Low birthweight linked to left-handedness

An international study coordinated by the University of Helsinki provides new information on the origins of handedness. The study stresses the importance of prenatal development, and especially very low birthweight, in the formation of handedness.

About every one person in ten in the world is left-handed. The mechanisms of handedness development, however, remain poorly known.

Previous studies have suggested left-handedness to be more common in twins and triplets than in singletons. Furthermore, preterm singletons have been reported to have increased odds of being left-handed in comparison to term-born singletons. However, these studies have not shown whether the cause was preterm birth or low birthweight.

A new study conducted by the University of Helsinki investigated the relation of both gestational age and birthweight to handedness. The work, based on over 2000 Japanese and Dutch triplets, was carried out in collaboration with Osaka University and Vrije Universiteit of Amsterdam.

The results were published on the 14th of May in the scientific journal Proceedings of the National Academy of Sciences of the United States of America.

“The birthweight of triplets is approximately 1.5 kg lower than that of singletons. Therefore triplet datasets offer a unique setting to investigate the effect of very low birthweight by controlling gestational age,” says Kauko Heikkilä, a PhD student at the University of Helsinki.

The study found that in both the Japanese and Dutch triplets, the birthweight of the left-handed infants was clearly lower than that of the right-handed ones. The effect of birthweight was very similar in both countries despite the lower prevalence of left-handedness in Japan than in the Netherlands. Moreover, a clear difference was seen in the birthweights between these countries, with lower average birth weights in Japan.

“The observation of the link between left-handedness and low birthweight in two very different cultures is an especially convincing result and enhances our former understanding of the importance of prenatal development in handedness formation, says Eero Vuoksimaa, docent of neuropsychology at the University of Helsinki, who led the study.

The study additionally found that the motor development of the left-handed infants was slightly delayed in comparison to the right-handed ones. The connection however seemed to be explained mainly by the lower birthweight. The researchers thus note that the effect of birthweight should be taken into account in all studies where differences in left- and right-handed persons are compared.

Additional information:

Eero Vuoksimaa, PhD

eero.vuoksimaa@helsinki.fi, tel. 02941 27586

Institute for Molecular Medicine Finland (FIMM), HiLIFE, University of Helsinki

Kauko Heikkilä PhD Student, Phil Lic.,

kauko.heikkila@helsinki.fi, tel. 0504151273

Institute for Molecular Medicine Finland (FIMM), HiLIFE, University of Helsinki

Original publication:

Kauko Heikkilä, Catharina EM Van Beijsterveldt, Jari Haukka, Matti Iivanainen, Aulikki Saari-Kemppainen, Karri Silventoinen, Dorret I. Boomsma, Yoshie Yokoyama, Eero Vuoksimaa. Triplets, birthweight, and handedness. Proceedings of the National Academy of Sciences of the United States of America.