As surprising as it might sound, it doesn't matter if you are right- or left-handed and if you're carrying baby in your arms or in a sling - most parents still choose the left hip!
Quite early in my babywearing journey I noticed that I usually wear my baby on the left side. It definitely had something to do with the fact that I wanted to have my right hand free to do what I needed to while holding the baby (like preparing lunch)...but even if I was not planning to do anything special, I still chose the left hip over the right one. It was intriguing to me, because as a right handed person I know that my right arm is stronger so theoretically I could carry my daughter on the right side for longer when not using the sling. But even if I pushed myself to keep her on the right side, after a minute or two she landed on the left hip again... :)
It was not until recently that I found out there is actually a lot of scientific evidence to back up this phenomenon (called left-side bias) and that is apparently quite common for mammals! According to the study in Nature Ecology & Evolution (see more below), humans are not the only species that "prefer" the left side. The research shows that mammal offsprings usually approach their mothers from the left side (either when they are scared or just walk next to their mother) and the reason for that might be because of the way the brain develops.
Mamal brain is divided into symmetrical left and right sides, called hemispheres. Each hemisphere is in charge of the opposite side of the body, so your right brain controls your left hand and the left brain controls the right hand. Each side of the brain also takes in sensory input from the opposite side of the body.
The left brain, often called the logical side, is in charge of the analytical thinking and handles for example reading, writing and calculations. The right brain is in charge of social processing and deals in images more than words. It controls our imagination and emotional intelligence as well as feelings of love and bonding.
So when we hold the baby on the left side, the sensory input send signals directly to the right side of the brain which can recognize our little ones' facial expressions, feelings and needs better and quicker that the other side. Also, when we hold the baby on the left side, they are closer to our heartbeat, which may help regulate temperature and keep them calm. And nothing is more comforting to the baby that listening to the heartbeat they are so familiar with from the time they were in their mother's womb!
The science behind the left side bias is logical and fascinating at the same time, yet of course there are parents that prefer to carry baby on the right side or don't have a side preference. As much as I appreciate how our brain and body work wonderfully together, I have a piece of advice - if you like to carry your baby on the hip, make sure to change the side each time - your back and hips will thank you... :)
The research mentioned in the post:
Karenina K, Giljov A, Ingram J, Rowntree VJ, Malashichev Y. Lateralization of mother–infant interactions in a diverse range of mammal species. Nature Ecology & Evolution. 2017;1(2):0030.