Objectives-The fall in the prevalence of left handedness with age has been attributed to either premature mortality or a cohort eVect of forced dextrality. Evidence for forced dextrality was sought to diVerentiate 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 eVects of forced dextrality on the writing hand. (J Epidemiol Community Health 1998;52:41-44) There are three main theories purporting to explain the fall in the prevalence of left handedness with age in cross sectional studies. These are:-(1) Forced used of the right hand in childhood because of pressures in the home and school have made many naturally left handed persons right handed in previous generations, but with liberalisation of attitudes this eVect is less pronounced so younger subjects are more likely to be left handed.(2) We live in a dextrally biased world in that most design (scissors, machinery, etc) is for right handed persons. Throughout their life a left handed person is subject to continual pressures to do things with the right hand and over the years this results in a shift in handedness to the right resulting in a fall in left handedness in older age groups.(3) Left handedness is a marker for decreased fitness to survive and left handers die prematurely resulting in fewer left handers present in older age groups.This last explanation of premature mortality has been expounded by Halpern and Coren.

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