Original subtitles

There's one thing you do every day in the privacy of your home

that you'd never dream of doing in front of strangers.

Get undressed.

So what's stopping you?

These eight volunteers are about to find out.

They face an unforgiving 48-hour ordeal as Horizon

exposes their minds and bodies to the problem of nudity.

It was more extreme than I imagined.

It didn't even occur to me that they were naked.

My heart rate shot up to a ridiculous degree.

It's quite relaxing actually being, you know,

walking round the house naked.

When that penny dropped and I knew what was happening,

that was just the most awful experience.

Why humans have a complex relationship with nudity

challenges scientists from Finland to Florida,

from Africa to California.

They're finding answers in unexpected places -

in the chest hair of Finnish students,

and in the extraordinary family history of lice,

in the sweat of an unusual African monkey

and in our instinct to stare at the human body.

With each discovery comes new insight

on what it means to be human and naked.

So would you strip in front of strangers?

What about on national television?

What IS the problem with nudity?

Just really really horrible, really horrible.

At this anonymous house in the heart of London,

Horizon has brought together eight complete strangers

to take part in a unique study.

From a ballet instructor to a data analyst,

a policeman to a history student,

they've travelled to London from all over the UK.

Ahead lies a series of tasks

designed to confront the volunteers' inhibitions

and challenge the way they think about the human body.

They'll be guided by a team of psychologists

led by Dr George Fieldman,

a specialist in the evolution of social and sexual relationships.

I think it's going to be a very interesting study.

This is the first time these people have ever been naked in public.

It'll be interesting to see

how they challenge their own taboos and society's taboos in this context.

I have no preconceptions.

Quite a few fears.

You know the nudity bit, and having an erection

or something like that, that's, that's probably one of my fears.

I suppose there's a fear that people will laugh,

that there'll be that element of ridicule or shock.

However they're expecting to feel about nudity,

there's only one way to know for sure -

and there will be no gentle introduction.

In the first task, half of our subjects

are going to end up completely naked

and they'll stay naked for the rest of the day.

In a basement room,

a mirror hangs on a dividing wall - but it's no ordinary mirror.

Behind it is a lone chair.

Whoever sits here can see straight through the mirror

and secretly watch what happens in the adjacent room.

First, we're pairing Foyez with Phil.

In a few moments, one of them will be naked.

They don't yet know who.

As they take their positions either side of the mirror,

we check for signs of stress.

Neither of them has ever stripped in public,

let alone surrounded by TV cameras.

With no idea what's in store, they wait for instructions.

INTERCOM: OK - please turn over and read your cards.

"Please stand in front of the mirror.

"When instructed, remove all your clothes,

"placing them in the basket provided.

"Please speak only in response to questions."

Unknown to Phil, Foyez sits back to watch.

INTERCOM: Please begin.

INTERCOM: Can you please rate your discomfort

on a scale from one up to ten?

Two.

Four.

INTERCOM: Do you notice anything unusual about the mirror?

I know Foyez is behind there,

but I have no problem standing here otherwise I wouldn't be here.

Under normal circumstances this would be quite uncomfortable.

I certainly wouldn't sit in a bar

naked next to a, next to a guy I didn't know.

Our subjects follow in pairs, each rating their own discomfort.

Kath from Dorset faces Alex from Manchester.

OK, ten.

Er...seven.

INTERCOM: Kath, do you notice anything unusual about the mirror?

I'm really hoping that's not a two-way mirror!

That experience was totally surreal.

I can't imagine anything that's ever happened before

or is going to happen after that will ever be like that again.

Lucy from Birmingham is up against Rosie.

INTERCOM: Rate your discomfort on a scale from one up to ten.

About five.

Three?

That was strange, that was strange.

I tried to just block them out and imagine I was in my bedroom at home,

somewhere comfortable.

Finally it's our oldest volunteer, Helen, watching Ian from Edinburgh.

Wandering up and down the stairs

with somebody you've just met the night before naked

walking behind you, and you sort of think "Wow!

"This is really different."

Most of our subjects owned up

to this being a pretty unpleasant experience,

but their physical responses were even more revealing.

The physiological data indicate that

everyone was more stressed naked than clothed.

The women might have been slightly more anxious in anticipation of

taking off their clothes than the men. The men seemed more anxious,

to judge by the data, when they were actually naked.

The interesting thing is that these people volunteered for this study.

They knew that it involved nudity and taking off their clothes

but in spite of that they were very anxious under these circumstances.

I felt like I wanted the floor to open up.

There was part of me that didn't want to be there.

I could feel my heart racing.

