Pick up your kid—baby, toddler, preschooler, kindergartener. Go on, we'll wait.
Now, look what side you're holding her on. Left side, right? How'd we know?
What the hell? (We know, right?) It's evolution, Mom—you're just being a mammal.
That's what researchers think, and a study published recently in the journal Nature Ecology & Evolution breaks it all down. For humans, the reason that we tend to hoist kids over to the left has nothing to do with keeping the dominant right hand free. Lefties know that, since they also tend to favor holding their babies on that side, too. Rather, it's because of the brain's structure.
Though you don't even know you're doing it, when you keep or hold your child on your left side, your left eye is closest to your offspring's left eye and this turns out to be key.
A New Scientist article explains that the right hemisphere of mammals' brains is where social cues and relationship building processes happen. It's also where the brain receives signals from the left eye.
When we carry our babies on the left side, mom and baby can better monitor each other's facial expressions and cues with their left eyes, University of Tasmania – Australia researcher Janeane Ingram and a group of others found. Same sorta goes for non-human mammals, they write in their study. Ingram and her group observed 175 mother-offspring pairs and 11,000 position choices. They found that when offspring approached their mothers, suckled or were moving forward in a pack or a herd, nearly 75 percent of the time they did it so that their mother was on their left side.
Ambling human kids? They also tended to also want their moms over to the left.
That switched when the babies were under threat (observed only in the non-human mammals!). In those cases, the mothers would position themselves so that their offspring where on their left side.
What Ingram and her colleagues hypothesize is that baby mammals made sure mom was on the left to better keep track of her, which increased their chances of survival. For human babies in our pampered existence, the threats are far fewer but the natural tendency is still there.
There might be another benefit: Ingram's group found that when baby whales' and horses' mothers were on their left, the little ones were more likely to rub up against the mamas' bodies (awwww) and also less likely to be accidentally left behind (whew!). Emotional bonding that pays off with survival.