How many times had I handed my toddler her spoon when I realized that I only gave it to her left hand? I’d heard stories about people forcing left-handers to be right, but could I coerce a right-hander to be a lefty. Looking down at the high chair, I noticed the spoon, the cup and even the bite sized strawberry pieces all poised on the left side of the tray -right where I placed them -with my right hand. Pushing everything to the other side, I watched her reach with her already red left hand -the right remaining motionless at her side.
Teasing out the nature verses nurture part of handedness is an enduring goal for scientists. The idea that handedness has something to do with brain function draws intrigue. Many studies (some more reputable than others) have correlated handedness with mental illness, sexual orientation and most recently, life span. But a non-biased indicator of true handedness is needed to make any association meaningful.
Hair whorl--that is the direction hair swirls on the head--has held long-time hopes for the field. While the clockwise nature of the swirl seems to match the rate of right-handedness (and counterclocwise, lefty-ness), a distinct association is missing. The order that teeth erupt on the other hand, does parallel handedness… maybe.
A Turkish group, reported a relationship between teething and handedness “there was a statistical difference between sequence of the tooth eruption and handedness”. The reports maintains that the down side to using dentition as a marker, as I also quickly came to realize, is that parents are not so good at keeping track of this specific milestone. The mouth schematic in our baby book has lines radiating from each tooth on one side only. The time of eruption and the type of tooth that broke first (the only data that I recorded), however, have no significance (with respect to side dominance that is). I peered over early baby pictures for a snap of the first toothy smile but to no avail. Likewise, the experiment consisted of few data points (92 individuals). More importantly, to me at least, was the puzzling way the data was analyzed and presented. After doing a few calculations myself, it seemed that the predictability of handedness with respect to tooth eruption was not much better than 50%. Now, I’m no statistician but I can usually read a table.
Orbak et al., 2007
Jansen et al., 2006
1 comment:
Interesting article! I, also, wonder what nature / nurture goes into being right or left handed. It's a micro for a lot of other questions on what is inherent in a person and what part upbringing plays. For instance, what makes one person question everything and another question nothing when it comes to the culture's belief system. I'm agnostic, others in my family don't question their religion - just one example of the differences.
Nice looking blog, Lisa!
Carol
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