Why did simply taking off their clothes

cause our volunteers such distress?

The links between nudity and sex may provide one answer.

There was just for a moment where I thought,

"This is very voyeuristic."

Obviously I'd have preferred a woman to be watching me.

But is sex the whole story?

I think the sexual thing is more when you've got bits of clothing on,

that's more sexual than just seeing somebody in the altogether.

For a deeper understanding of nudity, we must leave London

and ask a more fundamental question about the human body.

In a world where all other primates are covered with fur,

why don't we have any?

Many of the biggest questions about the evolution

of the naked body can be answered here in East Africa.

To anthropologists, this landscape is known as the cradle of mankind.

It's widely believed that here, modern naked humans

evolved around a quarter of a million years ago,

the last in a long line of primate ancestors.

Anthropologist Nina Jablonski

has spent twenty years researching the evolution of human skin.

Her research has brought her to Kenya

in an attempt to understand one of our skin's biggest mysteries.

Ever since people really got to grips with the idea of human evolution,

they've been trying to understand why we don't have any hair.

Fur is one of the great products of mammalian evolution.

Waterproof, insulating and protective,

it's an essential asset to any mammal.

So the question is,

why did we lose what had taken millions of years to gain?

From the moment Charles Darwin proposed

that humans were descended from apes,

scientists have puzzled over this question.

And the first of many theories came from Darwin himself,

150 years ago - that it was all down to sexual attraction.

Charles Darwin was one of the first to opine on these matters

and he felt quite strongly that humans became hairless

as a result of sexual selection,

actual preference for a hairless condition.

Well, really Darwin was positing that certain individuals, notably females,

would choose certain males because of their hairless condition,

and they would preferentially mate with them and so those individuals

who had less hair would be more reproductively successful,

so that's how he tied it in with his own theory of natural selection.

Simple though it sounds, Darwin's theory is still controversial.

But at Finland's Turku University,

Dr Markus Rantala aims to change that.

HEAVILY ACCENTED: Charles Darwin...

VOICEOVER: Charles Darwin was fascinated with sexual selection

and he always thought that all differences between races

and also different animals were mostly connected

with sexual selection, but there's no experimental evidence for that.

Dr Rantala is launching an international research project

to find out if Darwin's theory stands up.

Do women really find hairy men less attractive?

These prime male specimens

are making a great sacrifice to help Dr Rantala's research.

He's creating an unusual set of photographs.

First, of the men in their natural state.

But then the men have each agreed to have their bodies shaved

so that Dr Rantala can produce a second set of hairless photographs.

Dr Rantala plans to use these images to find out if people

really do have an underlying aversion to body hair.

And in London, our subjects are to provide some of the very first data.

OK, so we're going to be showing you some images of male torsos

and we'd just like you to rate the attractiveness

of the torsos on the sheet in front of you.

Of course, physique is going to affect their judgement.

But that's not important here.

Dr Rantala wants to see how each natural photo

fares against its shaved counterpart.

So how did our subjects vote?

They rated pictures one,

47 and 53 the most attractive.

All similar physiques, but not entirely hairless.

However, 60% of the torsos

were rated more attractive in the shaved photograph,

a marked preference for smoother skin.

In your ratings of attractiveness,

could you state something about the influence of body hair

in that assessment? Definitely for me it plays a big part.

I don't find body hair attractive. Don't? No.

Less body hair's not so bad but more body hair it's... No.

There was a twist to this test though.

Amongst the photos were four torsos they might recognise.

Alex,

Phil,

Foyez,

and Ian,

were all in the slide show with their chest hair intact.

And they didn't fare too well.

All placed in the bottom half of the ranking -

Alex at number 32,

Phil at 41,

Ian at 47,

and our hairiest subject, Foyez the policeman, ranked at 56,

the least attractive of all.

The fact I was rated last doesn't really bother me that much,

you know, because I'm quite confident as I am and...

when I look at myself in the mirror I think I'm pretty good,

so yeah, if other people's opinion is that, then yeah, more power to them.

I felt, I felt very sorry for Foyez because...

I, because my views are very strong on body hair,

and I, I felt a bit guilty after I'd said what I said

because he had identified himself as having quite a lot of body hair.

I am currently trying to improve the way I look,

but as for keeping the hair there,

the chest hair, no, that's mine to keep.

I like it. I ain't gone round boasting about it...

Anyway, to lose that, I'd feel somewhat emasculated.

This first test

seems to confirm that humans do find hairless bodies more attractive

and that supports Darwin's theory,

that over many generations where the least hairy men got all the girls,

the genes for hairiness all but died out.

