If you know an older left-handed person,
chances are they had to learn to write or eat with their right hand.
And in many parts of the world,
it’s still common practice to forcechildren to use their”proper” hand.
Even the word for rightalso means correct or good,
not just in English,but many other languages, too.
But if being left-handed is so wrong,
then why does it happenin the first place?
Today, about 1/10 of the world’spopulation are left-handed.
Archeological evidence showsthat it’s been that way
for as long as 500,000 years,
with about 10 % of human remains showing the associated differences
in arm length and bone density,
and some ancient tools and artifactsshowing evidence of left-hand use.
And despite what many may think,handedness is not a choice.
It can be predicted even before birth based on
the fetus’ position in the womb.
那么 如果惯用哪只手是天生的 那意味着它是遗传的吗？
So, if handedness is inborn,does that mean it’s genetic?
Well, yes and no.
Identical twins, who have the same genes, can have different dominant hands.
In fact, this happens as often as it does
with any other sibling pair.
But the chances of beingright or left-handed
are determined by the handednessof your parents
in surprisingly consistent ratios.
If your father was left-handedbut your mother was right-handed,
you have a 17% chanceof being born left-handed,
while two righties will have a left-handed child only 10 % of the time.
Handedness seems to be determinedby a roll of the dice,
but the odds are set by your genes.
All of this implies there’s a reason
that evolution has produced this small proportion of lefties,
and maintained itover the course of millennia.
And while there have been several theories
attempting to explain why handednessexists in the first place,
or why most people are right-handed,
a recent mathematical model suggests
that the actual ratio reflects a balance
between competitive and cooperativepressures on human evolution.
The benefits of being left-handed are clearest
in activities involving an opponent,
like combat or competitive sports.
For example, about 50 % of top hitters in baseball have been left-handed.
Think of it as a surprise advantage.
Because lefties are a minorityto begin with,
both right-handedand left-handed competitors
will spend most of their timeencountering
and practicing against righties.
So when the two face each other,
the left-hander will be better prepared against this right-handed opponent,
while the righty will be thrown off.
This fighting hypothesis, where an imbalance in the population results
in an advantage for left-handed fighters or athletes,
is an example of negativefrequency-dependent selection.
But according to the principlesof evolution,
groups that have a relative advantage tend to grow until that advantage disappears.
If people were only fighting and competingthroughout human evolution,
natural selection would lead to more
lefties being the ones that made it
until there were so many of them,
that it was no longer a rare asset.
So in a purely competitive world,
50% of the populationwould be left-handed.
But human evolution has been shaped by cooperation, as well as competition.
And cooperative pressure pushes handedness distribution in the opposite direction.
In golf, where performancedoesn’t depend on the opponent,
only 4% of top players are left-handed,
an example of the wider phenomenonof tool sharing.
Just as young potential golfers can
more easily find a set of right-handed clubs,
many of the important instrumentsthat have shaped society
were designed forthe right-handed majority.
Because lefties are worseat using these tools,
and suffer from higher accident rates,
they would be less successfulin a purely cooperative world,
eventually disappearingfrom the population.
So by correctly predicting the distribution
of left-handed people in the general population,
as well as matching datafrom various sports,
the model indicates that the persistence
of lefties as a small but stable minority
reflects an equilibrium that comes from competitiveand cooperative effects
playing out simultaneously over time.
And the most intriguing thing is what the
numbers can tell us about various populations.
From the skewed distribution of pawednessin cooperative animals,
to the slightly largerpercentage of lefties
in competitive hunter-gatherer societies,
we may even find
that the answers to some puzzles of early human evolution
are already in our hands.