Left-Handedness and Life Span

Dear Dr. Dave and Dr. Dee,

What are the statistics on the life expectancy of left-handed people?

Signed,

Need to know

Dear Need to know,

Life expectancy of left-handers and right-handers are the same.

There was a controversial study done in 1980 by Halpern & Coren that suggested left-handed people had an average life expectancy of 9 years less than right-handed people. However, data was based on second-hand information: surveys from the next-of-kin of recently deceased persons asking about right/left handedness and age of death. They found left-handed average age of death was 66; right-handed average age was 75.

Unfortunately, the Halpern & Coren study keeps getting quoted even though others have disputed the notion that left-handed people die earlier (Aggleton, et. al., 1994; Harris, 1993).

Second hand survey data used in the Halpern & Coren analysis is not a sound methodology. In addition, if a study reports a higher mortality rate (or any other "problem") in left-handers, then it's very important to look at the research design and how the data was analyzed. Simply checking the sample sizes will give a good indication of whether it was a real significant difference. If the sample sizes are large, and most studies have sample sizes of 1,000 or more, then there will be a "statistically" significant difference between groups (i.e.: Left versus Right) even if the amount of variance explained by the two groups is very small, which means there is no clinical significance. A statistical significant difference does not mean it has clinical or meaningful significance. Many statistical tests will give a significant difference only because of large sample sizes. Therefore, the measure of explained variance needs to be computed because it is unaffected by sample size.

The study by Aggleton, Bland, Kentridge, & Neave in the British Medical Journal had disputed the notion that left-handed people die earlier.

In addition, for a discussion of the Halpern & Coren study, see the link: http://bmj.bmjjournals.com/cgi/content/full/308/6925/408