In 1980, colleagues and I published a paper reporting high rates of right-handedness among older adults…85% right-handedness among 30-year-olds versus 98% among those age 80.¹ Our data came from a cross-sectional age sample where we measured the handedness of individuals of various ages at the same time.
Several theories try to explain this age difference. The first is generational differences. Older adults, who were originally left-handed, were subject to rightward handedness change attempts not experienced by younger adult left-handers. The older adults in our study were children in the early years of the 20th century when left-handers were often pressured to switch to right-handed writing. Substantial numbers of left-handers, now right-handers, in the older adult group would artificially increase the rate of measured right-handedness. A second theory, the elimination hypothesis, argues that left-handers because of the association between left-handedness and disease do not live as long as right-handers. Few left-handers survive into their older adult years which accounts for the high rate of right-handedness among older adults.
A third approach, the right-sided world hypothesis, maintains that right-hand use is an adaptation to living in a world arranged for right-handers. Both right- and left-handers become more right-handed as they age. A final theory, the differential aging hypothesis, proposes that the left hemisphere, which controls the movements of the right hand, ages more slowly than the right hemisphere. This differential aging effect supports a gradual shift of activities to the right hand because of the greater stability of left hemisphere movement control.
Because of my research history exploring age differences in rates of right- versus left-handedness, I was interested in the findings of a 2019 scholarly publication on this topic.² This recent study also used a cross-sectional age sample where the handedness of 30-year-olds was compared to that of 70-year-olds. The authors found higher rates of strong (consistent) right-handedness among the older as compared to the younger adults…79% versus 60% respectively. There was a 5% incidence of strong left-handedness in both age groups.
This study sheds new light on the various theories of age differences in rates of right- versus left-handedness. First, we can eliminate the elimination hypothesis as an explanation. A large pile of research papers published over the past 25 years report that left-handers live as long as right-handers. To further emphasize this point, both age groups had the same rates of strong left-handedness in this latest 2019 study. The current study results do not offer strong support for the generational differences approach either. Even if there were converted left-handers among the 70-year-olds, there is no obvious explanation for why these older adults displayed more consistent right-handedness than the younger adults.
These results are compatible with predictions from the the right-sided world and the differential aging theories. A right-hander may become more strongly right-handed with age, and a left-hander may start to use his/her right hand more frequently, to adapt to a world of tools and devices arranged for right-hand use. If the left hemisphere ages more slowly than the right, a person could rely more heavily on the left hemisphere/right hand control circuit and become more consistently right-handed across activities over time.
There is further evidence for the differential aging hypothesis from this study. The authors reported that older adults showed higher rates of strong right-footedness (i.e. the foot used to kick a ball) when compared to the younger adult group…49% versus 25% respectively. Movements of the right foot are also controlled by the left hemisphere. If the left hemisphere/right side control unit ages more slowly than that of the right hemisphere/left side, right hand and right foot preferences may increase over time and with age. This last point replicates my work in the 1980’s that also showed slightly higher rates of right-footedness among older as compared to younger adults.
¹Porac, C., Coren, S., & Duncan, P. (1980). Life-span age trends in laterality. Journal of Gerontology, 35, 715-721.
²Marcori, A.J., dos Santos Grosso, N., Porto, A.B., & Okazaki, V.H.A. (2019). Beyond handedness: assessing younger adults and older people lateral preference in six laterality dimensions. Laterality: Asymmetries of Body, Brain, and Cognition, 24, 163-175. https://doi.org/10.1080/1357650X.2018.1495725.