Paddy Steele, of Khandallah, asks :-

Why are most people 'right-handed' and only a few 'left-handed' - quite a disability in the use of some implements or instruments? Is it only human beings who exhibit this?

Thorsten Stanley, a paediatrician at the University of Otago Medical School, Wellington, responded.

Handedness, the preference for using one hand over the other, is partially genetic and partially environmental. Identical twins, who share exactly the same genes, don’t always share the same handedness.

Representing only 10 percent of the general human population, left-handers have been viewed with suspicion and persecuted across history. The word “sinister” even derives from “left or left-hand.”

Scientists have long wondered why left-handed people are a rarity. Stories about being slapped on the wrist for being a lefty aside, there must be some deeper, evolutionary reason. A new study, by mathematicians at Northwestern University using data from the sports world, suggests lefties are rare because of the balance between cooperation and competition in human evolution.

They concluded that the most important factor for an efficient society is a high degree of cooperation. The more social the animal—where cooperation is highly valued, for example in sharing the same tools— the more the general population will trend toward one side. In humans, this has resulted in a right-handed majority.

If societies were entirely cooperative everyone would be same-handed. But if competition were more important, one could expect the population to be 50-50. Their new model can predict accurately the percentage of left-handers in a group—humans, parrots, baseball players, golfers—based on the degrees of cooperation and competition in the social interaction.

The 90-10 right-handed to left-handed ratio in humans has remained the same for more than 5,000 years. It also explains the dominance of left-handed athletes in many sports where competition can drive the number of lefties up to a disproportionate level. Physical competition favors the unusual. For example, in a fight, a left-hander would have the advantage in a right-handed world.

Their model accurately predicted the number of elite left-handed athletes in baseball, boxing, hockey, fencing, and table tennis. More than 50 percent among top baseball players and well above 10 percent (the general population rate) for the other sports.

On the other hand, the number of successful left-handed PGA golfers is very low, only 4 percent. Their model also accurately predicted this.