Handedness in Children Born Extremely Preterm

A new study shows that being born extremely preterm may affect handedness.

Posted Jul 17, 2020

Globally, 10.6 percent of people are left-handers, while the remaining 89.4 percent are right-handers (Papadatou-Pastou et al., 2020). While it is known that handedness is influenced by both genetic and environmental factors, it is still not well understood why some people become left-handed and others become right-handed. 

One longstanding hypothesis is that the circumstances surrounding birth could affect handedness. It has been suggested that being born extremely early or under very stressful conditions could raise the chances of being left-handed or mixed-handed—the so-called “early insult” hypothesis for left-handedness (Satz, 1973). This theory assumes that early brain damage caused by birth complications could lead to a switch in handedness. As there are more people who are initially right-handed, there would be more “pathological” left-handedness than “pathological” right-handedness caused by this switch. Thus, the theory would assume that having birth complications would result in a higher amount of brain damage and a higher chance of being left-handed. 

But is this really the case? While previous studies have often investigated the link between birth complications and handedness, they often neglected to also obtain brain scans in order to assess whether or not there was any brain damage. A new study from The Netherlands (on which I am a co-author) now investigated whether being born extremely preterm affects handedness (van Heerwaarde et al., 2020).

In the study, the scientists evaluated handedness at school age (4-8 years) in 179 children that were born extremely preterm. The mean gestational age (the time between the last menstrual period of the mother and the child's birth) of the children was 26.5 weeks, compared to 38-42 weeks in full-term born babies. Importantly, the authors also obtained brain scans to assess brain damage.

The handedness assessment showed that in the children that were born extremely pre-term 19.6 percent were left-handers, 3.4 percent were mixed-handers and the remaining 77.1 percent were right-handers. This indicates that there were more left-handers and mixed-handers among children that were born extremely preterm than among children that were born full-term (in the general population, 89.4 percent of people are right-handers).

But were left-handedness and mixed-handedness also linked to brain damage? 

The scientists used magnetic resonance imaging to find out. Their brain scans showed that overall about 47 percent of children showed one or more lesions in the brain, which is substantially more than in full-term born babies. However, there was absolutely no statistical link between left-handedness and mixed-handedness and brain damage. The scientists then used another type of brain scan to assess damage in the white matter of the brain. Again, absolutely no relation to handedness was found.

Taken together, the study by van Heerwaarde et al. (2020) clearly shows that left-handedness and mixed-handedness are more common in children that were born extremely preterm than in children that were born full-term. Moreover, they also show that it is highly unlikely that brain damage causes left-handedness as no relation between handedness and brain damage was found. This suggests that other factors linked to being born extremely preterm are likely affecting handedness.

References

Papadatou-Pastou M, Ntolka E, Schmitz J, et al. (2020). Human handedness: A meta-analysis. Psychol Bull, 146(6), 481-524. 

Satz P. (1973). Left-handedness and early brain insult: an explanation. Neuropsychologia, 11, 115-117. 

van Heerwaarde AA, van der Kamp LT, van der Aa NE, de Vries LS, Groenendaal F, et al. (2020) Non-right-handedness in children born extremely preterm: Relation to early neuroimaging and long-term neurodevelopment. PLOS ONE 15(7),  e0235311.