Nancy L. Segal Ph.D.


Curious Twin or Sibling Pairs and School Separation

Unusual twins and twins' school separation are important to know about!

Posted May 01, 2014

(I). Twin and Twin-like Sibships and (II). Separating Twins at School

I. Twins fascinate us, especially the varieties among both identical and fraternal twins. About 25 percent of identical twin pairs result when a single fertilized egg divides relatively late, somewhere after about the eighth post-conceptional day. (Identical twins occur when the fertilized egg divides between the first two weeks after conceptionsplitting after that time can produce conjoined twins.) Such twins may show various reversed features, such as hand preference, hair whorl and/or fingerprint patterns, to name a few.

There are actually more fraternal twin than identical twin varieties. Some fraternal twins have different fathers, an event that can happen when women have sexual relations with different partners fairly close in time. These twins (called superfecundated twins) are genetically equivalent to half-siblings because they share one parent (mother), but not the other (father). The frequency of such sets is unknown because some pairs are never discovered. Another curious fraternal pair can occur when the twins’ parents are from different ethnic groups. Such twins may appear quite different if each twin takes after a different parent. The life experiences of these twins may also be quite diverse.

Another intriguing kinship is composed of same-age unrelated children reared together since infancy—what I have termed “virtual twins.” These pairs replicate the twin situation, but without the genetic link. I am studying such pairs at this time because they tell us how much shared environments affect behavioral development. Comparing these children with twins adopted together and apart allows a unique disentangling of genetic and environmental influences on human developmental traits. Most of the adopted apart and adopted together twins I am studying are from China. New pairs are welcome to join these studies.

II. Whether or not to separate young twins at school is a vexing question faced by all parents with multiple birth children. Some schools maintain mandatory separation policies for twins, believing that they will fail to develop a sense of individuality if kept together. Unfortunately, this view often rests on the unsystematic observations by administrators of a few twin pairs, rather than on systematically gathered data from many sets. A new study has addressed this question via data from a specially designed survey.

The study, conducted by Dr. Lynn Gordon, Professor of Elementary Education at California State University, Northridge, provides some important guidelines for teachers and educators. She determined that 71 percent of 131 elementary school principals advocate separating twins attending kindergarten for the first time. In contrast, this view was held by 49 percent of teachers, 38 percent of parents, and only 19 percent of young twins. In fact 100 percent of the young female identical twins wanted to remain together.

More revealing is that parents reported that 3 percent of the twins were “traumatized” and 17 percent were “somewhat traumatized” by early school separation. Most parents favored keeping young twins together, believing that school administrators should consider parents’ views in making their decisions. In fact, a number of states have passed, or are in the process of passing, legislation giving parents a meaningful voice in their twin children’s classroom placement. At the present time, 12 states have enacted laws, 10 states have sponsored bills, and two states have resolutions allowing parental input; see

One of the most important points to emerge from Dr. Gordon’s study is that the best policy is no policy! Teachers and administrators need to be flexible regarding twins’ school placement, taking each twins’ special circumstances into consideration. Schools do not follow the same set practices with regard to non-twins—twins deserve the same fair treatment. Dr. Gordons’ study can be read in full in the journal Educational Policy: Gordon, L. M. (2014). Twins and kindergarten separation: Divergent beliefs of principals, teachers, parents, and twins. Educational Policy. Advance online publication. doi: 10.1177/0895904813510778.

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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