About 66–72% of human mothers cradle their infants on their left side. Given that left-cradling exposes the baby’s face to the mother’s left visual field (i.e., mainly projected to her right hemisphere) and is altered by emotional states such as stress, maternal left-cradling was interpreted as reflecting right-hemispheric dominance for emotional processing. Whether this phenomenon is unique to human evolution is still in debate. In the present study we followed 44 olive baboon (Papio anubis) mothers and their infants in different social groups. We found that a maternal cradling bias exists and is predominantly towards the left in a similar proportion as in humans, but shifts toward a right bias in mothers living in high density groups. The sensitivity of left-cradling to social pressure highlights its potential links with the mother’s stress as reported in humans. Our finding clearly illustrates the phylogenetic continuity between humans and Old-World monkeys concerning this lateralization and its potential links with hemispheric specialization for emotions, inherited from a common ancestor 25–35 million years ago.

大约66-72％的人类母亲在左侧抱着婴儿。鉴于左摇篮使婴儿的脸暴露于母亲的左视野（即主要投射到她的右半球），并且由于诸如紧张等情绪状态而发生了变化，因此母亲的左摇篮被解释为反映了右半球在情感加工中的主导地位。这种现象是否是人类进化所独有的，仍存在争议。在本研究中，我们追踪了44个橄榄狒狒（Papio anubis）不同社会群体中的母亲及其婴儿。我们发现，存在一个母亲的偏见，并且其向左的偏向与人类相似，但是在生活在高密度群体中的母亲中，偏向右偏。左摇篮对社会压力的敏感性凸显了它与人类所报道的母亲压力的潜在联系。我们的发现清楚地说明了人类和旧世界猴之间在这种侧向化方面的系统发育连续性，以及它与半球情感特化的潜在联系，这是从25-3500万年前的共同祖先继承而来的。