​Mark J. Panaggio

Around 90% of humans are right-handed. However, few other species exhibit strong hand (or paw) preferences, and those that do seem to be about 50-50 right vs. left.  This population-wide preference seems to be universal as every human population in recorded history has been predominantly right-handed.  

 

In many countries, left-handedness is actively discouraged.  In parts of the U.S., as recently as the 20th century, left-handers were often forced to write with their right hand in school.  The thinking is that there are clear benefits to conformity. If everyone was right-handed, we could all write from left to right without smearing the ink. We could all use right-handed tools and equipment with no need for creating left-handed versions.  Why can't we all just use our right hands?

 

The causes of individual handedness remain uncertain. There seems to be a genetic component as left-handed parents are much more likely to have left-handed children (even if you account for environmental factors), but it can't be exclusively genetic because identical twins occasionally prefer different hands. Due to these complex factors that lead to left-handedness, it is next to impossible to get a person to permanently switch even at an early age.  

 

Even though pressuring individuals to switch is mostly ineffective, one might expect that these pressures and the benefits of conformity would cause left-handers to gradually disappear over many generations. However, this is not what seems to be happening. Archaelogical studies  (looking at bone densities, art work, etc.) suggest that this 9:1 ratio has remained relatively constant for millenia.

 

Why is this? Why do only 10% of humans prefer our left hands instead of 50% or even 0%?  Why don't other species show this same bias?  

 

One theory is that left-handers experience a competitive advantage in confrontational settings.  This is particularly evident in sports.  Left-handers seem to be over represented at the highest levels of many different sports.  In combat sports like boxing, MMA and fencing, left-handers have the element of surprise in the sense that they are used to facing right-handers, but right-handers have comparatively little experience against lefties.  Can this competitive advantage explain the resilience of the left-handed minority?

 

Together with Danny Abrams (Northwestern University) I developed a mathematical model demonstrating that the balance between competition and cooperation can lead to different equilibrium handedness ratios.  In particular, we were able to show that a large degree of cooperation, which characterizes human populations but not most animals, is needed to maintain biased (90%-10%) populations.  We also demonstrated that the balance of competition and cooperation can also explain the distribution of hand-preferences among professional athletes.

 

Many questions about the interplay between competition, cooperation and handedness remain. Are these competitive and/or cooperative advantages are enough to change the gene pool?  Will a reduced reliance on physical confrontation in our society over the last few centuries lead to changes in the distribution of handedness? How significant of an advantage do left handers experience? Can this advantage be quantified?  Why do some sports seem to be more biased than others?