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Fetuses and newborns: What behaviors predict handedness?

By: , Posted on: May 4, 2016


Baby sucking right thumb

My second post in honor of Mother’s Day this month deals with other pregnancy factors that may predict later handedness of the child.  One of these is fetal thumb sucking.  Recently, an expectant mother posted an ultrasound image of her fetus on a Facebook page devoted to news of and discussion about left-handedness.  The fetus appeared to be sucking his/her thumb. The mother asked if this behavior could be a predictor of the eventual handedness of the child.  Researchers have studied the side of fetal thumb sucking behavior as a potential predictor of handedness side.  The results indicate that fetuses who suck the right thumb become right-handers while those that suck the left thumb become either right- or left-handed.  Fetal sucking of the right thumb is a better predictor of right-handedness than is fetal sucking of the left thumb a predictor of left-handedness.

There is more room for the head of the fetus if it is positioned to the left side of the uterus or to the left of the mother’s body midline.  By the final weeks of pregnancy, the fetus is in a head-down position with the right ear facing out. The majority of births occur with a left-sided presentation and this orientation is a predictor of eventual right-handedness in children.  This connection makes sense since most people are right-handed and most births occur with a left-sided presentation.  The more infrequent right birth presentation is only weakly associated with the eventual development of left-handedness.

Newborns also display a head-turning bias when lying on their backs.  Most newborns (65%) lie on their backs with their heads turned toward the right side.  The remaining number either turn their head to the left (15%) or show both right- and left-sided turning behaviors (20%).  A 2013 study reported that infants prefer to turn their heads toward the side of the thumb sucked as a fetus. Head-turning bias also predicts handedness for reaching during the early months of life.  Right head-turning infants reach for objects with the right hand while left head-turning infants reach with the left hand.

Side biases occur before birth as shown by research on fetal thumb sucking. Fetal thumb sucking occurs as part of a normal pregnancy and it appears to be a more reliable predictor of right- rather than left-handedness.  However, the development of a stable hand preference takes time. Infants show inconsistent hand preferences when reaching for or grasping objects.  Consistent use of either the right or left hand for these movements does not start to appear until around the first year of life.  For this reason, mothers (and fathers) have an opportunity to influence the handedness of the child by encouraging the infant to imitate their own handedness or by placing objects selectively into the infant’s right or left hand.  The fact that infants of left-handed mothers show more left-handed behaviors than infants of right-handed mothers supports the developmental influence on handedness of these parental behaviors. This was originally posted on the author’s blog page. You can access it here.



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.



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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