Is forced dextrality an explanation for the fall in the prevalence of sinistrality with age? A study in northern England.

OBJECTIVES The fall in the prevalence of left handedness with age has been attributed to either premature mortality or a cohort effect of forced dextrality. Evidence for forced dextrality was sought to differentiate between these competing theories. DESIGN 6097 Edinburgh handedness inventories were used to calculate laterality quotients (LQ) with and without the questions relating to writing and drawing. These questions might be expected to be most influenced by forced dextrality. SETTING The study was performed in a small industrial town in Lancashire, UK. PARTICIPANTS Using the British family practitioner service where over 95% of the population are registered with a general practitioner a response rate of 82.17% was obtained with respect to the Edinburgh Inventory. RESULTS Questions about writing and drawing on the Edinburgh Inventory contributed to the positivity (right handedness) of the mean LQ, but equally across the ages. When a negative LQ was used to define left handedness the prevalence of left handedness fell from 11.2% at age 15 to 4.4% at age 70. Removal of the questions about writing and drawing caused the prevalence of left handedness to fall from 10.5% at age 15 to 4.95% at age 70. CONCLUSIONS Less than 20% of the fall in the prevalence of left handedness was accounted for by questions relating to writing and drawing. The fall in the prevalence of sinistrals in older age groups is not adequately explained by cohort effects of forced dextrality on the writing hand.