Why measure handedness
There are many reasons to study the development of hand preference in infants. For one thing, being left-handed can be an advantage in one-on-one sports such as tennis. For example, Rafael Nadal is a left-handed player and with this natural advantage, he is now a star tennis player with many successful matches to his name. The advantage being, in a population with a left-handed minority and a right-handed majority, the left-handed Rafael Nadal plays most of his matches against right-handed opponents and is therefore well-practiced at dealing with this asymmetry. A right-hander plays the majority of his matches against other right-handers. In conclusion, when confronted with left-handers, they are less practiced, thereby giving the lefty a natural advantage.
Measuring handedness in infancy
Recently, Nelson et al. presented “Unimanual to bimanual: Tracking the development of handedness from 6 to 24 months”. One of their findings was that, as infants, 39% of these children showed preference for the right hand, whereas 61% had no hand preference. In toddlerhood, this 39% increased to 97%. This study provides some evidence that handedness might begin to stabilize earlier than traditionally assumed. However, in order to confirm this conclusion, more research was required.
Nelson et al. were particularly interested in asymmetric bimanual actions. Nelson et al. explain: The current study is the first longitudinal attempt to connect unimanual and bimanual preferences, and emerging handedness patterns, over repeated monthly assessments in a large group of developing children (N = 38). They focused on actions where two hands work together to achieve a goal, a skill known as Role-Differentiated Bimanual Manipulation (RDBM).
The development of a series of tasks that should reliably elicit RDBM actions was an important part of this study. To test the tasks, the researchers set up experiments in their laboratory, Greensboro Infant Development Center. In the experiment, the passive hand stabilized or held the item for the active hand to perform the action. For example, when a child wanted to remove a ball from a tube the passive hand stabilized the tube while the active hand removed the ball from the tube. Test sessions were recorded with two cameras providing adequate views of the child’s actions. Video coding was done offline afterwards with The Observer XT coding and analysis software. After analyzing the data, they concluded that their series indeed elicited RDBM actions in toddlers as well as adults.
As the series was found reliable, Nelson et al. tried to connect hand-use preferences from this new RDBM series to prior unimanual hand use data collected when the same children had been observed as 6-14 month old infants. The results, as mentioned earlier, were the outcome of this longitudinal study. In addition to the conclusion that development of handedness could stabilize earlier in life than previously thought, the researchers also indicate that there is no single pattern found to get to stabilization of handedness. They actually found 5 patterns leading to 97% lateralized handedness at the end of their study.
The researchers close their presentation in Infant Behavior and Development with a statement about future research: future work should explore the implications of multiple trajectories and potential differences in developmental timing, as it pertains to handedness specifically, but also how it shifts our notions of development in general as we move as a field away from a ‘one size fits all’ model toward understanding individual differences and outcomes.
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- Nelson, E.L.; Campbell, J.M.; Michel, G.F. (2013). Unimanual to bimanual: Tracking the development of handedness from 6 to 24 months. Infant Behavior and Development, 36, 181-188.