The left hand of deathness

Since the start of the year a long time obsession with islands and archipelagos was given a bit of an airing -I, I, II, III, IV.  But I also seem to have been picking on left-handers a bit I, II, III. Not picking on really, but poking at the idea that left-handedness is something of potential interest.  Left-handers seem to rise to the top of several key professions: 6 of the last 12 President's of the US [3xL] have been southpaws including the present incumbent. There is pretty good data supporting the hypothesis that lefties do better at one vs one sports like tennis, boxing, and baseball: maybe 3x over-represented in competition. Some pundits have tried to rationalise this excess by invoking better spatial awareness because the wiring of the southpaw brain and its left-right cross-over is arranged advantageously.  But I think it's more likely that it is an example of frequency dependent selection.  Many (most? all?) natural human populations seem to support a little over 10% lefties.  If you're in the minority group you almost always encounter right-handers and learn to cope with their stance and heft from an early age.  Right-handers also have most of their matches against fellow right-handers and so can be disconcerted when their opponent is a mirror of normal.  This advantage plays in favour of left-handers but only if they are in the minority - as they become more common their advantage evaporates.

In skirting round the Edge of Leftness, I came across several uncritical references to the fact that left-handers have a dramatically shorter lifespan and there were ready explanations for this 'observation': that southpaws didn't fare well in a right-handed world and did daft, dangerous and non-dextrous things with scissors, knives and cars which finished them off early.  The data for this deficit stems from a study of death records in California carried out by Diane Halpern and Stanley Coren and published in the prestigious New England Journal of Medicine NEJM in 1991. In California everything is the in public domain and Halpern and Cohen obtained a long list of dead people, their age at death and their addresses.  They then contacted the grieving rellies to ask if the deceased was left- or right-handed.  They found the average age at death was 56 years for southpaws but a full nine years longer at 65 among right-handers.  This is a staggering discrepancy: the only way to reliably achieve such a shortening of lifespan is to smoke 120 (!) cigarettes a day - one every 8 minutes and they say it takes 5 minutes to smoke one.

It was so unexpected and so contrary of people's own experience, that lots of competent medical practitioners and statisticians rounded on the data and found a notable bias that had escaped the attention of Halpern and Cohen in their direct and obvious tally.  The issue being that old people had been born a long time ago . . . when customs were different.  In Victorian and Edwardian times, left-handedness was seen as a defect, a sin or worse and children were forcibly converted as effectively as the Conquistadors made good Catholics out of Aztecs, Incas and Mayans or with rather less success some Christians seek to make homosexuals love the opposite sex.  The Beloved's mother was forced by the Good Sisters to write and sew with a her right hand in the 1940s. So there was a notable deficiency of left-handers among octogenarians, not because they were dead but because they wrote with their right hand and were categorised dexter for the study by their surviving relatives.  As the 20th century meandered on towards a more tolerant and inclusive society, beating children into writing with their wrong hand diminished as an acceptable practice and the number of left-handers bounced up to the normal/natural 10-12%. Graph by Chris McManus via BBC. People dying in their fifties were thus more likely to be (seen as) left-handed than those who were older.

In the demolition of Halpern and Cohen's some nice analogies were bought out. The average age at which Harry Potter readers die is much less than readers of Charles Dickens but you don't (or shouldn't) conclude that Voldemort kills HP fans with The Dark Arts - no it's just that Pottering is really only a task fit for children.  Likewise you could, but shouldn't, conclude that creches are much more dangerous than parachute training school because the average age of death in the former is much less than in the latter.  I suggest that if it is completely clear that Halpern and Cohen were a) wrong and b) it was obvious to you, then you are deluding yourself with 20/20 hindsight.  More importantly, I exhort you (and more importantly me) to be more aware of the subtle bias and other unconsidered certainties that we just can't see.