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It's 1878.

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Sir Francis Galton
gives a remarkable talk.

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He's speaking to the Anthropologic
Instisitute of Great Britain and Ireland.

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Known for his pioneering work
in human intelligence,

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Dalton is a brilliant polymath.

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He's an explorer,

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and antrhopologist,

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a sociologist,

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a psychologist,

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and a statistician.

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He's also a Eugenist.

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In this talk,

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he presents a new technique

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by which he can combine photographs
and produce composite portraits.

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This technique could be used
to characterize different types of people.

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Dalton thinks that if he combines
photographs of violent criminals,

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he will discover the face of criminality.

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But to his surprise,

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the composite portrait that he produces

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is beautiful.

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Galton's surprising finding
raises deep questions.

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What is beauty?

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Why do certain configurations of line
and color and form excite us so?

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For most of human hisotiry,

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these questions have been approached
using logic and speculation,

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but in the last few decades,

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scientists have addressed
the question of beauty

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using ideas from evolutionary psychology
and tools of neuroscience.

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We're beginning to glimpse
the why and the how of beauty,

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at least in terms of what it means
for the human face and form.

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And in the process,

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we're stumbling upon some surprises.

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When it comes to seeing
beauty in each other,

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while this decision is certainly
subjective for the individual,

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it's sculpted by factors that contribute
to the survival of the group.

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Many experiments have shown

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that a few basic parameters contribute
to what makes a face attractive.

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These include averaging, symmetry
and the effects of hormones.

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And let's take each one of these in turn.

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Galton's finding

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that composite or average faces
are typically more attractive

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than each individual face
that contributes to the average

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has been replicated many times.

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This laboratory finding fits
with many people's intuitions.

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Average faces represent the central
tendencies of a group.

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People with mixed features
represent different populations,

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and presumably harbor
rare genetic diversity

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and adaptibility to the environment.

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Many people find
mixed-race individuals attractive

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and inbred families less so.

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The second factor that contributes
to beauty is symmetry.

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People generally find symmetic faces
more attractive than asymmetic ones.

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Developmental abnormalities
are often associated with asymmetries,

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and in plants, animal and humans,

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asymmetries often arise
from parasitic infections.

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Symmetry it turns out,

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is also an indicator of health.

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In the 1930s,

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a man named MAximallian Fector Rowis

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recognized the importance
of symmetry for beauty

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when he designed the beauty micrometer.

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With this device,

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he could measure minor, asymmetric flaws

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which he could then make up for
with products he sold from his company,

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named brilliantly after himself:

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Max Factor,

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which as you know is one of the world's
most famous brands for makeup.

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The third factor that contributes
to facial attractiveness

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is the effect of hormones.

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And here, I need to apologize
for confining my comments

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to heterosexual norms.

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But estrogen and testosterone
play important roles

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in shaping features
that we find attractive.

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Estrogen produces features
that signal fertility.

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Men typically find woman attractive

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who have elements of both
youth and maturity.

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A face that's too baby-like might
mean that the girl is not yet fertile,

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so men find women attractive

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who have large eyes, full lips
and narrow chins

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as indicators of youth,

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and high cheekbones as an
indicator of maturity.

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Now, testosterone produces features
that we regard as typically masculine.

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These include heavier brows,

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thinner cheeks

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and bigger, squared-off jaws.

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But here's a fascinating irony.

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In many species,

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if anything,

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testosterone suppresses the immune system.

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So the idea that testosterone-infused
features are a fitness indicator

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doesn't really make a whole lot of sense.

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Here, the logic is turned on its head.

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Instead of a fitness indicator,

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scientists invoke a handicap principle.

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The most commonly cited
example of a handicap


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is a peacock's tail.

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This beautiful but cumbersome tail
doesn't exactly help the peacock

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avoid predators and approach peahens.

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Why should such an extravagant
appendage evolve?

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Even Charles Darwin,

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in an 1860 letter to Asa Gray wrote

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that the sight of the peacock's tail
made him physcially illl.

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He couldn't explain it with his
theory of natural selection,

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and out of this frustration,

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he developed a theory of sexual selection.

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On this account,

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the display of the peacock's tail
is about sexual enticement,

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and this enticement means
that it's more likely

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that the peacock will mate
and have offspring.

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Now, the modern twist on this
display argument

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is that the peacock is also
advertising its health to the peahen.

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Only especially fit organisms
can afford to divert resources

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to maintaining such
an extravagant appendage.

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Only espeically fit men can afford
the price that testorone levies

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on their immune system.

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And by anaology,

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think of the fact that only very
rich men can afford

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to pay more than $10,000 for a watch
as a display of their financial fitness.

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Now, many people hear these kinds
of evoulutionary claims

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and think they mean that we somehow
are unconciously seeking mates

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who are healthy,

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and I htink this idea
is probably not right.

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Teenagers and young adults are not
exactly known for making decisions

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that are predicated on health concerns.

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But they don't have to be,

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and let me explain why.

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Imaging a population in which people
have three different types of preferences:

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for green, for orange and for red.

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From their point of view,

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their preferences have nothing
to do with health,

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they just like what they like.

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But if it were also the case

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that these preferences are associated
with the different likelihood

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of producing offspring,

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let's say in a ration of three
to two to one,

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then in the first generation,

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there would be three greens
to two organges to one red,

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and in each subsequent generation,

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the proportion of greens increase

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so that intention rations 98 percent
of this population has a green preference.

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Now, scientists coming in
and sampling this population

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disovers that green preferences
are universal