Reduced incidence of left-handedness in clinically diagnosed dementia of the Alzheimer type.

Although it is generally held that about 10% of the population is left-handed, reported figures vary widely due to differences in handedness classification criteria and subject characteristics. Among those population studies that have used the same handedness classification criteria, a consistent relationship between increasing right-hand preference and increasing age has been reported. One recent study of Alzheimer's disease (AD) reported a higher incidence of left-handedness in early onset relative to late onset cases. In the present study we examined handedness patterns in three elderly groups; normal (N = 217), depression (N = 73), and AD (N = 114). Our results indicated a reduced frequency of left handedness in AD (2.6%) relative to control (11.1%) and depression (13.7%) groups. Within the limited age range we studied (60-80 years), no relationships were found between age and handedness preference either within or across the groups. Furthermore, for the AD group there was no relationship between severity of global impairment and strength of handedness. Our results suggest that compared to right handers, left handers are less vulnerable to the cognitive changes associated with AD. Nevertheless it is also possible that left handers are overrepresented among early onset dementia patients and die before entering the pool of senile dementia patients. Further work is required to determine if early and late onset AD are associated with different incidences of left handedness.