But Darwin's theory isn't enough.

By the rules of evolution, it simply doesn't make sense on its own.

Well really, for much of our history as primates

and in the early history of our own lineage,

having hair was probably extremely important

and the lack of hair would have been considered a sign of illness

or, or certainly undesirability.

Our ancestors would have been attracted to healthy mates

by their thick glossy fur.

They wouldn't look twice at one with balding fur.

That's how evolution weeds out weakness and disease.

So before naked skin could become attractive to humans,

it must have become beneficial to lack hair.

Only then could a balding ape

be considered a good prospect as a mate.

One then has to think about,

well what is the good reason rooted in natural selection

that can lead us to understand the evolution of hairlessness?

What was it that gave humans greater reproductive success

as a result of being hairless?

Professor Jablonski believes the answer lies millions of years ago,

with our earliest and furriest ancestors.

While homo sapiens, the hairless modern human,

evolved a quarter of a million years ago,

the human family tree stretches much further back,

to around six million years ago,

when a distant ancestor split from the chimpanzee line.

It was almost certainly covered in fur.

What could have happened since then to prompt the loss of our fur

while so many other animals kept theirs?

All of these animals that we see around us

have a lot of hair for a very good reason.

It may seem stupid that a fur coat

is actually a good thing to have in this hot, open, sunny environment,

but in fact it is a good thing.

Still naked, Ian from Edinburgh

is about to find out why fur is so useful in the heat.

He's joined by Alex in a challenge

that's not going to be comfortable.

They're standing in the intense heat of industrial radiators

while a thermal camera reveals how their bodies are affected.

On the right, Ian's skin is hot, turning from yellow to red,

while those white patches on Alex's clothes show they're even hotter.

But Alex's loose clothes are actually protecting

his body from the heat. Underneath, his skin stays cool.

This is exactly how fur

protects most animals from the heat of the sun.

But one theory suggests our ancestors found a better way,

that by combining three remarkable attributes,

their fur became redundant.

First, they stood upright.

Next, they were very active,

ranging great distances on the open savannah.

And third, just as Alex and Ian are now, they began to do something

no other animal can match...

to sweat profusely.

It's this unique solution to keeping cool that drove the loss of our fur,

according to Professor Peter Wheeler.

Humans rely on whole body cooling, and their combination of a naked skin

and highly developed sweat glands enables them to lose heat at a rate

not approached by any other mammal.

Humans are the sweatiest creatures in history.

Our skin contains the most sweat glands,

and at nearly a litre an hour,

produces the greatest volume of sweat of any animal.

The surface of our entire body is an active cooling system.

This means that the human can lose heat

at a rate in excess of one kilowatt.

Now that's the amount of heat put out by a one bar electric fire.

Alex and Ian are both dripping,

but Alex's shirt soaks up the sweat,

just like fur would, and he feels little benefit.

If you possess body hair, you still can lose heat by sweating,

but it's less effective because the airflow over the skin surface

is greatly reduced, reducing the rate at which water is going to evaporate.

Peter Wheeler believes sweating into a fur coat

was no use to our ancestors.

By losing their body hair, what they're able to do is evaporate water

both more efficiently and effectively from the skin surface.

While the sweat soaks into Alex's shirt,

it can evaporate freely from Ian's naked skin,

and with it, goes all the excess heat generated by his active body.

The more he sweats, the more he cools.

The theory that sweating drove our loss of fur is persuasive,

but it's impossible to prove.

That's why Professor Jablonski is in Kenya.

She believes that out on the African savannah,

there's living evidence

that supports the controversial sweat theory.

She's on the trail of a very unusual primate

which appears to be following our own evolutionary footsteps,

the Patas monkey.

Patas are really of interest to me because they live in open

environments very much like those in which we imagine early members

of our own genus, the genus homo, to have lived,

and they really provide something of a model for how

we think humans may have moved during early parts of their evolution.

Patas and humans share many fascinating characteristics,

because they have similar body proportions

in that Patas have relatively long limbs compared to other monkeys.

They're very good at walking long distances.

They range very, very widely,

they have a larger home range than any other primate.

But Patas monkeys share another of the attributes

that made our ancestors' fur redundant.

As well as being very active,

they've followed our solution to keeping cool.

Most primates don't sweat very much, but Patas monkeys sweat copiously,

and that's what gets anthropologists really excited about studying them.

It's this ability to sweat that suggests to Professor Jablonski

that right before our eyes,

the Patas monkey is echoing our own early evolution.

