Left-handedness is caused by both genetic and non-genetic factors.

One factor that has been discussed to affect handedness in children is post-traumatic stress in mothers.

A new study investigated the effect of post-traumatic stress in mothers on handedness in toddlers.

Higher post-traumatic stress in mothers during pregnancy was linked to higher mixed-handedness in children.

What are the causes of left-handedness?

What causes someone to be left-handed, mixed-handed, or right-handed? It is well-known that genetics has a substantial influence. Children of two left-handed parents are more likely to be left-handed themselves than children of two right-handed parents.

But genetics is not the whole story. Studies have shown that several factors before and during birth and during early life also influence whether someone becomes left-handed or not. For example, handedness is influenced by breastfeeding, birthweight, season of birth, and being part of a multiple birth (De Kovel et al., 2019). A factor that has also been discussed to affect handedness is maternal stress, but it has so far not been investigated well in psychological studies.

A new study on stress before birth in mothers and handedness in their children

A new study, just published in the scientific journal Developmental Psychobiology, has investigated the role that stress in mothers has for handedness in their children (Hamaoui et al., 2026). In the study, titled “Prenatal Maternal Stress and Weak Handedness in Early Childhood: The Iowa Flood Study,” first author Jad Hamaoui from the University of Montreal in Canada and the rest of the research team analyzed data from the so-called Iowa Flood Study. In 2008, a major flood hit the U.S. Midwest and caused almost 10 billion dollars in damage, making it one of the worst natural disasters in the state of Iowa. Following these events, the Iowa Flood Study collected psychological data from pregnant women who were exposed to this flood over a longer timeframe, as well as data from the children of these women. The idea behind this study was that this research design allowed for assessing the effects of maternal stress on psychological and behavioral variables in their children.

Overall, data from 217 mothers and their toddlers were collected in this unique dataset. To assess the timing of stress exposure during pregnancy, the toddlers were grouped into those with first-, second-, or third-trimester exposure.

Children’s handedness was assessed using a short questionnaire with five items that the mothers filled out. The items were using a spoon, drawing, throwing, hitting things, and stacking blocks. Based on the individual handedness questionnaire answers, the scientist calculated a lateralization quotient (often abbreviated LQ) for each toddler that ranged between -100 (100 percent left-handed) to 0 (mixed-handed or ambidextrous, no clear preference for one hand) to +100 (100 percent right-handed). Based on this score, each toddler was classified as left-handed, right-handed, or not having a clear preference (mixed-handed or ambidextrous).

Moreover, stress in the mothers was assessed objectively (e.g., by asking about physical injuries) and subjectively by assessing post-traumatic stress symptoms using a questionnaire.

Results of the study: Post-traumatic stress in pregnant mothers affects handedness in toddlers

Overall, about 83 percent of the toddlers were right-handed, 12 percent were left-handed, and 5 percent were mixed-handed or ambidextrous. Interestingly, the scientists found that post-traumatic stress in mothers affected handedness in a trimester-dependent way. For toddlers that were in the first trimester of pregnancy when the flood hit Iowa, male, but not female, toddlers showed a higher probability of not having a clear hand preference when mothers experienced higher post-traumatic stress. No handedness effect was found for second-trimester toddlers. For third-trimester toddlers, both male and female toddlers showed a higher probability of not having a clear hand preference when mothers experienced higher post-traumatic stress.

Taken together, the results of the study confirm the idea that post-traumatic stress in mothers during pregnancy affects the handedness of their children. However, the effect depends on the trimester of pregnancy during which the traumatizing event happens and the children’s gender.

de Kovel, C. G. F., Carrión-Castillo, A., & Francks, C. (2019). A large-scale population study of early life factors influencing left-handedness. Scientific reports, 9(1), 584. https://doi.org/10.1038/s41598-018-37423-8

Hamaoui, J., Zhang, H., King, S., & Castellanos-Ryan, N. (2026). Prenatal Maternal Stress and Weak Handedness in Early Childhood: The Iowa Flood Study. Developmental psychobiology, 68(2), e70143. https://doi.org/10.1002/dev.70143

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