Maybe the right-hand rule isn’t arbitrary and is due to the cognitive properties of the left side of the brain.
Many Mammal Mamas Carry Kids on the LeftRegardless of whether they are left- or right-handed, human moms tend to cradle their babies on the left side of their bodies, especially in the early months. This left-handed bias likely has to do with the human brain’s lopsided layout: sensory information on the left side of the body is processed on the right side of the brain. The brain’s right hemisphere is also where emotions are processed, so holding and observing the baby on the left may help transmit social information to the right side more efficiently. Babies seem to prefer to keep their mother in the left visual field, too. Fascinatingly, researchers recently documented left-side bias in non-primate mammal mothers. Observed off the coast of a Russian island, walrus moms tend to keep their babies on the left while bobbing along the waves, and their calves swam over to their mother’s left side before diving to suckle. Ditto for flying fox moms dangling from tree branches in Sri Lanka who seemed to favor keeping their babies on the left.