I never noticed until recently that several left-handed people are either a twin or triplet. Scarlett Johansson has a twin brother Hunter Johansson. Mary-Kate is the left-handed Olsen twin. Doubles tennis star Bob Bryan plays and writes left-handed. Drew Scott of the Property Brothers is a twin and a southpaw. Left-hander Daniel Sedin played his entire National Hockey League career on the Vancouver Canucks with his twin brother Henrik.
A recent study, Handedness in Twins: Meta-Analyses (Pfeifer, L.S., Schmitz, J. et al), integrated data from 59 previous research projects and showed that yes, there is a higher percentage of left handers amongst twins compared to singletons. However, a key finding is that a newborn’s birth weight could be a factor in whether that person becomes left-handed. One of the study’s lead authors, Lena Pfeifer from Ruhr University Bochum in Germany, says, “Initial evidence implies it might be certain pre or perinatal environmental factors that might have a huge impact. We also discussed that in our paper there were studies that demonstrated these factors are birth weight, or more specifically lower birth weight, as well as being part of the multiple birth.”
This resonated with me. I am left-handed and happen to be a triplet. My birth weight was 3.12 pounds. Multiples have a higher chance of a pre-mature birth and having a lower birth weight. My mom delivered the three of us just shy of 34 weeks into her pregnancy.
Ms. Pfeifer points to a Finnish study, Higher prevalence of left-handedness in twins? Not after controlling birth time confounders (Heikkilä, K. et al, 2015), where researchers found, “The difference in prevalence of left handedness between multiples and singletons disappears when controlling for birth weight. These same researchers also had a study on triplets, Triplets, birthweight, and handedness (Heikkilä, K. et al, 2018), and found left handers among the study sample had significantly lower birth weight than right handers. So, whenever you control for the factor of birth weight or the factor of gestational age, then you do not find these differences anymore. Lower birth weight seems to be a factor contributing to left-handedness whether it is a twin, triplet, or a singleton.”
The lead author of these two studies is Kauko Heikkilä with the University of Helsinki. Mr. Heikkilä tells Southpaw Essentials that the “foremost evidence in my studies points to very small birth weight being one possible correlate” to left handedness. In his 2018 study, Mr. Heikkilä states, “The prevalence of left-handedness increases with decreasing birth weights.” He says left handedness continues to be a mystery, but “to see the association with birth weight, we either need very big datasets, babies with very low birth weight (which are very few), or multiples (who are fewer in comparison to singletons and have lower birth weights).”
So, while the evidence is not 100% definitive, it would be very interesting to create a study looking at birth weight, premature birth and laterality to determine whether these all correlate. Ms. Pfeifer says,
“The gold standard in research is to design a true experiment in which you manipulate an independent variable and then you see how it affects a dependent variable so that you have a direct causal influence. This is what we actually would need to conclude how far birth weight contributes to handedness.”