The image of a mother holding, or “cradling,” an infant in her arms is the centerpiece of holiday season in Christian belief.
Why do so many mothers around the world - whether left-handed or right-handed -cradle infants on the left side of the body? Their left arm wraps around the infant, holding it near the left breast, while the right hand stays largely free.
Behavioral scientists have watched cradling habits in parents and children in shopping malls and other public places. About 80 percent of mothers cradle a newborn on the left.
About 80 percent of fathers also cradle on the left. Nobody knows whether that's male instinct, or just Dad following Mom's baby-holding instructions.
Little girls cradle their dolls in either arm until age 6. Then they shift into the left-sided mode.
The same left-sided preference exists among humanity's closest relatives in the animal kingdom - chimpanzees, gorillas, and orangutans. In the wild as well as in zoos, about 80 percent of the members of the Great Ape family cradle on the left.
Left-sided cradling is not a practice that developed in modern times, so moms could more easily answer the telephone or keyboard at the computer while holding a baby.
Ancient statues and paintings in museums and books show that mothers have cradled on the left for thousands of years. One survey of more than 400 Madonna-with-child paintings in European art galleries showed left-armed infant carrying in more than 80 percent of the cases.
A passage in the Talmud, the collection of Jewish religious and civil laws that originated in the 1200s B.C., advised women to cradle an infant on the left side when beginning breast feeding. “The source of all understanding is from the left side,” the Talmud explained.
What's so important about left-sided cradling that it has become the preferred, standard, default way of holding infants for humans and their closest relatives?
Researchers have been debating that question for almost 170 years.
Rejected is the obvious explanation - that moms cradle on the left to keep their dominant hand free to battle enemies or tend the child. If that were true, left-handed moms would cradle on the right to keep their dominant left hand free.
The “heartbeat hypothesis” also has fallen from favor. American psychologist Lee Salk originated the idea. He reasoned that newborns need the comforting sound of the mother's heart that they heard constantly in the security of the uterus before birth. Salk thought the heart sound helped to soothe and reassure the infant, and perhaps had a role in stimulating normal brain develop-ment.
Cradling on the left would ensure maximum exposure to the “lub-dub, lub-dub, lub-dub” of the heart, which extends into the left side of the chest.
Psychologists later recognized that mom's voice has a much more important role in both comforting the infant and stimulating growith of connections between nerve cells in the brain.
Many now embrace the “mom's voice” hypothesis about left-sided cradling.
By cradling an infant on the left, the mother can speak directly into the infant's left ear. Sounds from the left ear are processed by the right side of the brain. In infants, the right brain is more developed and better able to process sounds from the mother.
Experts believe that infants might not hear the mother's voice in the same way if it entered the right ear. And the infant might not get the same benefits from these mother-to-child information downloads that are so critical for wiring together nerve cells in the brain.
Michael Woods is the Blade's science editor. Email him at firstname.lastname@example.org.