J Epidemiol Community Health 64:36-40 doi:10.1136/jech.2009.091629
  • Research report

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

  1. T J C Polderman1,2,
  2. M Bartels1,
  3. F C Verhulst2,
  4. A C Huizink2,3,
  5. C E M van Beijsterveldt1,
  6. D I Boomsma1
  1. 1
    Department of Biological Psychology, VU University Amsterdam, Amsterdam, The Netherlands
  2. 2
    Erasmus University Medical Center - Sophia Children’s Hospital, Department of Child and Adolescent Psychiatry, Rotterdam, The Netherlands
  3. 3
    Department of Education, University of Amsterdam, The Netherlands
  1. Correspondence to Dr T J C Polderman, Department of Biological Psychology, VU University Amsterdam, Van der Boechorststraat 1, 1081 BT, Amsterdam, The Netherlands; jc.polderman{at}
  • Accepted 20 July 2009


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.


  • Funding This work was supported by NWO 904-57-94, NWO/SPI 56-464-14192; NWO 480-04-004; NIMH, RO1 MH58799-03. TJCP is funded by Sophia Foundation for Scientific Research (SSWO, no. 562).

  • Competing interests None.

  • Ethics approval The Netherlands Twin Register has received ethical approval from the VU University Medical Ethics Committee.

  • Provenance and Peer review Not commissioned; externally peer reviewed.