physical trait, including intelligence, personality, height and weight. Perhaps these observations need to be communicated more clearly outside the research community.

A second explanation as to why identical twins are more often misjudged as fraternal than the reverse is that individual differences in appearance and behavior are highly celebrated in most societies. Thus, the concept of genetic identically identical individuals may be disturbing to some twins and their families who suspect that identical twins will struggle over issues of identity and selfhood.  Several identical twins who participated in the Minnesota Study of Twins Reared Apart had such concerns prior to meeting their co-twins, but their worries proved unfounded upon reunion when they realized that they and their co-twins were not exactly alike in every measured trait.

It is important that parents of same-sex twins be informed of their children’s twin type soon after their birth.  This information can help parents and ultimately teachers, family members, friends and twins themselves to understand their similarities, differences preferences and predispositions. DNA testing should be a routine part of twin deliveries and needs to be covered by health insurance plans. 

I am grateful to Susan and Sarah for sharing their story with me and allowing me to share it with readers of TRHG.



Race, R.R., and Sanger, R. (1975). Blood groups in man. Oxford: Blackwell Scientific Publications.

About the Author

Nancy L. Segal, Ph.D., is a professor of psychology and the Director of the Twin Studies Center, at California State University, Fullerton.

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