Mother’s Day is on May 8th this year. To recognize this holiday, I am writing a series of posts on research about maternal influences on left-handedness in children. A recent comment on a left-hander Facebook page asked if older mothers give birth to more left-handed children than younger mothers. Maternal age at pregnancy is a known risk factor for various birth complications. Mothers older than age 35 have higher risks of pre-mature births, of bearing infants with low birth weights or of requiring emergency Caesarean deliveries, to name a few of the birth risk examples. However, what does this have to do with handedness? How is left-handedness connected to the birth complications that occur at higher rates among older mothers?
The research history on this issue dates back to the 1970’s. At that time a researcher proposed that brain damage caused by a lack of oxygen could occur during a high risk birth. The left hemisphere of the brain that controls the movements of the right hand is more likely to be damaged by oxygen deprivation because it has a more active metabolism and a greater need for oxygen than the right hemisphere that controls the movements of the left hand. Since the left hemisphere is more vulnerable to brain damage, oxygen deprivation during the birth process switches handedness control from the left to the right hemisphere. Right hemisphere control causes left-handedness in the infant. Older mothers are at higher risk of birth complications that cause oxygen deprivation, brain damage and a switch of handedness from the right to the left hand. This theory established a connection between a mother’s age at the time of birth and the increased rate of left-handedness in children. High birth risk older mothers produce more left-handed offspring.
Researchers collected data on high risk births and increased rates of left-handedness in children born with these risk conditions over the next decade. The research effort produced many studies but the findings were diverse and conflicting. Finally, in 1989, colleagues and I produced a meta-analysis of over 40 studies from the scientific literature that explored the relationship between birth risk and left-handedness. A meta-analysis is a special type of research project where data are collated from published studies and examined for patterns that either confirm or dispute a given theory. In this case the theory is that high risk births produce more left-handers. Our meta-analysis did not reveal a connection between a mother’s age at the time of birth and increased rates of left-handedness in children.
Research on the topic of birth risk factors and handedness continues. However, the theory that connects birth complications, oxygen deprivation-induced brain damage and handedness switches to the left side continues to be controversial. There is more research support for the effect of other pregnancy factors, other than a mother’s age, on the side of handedness. These are discussed in the next post.