Mylène Gorissen
As you are probably all aware of, people have a favour for using either their right or their left hand. This is the result of lateralisation (division) of the brain in two hemispheres, with each its own distinct functions. However, what most of you probably are not aware of is that there is also a lateralisation in the way humans carry their new-born children. Decades of research shows that mothers prefer to carry their baby on the left side of their body using their left arm [1]. It is still unknown which factors determine the development of this asymmetry of the nervous system, but there are several theories explaining the lateralisation of cradling a child [1].
The first to study lateralisation in cradling was Salk in 1960. He found a general left-side bias across almost three hundred tested mothers: 83% of them cradled on the left body side, irrespective of their preferred hand side [1]. He concluded that by carrying a baby on the left body side, the child is able to hear it’s mother’s heartbeat because of the anatomical location of the heart. Nonetheless, the heartbeat hypothesis may be inconclusive, since a case-study performed in a mother with her heart on the right body side (a congenital condition called dextrocardia) showed that she also carried her child on the left side of the body [2].
Two other major hypotheses were generated to explain the leftward asymmetry in cradling. The first being emotional bias, the second being motor bias [1, 3]. The emotional bias hypothesis explains the asymmetry of cradling by the fact that emotional processing takes place mainly in the right hemisphere [1]. Parents prefer to hold the child in their left visual field, which is projected onto the right hemisphere [1]. This effect seems to be associated with the child’s age, because in mothers, the strongest preference to hold the baby on the left is being found directly after birth [2]. This bias decreases after 12 weeks. The idea is that when the baby is born, mothers may have a stronger left-side bias driven by emotions [2]. As the baby ages, mothers start to multitask. This brings us to the second hypothesis, namely the motor bias hypothesis. From an evolutionary perspective, a leftward cradling bias may lead to a natural selection of right-handedness, because this allows mothers to multitask while carrying their baby [2]. Furthermore, a recent meta-analysis on this topic describes that males are less left-lateralized during infant holding compared to women [1]. This could be explained by the emotional bond mothers have with their new-born children. Moreover, the emotional and motor hypotheses may be connected with each other, as the emotional state seems to alter the side of embraces [4]. In a study by Packheiser et al., participants performed more left-sided embraces in emotional situations like reunions and farewells, compared to more neutral situations like greetings [4]. So, when socializing, if you are doubting which way to go, left is emotionally right.
[1] Packheiser, J., et al. Handedness and sex effects on lateral biases in human cradling: Three meta-analyses. Neurosci Biobehav Rev 104, 30-42 (2019).
[2] Todd, B. & Butterworth, G. Her heart is in the right place: an investigation of the ‘heartbeat hypothesis’ as an explanation of the left side cradling preference in a mother with dextrocardia. Early Dev. Parent. 7, 229-233 (1998).
[3] Ocklenburg, S., et al. Hugs and kisses – The role of motor preferences and emotional lateralization for hemispheric asymmetries in human social touch. Neurosci Biobehav Rev 95, 353-360 (2018).
[4] Packheiser, J., et al. Embracing your emotions: affective state impacts lateralisation of human embraces. Psychol Res 83, 26-36 (2019).