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We like to think of romantic feelings

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as spontaneous and indescribable
things that come from the heart.

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But it's actually your brain

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running a complex series of calculations
within a matter of seconds

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that's responsible
for determining attraction.

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Doesn't sound quite as poetic, does it?

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But just because the calculations
are happening in your brain

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doesn't mean those warm,
fuzzy feelings are all in your head.

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In fact, all five
of your senses play a role,

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each able to vote for,
or veto, a budding attraction.

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The eyes are the first
components in attraction.

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Many visual beauty standards vary
between cultures and eras,

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and signs of youth,
fertility and good health,

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such as long lustrous hair,

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or smooth, scar-free skin,

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are almost always in demand

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because they're associated
with reproductive fitness.

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And when the eyes spot
something they like,

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our instinct is to move closer

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so the other senses can investigate.

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The nose's contribution to romance

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is more than noticing perfume or cologne.

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It's able to pick up
on natural chemical signals

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known as pheromones.

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These not only convey

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important physical or genetic information
about their source

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but are able to activate a physiological
or behavioral response in the recipient.

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In one study, a group of women

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at different points
in their ovulation cycles

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wore the same T-shirts for three nights.

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After male volunteers
were randomly assigned

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to smell either one of the worn shirts,
or a new unworn one,

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saliva samples showed
an increase in testosterone

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in those who had smelled a shirt worn
by an ovulating woman.

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Such a testosterone boost

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may give a man the nudge to pursue a woman

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he might not have otherwise noticed.

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A woman's nose is particularly attuned

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to MHC molecules,
which are used to fight disease.

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In this case, opposites attract.

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When a study asked women to smell T-shirts
that had been worn by different men,

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they preferred the odors of those
whose MHC molecules differed from theirs.

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This makes sense.

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Genes that result in a greater
variety of immunities

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may give offspring
a major survival advantage.

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Our ears also determine attraction.

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Men prefer females
with high-pitched, breathy voices,

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and wide formant spacing,
correlated with smaller body size.

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While women prefer low-pitched voices
with a narrow formant spacing

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that suggest a larger body size.

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And not surprisingly,

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touch turns out to be crucial for romance.

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In this experiment,
not realizing the study had begun,

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participants were asked
to briefly hold the coffee,

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either hot or iced.

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Later, the participants read a story
about a hypothetical person,

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and were asked to rate their personality.

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Those who had held the hot cup of coffee

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perceived the person
in the story as happier,

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more social, more generous
and better-natured

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than those who had held
the cup of iced coffee,

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who rated the person as cold,
stoic, and unaffectionate.

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If a potential mate has managed
to pass all these tests,

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there's still one more:

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the infamous first kiss,

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a rich and complex exchange
of tactile and chemical cues,

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such as the smell of one's breath,

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and the taste of their mouth.

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This magical moment is so critical
that a majority of men and women

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have reported losing
their attraction to someone

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after a bad first kiss.

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Once attraction is confirmed,
your bloodstream is flooded

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with norepinephrine,

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activating your fight or flight system.

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Your heart beats faster,

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your pupils dilate,

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and your body releases glucose
for additional energy,

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not because you're in danger
but because your body is telling you

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that something important is happening.

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To help you focus,

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norepinephrine creates
a sort of tunnel vision,

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blocking out surrounding distractions,

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possibly even warping your sense of time,

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and enhancing your memory.

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This might explain why people
never forget their first kiss.

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The idea of so much of our attraction

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being influenced by chemicals
and evolutionary biology

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may seem cold and scientific
rather than romantic,

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but the next time
you see someone you like,

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try to appreciate how your entire body
is playing matchmaker

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to decide if that beautiful
stranger is right for you.