TODAY I WANT to write about two mysterious human attributes – why we swing our arms when we walk and why we are superstitious. One of the mysteries, the arm-swinging, has recently been explained, but the other remains a somewhat open question, writes
WILLIAM REVILLE
Why humans naturally swing their arms in opposition to their legs when they walk has long been a mystery. The arms do not touch the ground (if you are having any luck at all with your relative anatomical proportions) and, so, the swinging didn’t seem to serve any mechanical function. One explanation was that arm- swinging remains hard-wired into the human nervous system since the time our ancestors walked on four legs.
The mystery was recently solved by Steven Collins, a biomechanical engineer, and others and published online in July in the journal Proceedings of the Royal Society B(rspb.royalsocietypublishing.org). The researchers studied 10 people asked to move their arms in four different ways as they walked – swinging normally, arms held motionless at their sides, arms bound to their sides, and swinging anti-normal (ie in synchrony with each leg) – and they measured the work done by the shoulder muscles as well as the overall effort of walking.
Neither normal nor abnormal arm-swinging require much effort from the shoulder muscles; rather the swinging tends to be a natural consequence of the twisting of the body due to walking. Normal arm-swinging made walking easier. Voluntarily holding the arms steady increased the effort of walking by 12 per cent (equivalent to carrying a 10kg backpack), but forcing the arms to swing anti-normal increased the effort of walking by 26 per cent.
Collins explains that walking naturally tends to make your body spin around a vertical axis. Normal arm-swinging cancels out this effect and consequently leg muscles don’t have to work to counter the spinning. On the other hand anti-normal swinging doubles the spinning tendency, forcing leg muscles to work hard to cancel the spinning movement.
Collins concludes that his results “puts to rest the theory that arm-swinging is a vestigial relic from our quadrupedal ancestors. Instead arm-swinging is a sensible part of an economic gait on two legs”.
And now to superstition. Many readers will have noted that Tiger Woods wore a red T-shirt when competing with Padraig Harrington in the final round of the Bridgestone Invitational Golf Tournament on August 9th, and also when competing in the final of the US PGA Championship against YE Yang on August 16th. Tiger always wears a red shirt when competing on a Sunday, believing it to be a lucky charm. Most people indulge their own little superstitions even though they acknowledge they make no rational sense. I greatly dislike dating any document as the 13th of the month. How did superstitious habits evolve? Emma Young offers her thoughts on this subject in August's New Scientist(No 2,720).
Our brains have evolved to detect order in the environment and to expect that every outcome has a cause. Some cause and effect relationships are uncertain, but must be responded to and this leaves us prone to superstition. For example, a rustle in the grass could either be the wind or a deadly poisonous snake. Because it is usually the wind didn’t mean that our ancestors could afford to be blasé about such sounds. It made more sense to assume that all rustling sounds signalled mortal danger and evolution through natural selection therefore favoured the inclusion of this superstitious belief into the hard-wiring of the brain.
Some psychologists believe religion became established because of the evolutionary benefit of superstition. For example, RI Dunbar proposes ( Oxford Handbook of Evolutionary Psychology, Oxford University Press, 2007) that the main function of religion is to promote social cohesion by persuading people in a community to fall in with the rules of the official authority, and that this is partly achieved by harnessing our natural inclination to believe in Gods that can influence our lives.
Environment and culture influence individual levels of superstition. People who live in danger are far more likely to carry a lucky charm than people who live in settled areas and church attendance markedly increases in hard economic times. Also, it is probably no random coincidence that the “moving statues” phenomenon in Ireland in 1986 occurred at a time when the gravest disquiet was being openly expressed about the health of the national economy.
William Reville is associate professor of biochemistry and public awareness of science officer at University College Cork