Introduction: Handedness is reported to originate early in pregnancy, but mechanisms of development are not well known. Understanding associations of laterality could potentially help to understand brain development better. Earlier studies have indicated that very-low-birthweight (<1500 g) or extremely-low-birthweight (<1000 g) singletons have higher prevalence of left-handedness than children of normal birthweight. Birthweight is naturally smaller in multiples with twins and triplets being about 1 kg and 1.5 kg smaller than singletons, respectively. Studies have indeed indicated higher prevalence of left-handedness in multiples compared to singletons. However, it is not known if birthweight is related to handedness in triplets.
Methods: We studied birthweight and handedness in two large samples of triplets from Japan (N = 1305 individuals) and Netherlands (N = 892 individuals). We used logistic regression to test if birthweight was related to handedness (left versus right excluding ambidextrous participants). Clustered data (triplets within families) was taken into account in all analyses. Motor development of the children was tested for associations with handedness and birthweight.
Results: Left-handers had (Japan M = 1599 g; The Netherlands M = 1794 g) significantly (ps <. 01) smaller birth weight compared to right-handers (Japan M = 1727 g; The Netherlands M = 1903 g).
The association between birth weight and handedness remained significant in both data sets when birth order, sex and infertility treatment were in the same model. Among Japanese triplets the association also remained when adjusting by gestational age. In Dutch triplets both birthweight and gestational age had a significant association with handedness. However, when these two were simultaneously in, the two associations became nonsignificant. In accordance with earlier twin studies zygosity did not have an association with handedness. Left-handedness was related to delayed motor development but this association was not evident when adjusting for birth weight.
Conclusions: We showed that lower birthweight is associated with higher prevalence of left-handedness in triplets. Because of two datasets the result could be replicated. Left-handers had delayed motor development in comparison to right-handers, but the difference vanished when adjusted by birth weight. Our results stress the importance of multiples and birthweight in handedness studies.
Developmental Disorders (e.g. ADHD) , Neuropsychology (e.g. Dyslexia, Handedness, Language)
