Babies with very low birth weights more likely to be left-handed: study

The lower a baby’s weight at birth, the more likely that baby will be left-handed, according to a new study.

The research, published in the journal Proceedings of the National Academy of Sciences of the United States of America, builds on previous research showing a link between very low birth weights and left-handedness. The researchers studied over 2,000 babies — all triplets — from the Netherlands and Japan, and found strong evidence of a connection.

Approximately, 10 per cent of people are left-handed, at least in countries where left-handedness is socially acceptable, according to the paper. Among the babies that were part of this study, 9.6 per cent of the Japanese triplets and 13.9 per cent of the Dutch triplets were left-handed.

But the left-handed babies weighed less than the right-handed ones. For the Japanese infants, those who were left-handed weighed 1,599 grams on average at birth, while right-handed ones weighed an average of 1,727 grams. Similar results were seen among the Dutch infants, who were a little bit larger.

Babies actually develop their hand preference in the womb, said study co-author Kauko Heikkilä, at the University of Finland, in an email. The earliest signs of this come at 10 weeks of gestation, and again at 15 weeks, when some babies can be observed sucking their thumbs on ultrasound images.

The researchers decided to focus on triplets for this study, Heikkilä said, because their previous research had found that left-handedness was more common among twins than single births.

Other studies have found a link between birth weight and left-handedness in single births, but the likelihood only increases when the baby weighs less than 1,500 grams, he said.

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Triplets are even more likely than twins to have low birth weights, so studying them helped the researchers confirm the link between low weight and hand preference.

The study also found that left-handed babies reached certain motor development milestones later than the right-handed babies; however, this may just be due to their lower birth weights rather than their hand preference.

Heikkilä said that we still don’t really know much about what causes left- and right-handedness. “At best we may only hope that triplets, and our finding, will in future help understand the origins of handedness better.”