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Mother’s age and left-handedness
Mother’s Day is on May 8th this year. To recognize this holiday, I have written 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 will be discussed in the next post, Fetuses and newborns: What behaviors predict handedness? This was originally posted on the author’s blog page. You can access it here.
ABOUT THE AUTHOR:
Clare Porac received her Bachelor of Arts degree in sociology from Duquesne University and her MA and PhD degrees in psychology from the New School for Social Research, and is presently Professor of Psychology at Penn State Erie. She has authored or coauthored 63 research articles and has presented 66 conference papers on her human laterality research; she has an additional 55 publications and 50 conference papers on other topics. You can read her full biography here and access an interview and podcast with her called “Penn State Behrend Professor Debunks Left-Handed Myths” here.
ABOUT THE AUTHOR’S BOOK:
Professor Porac’s new book, Laterality: Exploring the Enigma of Left-Handedness, is a comprehensive overview of scientific research on laterality that not only tells us what is true, but also debunks commonly held misperceptions. Each chapter is based on a question or questions covering diverse topics such as genetic and biological origins of handedness, familial and hormonal influences on handedness, and the effects of a majority right-handed world on the behaviors of left-handers.
If you would like to purchase a copy of Laterality: Exploring the Enigma of Left-Handedness at up to 30% off the list price and free global shipping, visit the Elsevier Store. Apply discount code STC315 at checkout.
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