WEBVTT

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If you know an older left-handed person,

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chances are they had to learn to write
or eat with their right hand.

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And in many parts of the world,

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it's still common practice to force
children to use their "proper" hand.

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Even the word for right
also means correct or good,

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not just in English,
but many other languages, too.

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But if being left-handed is so wrong,

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then why does it happen
in the first place?

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Today, about 1/10 of the world's
population are left-handed.

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Archeological evidence shows
that it's been that way

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for as long as 500,000 years,

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with about 10% of human remains

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showing the associated differences
in arm length and bone density,

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and some ancient tools and artifacts
showing evidence of left-hand use.

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And despite what many may think,
handedness is not a choice.

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It can be predicted even before birth
based on the fetus' position in the womb.

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So, if handedness is inborn,
does that mean it's genetic?

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Well, yes and no.

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Identical twins, who have the same genes,
can have different dominant hands.

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In fact, this happens as often as it does
with any other sibling pair.

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But the chances of being
right or left-handed

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are determined by the handedness
of your parents

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in surprisingly consistent ratios.

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If your father was left-handed
but your mother was right-handed,

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you have a 17% chance
of being born left-handed,

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while two righties will have
a left-handed child only 10% of the time.

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Handedness seems to be determined
by a roll of the dice,

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but the odds are set by your genes.

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All of this implies there's a reason

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that evolution has produced
this small proportion of lefties,

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and maintained it
over the course of millennia.

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And while there have been several theories

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attempting to explain why handedness
exists in the first place,

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or why most people are right-handed,

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a recent mathematical model

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suggests that the actual ratio
reflects a balance

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between competitive and cooperative
pressures on human evolution.

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The benefits of being left-handed

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are clearest in activities
involving an opponent,

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like combat or competitive sports.

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For example, about 50% of top hitters
in baseball have been left-handed.

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Why?

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Think of it as a surprise advantage.

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Because lefties are a minority
to begin with,

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both right-handed
and left-handed competitors

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will spend most of their time
encountering

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and practicing against righties.

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So when the two face each other,

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the left-hander will be better prepared
against this right-handed opponent,

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while the righty will be thrown off.

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This fighting hypothesis,

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where an imbalance in the population

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results in an advantage for left-handed
fighters or athletes,

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is an example of negative
frequency-dependent selection.

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But according to the principles
of evolution,

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groups that have a relative advantage

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tend to grow until
that advantage disappears.

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If people were only fighting and competing
throughout human evolution,

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natural selection would lead to more
lefties being the ones that made it

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until there were so many of them,

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that it was no longer a rare asset.

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So in a purely competitive world,

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50% of the population
would be left-handed.

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But human evolution has been shaped
by cooperation, as well as competition.

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And cooperative pressure

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pushes handedness distribution
in the opposite direction.

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In golf, where performance
doesn't depend on the opponent,

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only 4% of top players are left-handed,

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an example of the wider phenomenon
of tool sharing.

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Just as young potential golfers

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can more easily find
a set of right-handed clubs,

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many of the important instruments
that have shaped society

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were designed for
the right-handed majority.

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Because lefties are worse
at using these tools,

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and suffer from higher accident rates,

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they would be less successful
in a purely cooperative world,

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eventually disappearing
from the population.

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So by correctly predicting
the distribution

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of left-handed people
in the general population,

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as well as matching data
from various sports,

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the model indicates

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that the persistence of lefties
as a small but stable minority

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reflects an equilibrium

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that comes from competitive
and cooperative effects

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playing out simultaneously over time.

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And the most intriguing thing

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is what the numbers can tell us
about various populations.

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From the skewed distribution of pawedness
in cooperative animals,

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to the slightly larger
percentage of lefties

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in competitive hunter-gatherer societies,

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we may even find that the answers
to some puzzles of early human evolution

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are already in our hands.