No effect of classroom sharing on educational achievement in twins: a prospective, longitudinal cohort study

Background: A returning dilemma for families with multiple births is whether twins should share the same, or a parallel classroom, or in other words, whether they should be separated at school or not. This study investigated the effects of sharing a classroom during primary school on cognitive achievement in twins.
Method: Subjects were 839 monozygotic and 1164 dizygotic twin pairs who were registered at birth at The Netherlands Twin Register. A prospective, longitudinal study design was used with educational achievement at age 12 years, measured with a standardised test (CITO test), as outcome measure.
Results: Most twin pairs (72%) shared a classroom during their schooling, 19% were in separate, but parallel, classes, and 9% "partly" shared a classroom. Twins who were in parallel classrooms had higher CITO scores (mean 539.51; SD 8.12), compared to twins who shared a classroom (537.99; SD 8.52). When controlling for socioeconomic status, and externalising problems before starting primary school (age 3), there was no significant difference in educational achievement between separated and non-separated twin pairs (p = 0.138). In addition, there was no interaction with sex or zygosity of the twins (p = 0.798).
Conclusion: There is no difference in educational achievement between twins who share a classroom and twins who do not share a classroom during their primary school time. The choice of separation should be made by teachers, parents and their twin children, based on individual characteristics of a twin pair.