Mammals keep our offspring on our left side so we can more easily identify their emotions
By the time my son was two-years-old, my left tricep was in great shape. Neither of my kids are petite, and they both loved to be carried around, giving my arms and shoulders plenty of exercise. Being right-handed, I tried to keep the kids perched on my left arm most of the time, leaving my “good” hand to interact with the rest of the world. There may have been a more tender side to this preference though, as researchers are building the case that the left-arm bias may have actually been about accommodating the right side of my brain, not my hand.
Looking closely from the left
According to this study, the key reason to hold a baby in your left arm is so their face will be more clearly visible to your left eye. That eye is connected to the right hemisphere of your brain, which is more directly associated with processing spatial and emotional information. You probably don’t notice any that gap in your day to day activity, but most of the stimuli you’re looking at isn’t being clutched right next to your face (and pulling on your glasses). So by picking up small changes in your baby’s emotional state more easily, a parent can either head off trouble or just provide emotional feedback a bit faster.
Now even if parents have been proven to more accurately assess their babies’ moods when relying on their left eye, that doesn’t prove that emotional communication is the root cause behind this pattern. Maybe our right hemispheres became attuned to facial expressions because so many of us were already right-handed, and needed to access the ancient equivalent of a cell-phone all day? To test this, researchers looked at other mammalian parents to see if they had a similar preference in how they viewed their kids. Non-primates in particular were of interest, as their forelimbs should make less of a difference in how they interact with their offspring.
Gauging the maternal gaze of bats and walruses
The two other mammal species didn’t have much in common, except for the fact that they were known to spend some time looking at the faces of their offspring. Walruses and flying fox fruit bats were both observed interacting with their young, and as expected, they were more likely to orient themselves to keep their kin in their left visual field. In the case of the walruses, researchers noted that interactions from the animals’ left side weren’t only more common, but they lasted longer than engagement from the right.
In all of these cases, this pattern turned up in the babies as well. Offspring tended to approach their parents from the left, and when facing a parent could watch with their left eyes, just like their mothers.
Looking and leaning when kissing
Looking back to humans, a separate study may have inadvertently found a similar connection between adult humans. Adult couples were asked to kiss their spouse, and then record their descriptions of how the kiss occurred. Among other trends, researchers found that most people prefer to avoid mirroring their partner’s head position, and will tilt their head in the opposite direction, usually to each person’s right.
This was attributed to the handedness of each participant, and who initiated the kiss (usually men in heterosexual couples.) However, it seems like the tension between handedness and emotional observation is relevant here, because tilting your head to the right means that you’re giving your left eye the best view of your partner’s face. It seems fair to say that reading the emotional state of the person you’re about to kiss would be important, giving people all the more reason to lean right as they smooch. Unfortunately, this study didn’t specifically mention how often folks were kissing with their eyes open or closed.
Source: Mammals prefer to cradle babies on the left, study demonstrates by Nicola Davis, The Guardian