For over 60 years, laterality researchers have been intrigued by the observation that people tend to hold and cradle babies on the left side of the body (as shown in this illustration). The behavior is observed in adults as well as children and in non-human species such as chimpanzees. I discussed the left cradling bias in two previous blog posts, ¹ but a recent study, using meta-analytic techniques, has shed new light on this side bias in cradling. ² Meta-analysis examines the published literature on a topic and offers valuable insights into consistent patterns that emerge when many studies are used as data.
The authors argued that 60 years of research have produced inconclusive results because of the small sample sizes and inconsistent experimental designs plaguing the published literature on this topic. For consistency, they restricted their analysis to English, German, and French language studies using adult, human participants with no health concerns. This procedure resulted in a database of 40 studies published between 1960 and 2012. The authors sought to answer three questions concerning the left cradling bias. First, does it exist at a population level? Second, are there reliable handedness differences? Are left-handers less likely to exhibit this bias because cradling on the left side would restrict the movements of their preferred hand? Third, are there reliable sex differences in cradling side?
Their data confirmed the left cradling bias exists at a population level. Over all the participants, 69% showed a preference for cradling on the left. However, there were both handedness and sex differences in the strength of this preference. Among the 592 identified left-handers 59.6% preferred to cradle on the left side. The 6207 right-handers showed a stronger bias with 71.7% preferring the left side. The data from 3940 males indicated that 56.9% favored the left side while among the 8028 females the percent left preference was 66.7%. The strongest left-side bias was found among right-handers and female participants. Unfortunately, there was too little data to investigate whether the sex difference varied for left- and right-handers.
Both of these findings presented interpretation challenges for the authors. Right-handers showed a stronger left cradling bias than left-handers. This makes sense because holding a baby on the left side keeps the preferred right hand free for coordinated movements. However, the fact that a majority of left-handers also cradle on the left weakens the argument that babies are always held in a way that frees the preferred hand for other activities. The authors concluded that handedness side does affect cradling behavior, but it is not the sole cause since the left-side bias was found in both left- and right-handers.
The authors also had difficulty explaining the sex difference in left cradling bias strength. They speculated the sex difference favoring a stronger left bias in females could be based on differences in experience. Women have more experience cradling babies. Women may also form stronger emotional bonds with babies when compared to men. The second explanation is based on the speculative idea that the right hemisphere processes information from the left visual field and it is also specialized for processing emotion. Babies are held on the left to facilitate emotional bonding based on right hemisphere processing. However, this latter explanation does not specifically address why women are more likely to cradle on the left when compared to men.
Although the explanations for the study’s results are speculative, the meta-analysis of 40 separate published papers produced three significant findings. The left cradling bias exists at a population level and at higher rates for women and right-handers. This outcome gives consistency to an otherwise contradictory literature on this topic, and it also offers guidelines for future research. One possible line of inquiry, for example, would ask whether the sex difference in the left cradling bias exists for both left- and right-handers.
¹ Holding babies: Why is the left side favored? May 17, 2016; The left side is on hold. February 3, 2021.
² Packheiser, J., Schmitz, J., Berretz, G., Papadatou-Pastou, M., & Ocklenburg, S. (2019). Handedness and sex effects on lateral biases in human cradling: Three meta-analyses. Neuroscience and Biobehavioral Review, 104, 36-42. https://doi.org/10.1016/j.neubiorev.2019.06.035