It was survival of the best sweaters

that really was part of our evolutionary process.

We're talking about early members of the human lineage

that had a little bit less hair, that had more productive sweat glands,

and those individuals would have had an incremental reproductive advantage

over others, and that made the difference in human evolution,

driving the loss of body hair and an increase in the number

'and activity of sweat glands.'

Oh, I've got a great view!

But if Patas monkeys are following our evolutionary steps,

why do they still have fur?

Even though they do have hair,

their hair is very different from that of other monkeys.

It's not as dense, it's quite coarse, and so when they sweat,

they can also lose heat through evaporation just like humans can.

They may have thinner fur,

but Professor Jablonski believes Patas will never go naked,

because they lack one crucial feature that made all the difference

to our ancestors - walking upright.

On all fours, Patas are exposed to too much sun.

Despite their ability to sweat,

they can't afford to lose their protective fur.

If humans had been quadrupedal,

walking on all four legs, we probably wouldn't have lost our fur,

but humans became bipedal,

and when they did come out in this open sunny environment,

there was every reason for them to lose their fur,

and they lost it on their front and on their backs,

but they did retain a little bit of it in a very strategic position,

right on the tops of their heads.

This final evidence convinces Professor Jablonski

that uniquely equipped against the heat of the sun,

our naked ancestors had a huge advantage

over their hairy relatives.

And that made them the first primate

able to exploit the harsh environment of the open savannah.

But to the scientists behind the sweating theory,

going naked had an even bigger pay-off for mankind,

and particularly for our brains.

The human brain produces something like 20 watts of heat.

That doesn't sound very much but if you put a 20-watt light bulb

in a small box the size of the skull, it's soon going to overheat.

One or two degrees and it starts to impair brain functioning.

Three or four degrees, and it's usually fatal.

This risk of overheating

drastically limits the size of most animals' brains, but not ours.

It's probably no coincidence today that the mammal

that's got the largest brain relative to its body size, that is humans,

also possess the most powerful cooling system

of any mammal to protect it.

It was this superior cooling system

that would change the course of evolution.

Without losing hair, without our sweatiness,

we wouldn't have been able to evolve the big brains

that characterise us today,

and that really make us the modern human species that we are.

Essentially, being hairless was the key to so much of human evolution.

The next question is, when did we reach this pinnacle of evolution?

Were modern humans the first naked ape,

or our older cousins, the Neanderthals?

It could have been an earlier ancestor from the genus homo

that lived between one and two million years ago.

Was it Australopithecus, living two million years before that?

Or even Ardipithecus,

the species that branched from the chimpanzee line

six million years ago?

It's a question that's never been answered,

because, although fossils tell us so much about our ancestors' bodies,

the length of their limbs, the size of their brains,

one thing that is never preserved is their skin.

But there is a surprising new source of evidence.

It's not from humans or from apes, but from a quite different creature

that's been our constant companion throughout evolution.

Florida University geneticist Dr David Reed has found that

the chequered family history of human lice is very revealing.

When you think of human hair, that's the habitat of these lice,

and it offers us the opportunity to study these lice

and how they've evolved with their hosts.

If you look at the genetics of these lice,

written in their genetic code is our own evolutionary history.

Watershed events like losing the complete body hair on a host

would have a huge impact on where parasites can go on the body,

and whether parasites persist or not.

Dr Reed has been doing some unusual genetic detective work.

So we collect these parasites from humans,

and then we bring them back to the lab.

Then we extract the DNA from these lice,

and we magnify that DNA through DNA sequencing.

We can use those DNA sequences to build

evolutionary trees that describe the relationships of these lice.

The family tree that Dr Reed has built for lice

provides a mirror for the tree of human evolution.

And we see that if you date when chimpanzee and human lice diverged,

it matches up perfectly

with when humans and chimps last shared a common ancestor.

What most interests Dr Reed is the extraordinary relationship

between humans and lice.

Generally each primate species only has a single louse species.

Humans are somewhat unique in that we have three types of lice.

The louse that most people would be familiar with

that occurs on humans is of course the head louse.

We think that this is the ancestral type of louse

that we've had all along throughout our evolutionary history.

With us for six million years, the head louse originally lived

all over the bodies of our earliest hominid ancestors.

But if that hominid loses all of its body hair,

and retains perhaps only head hair that's suitable habitat,

now you have one refuge on the entire body.

So how does Dr Reed explain our next species of louse,

the pubic or crab louse?

The crab louse is somewhat different from the human head louse

in terms of its size and shape,

and it's well adapted to holding onto hairs

that are much larger in diameter and much farther apart in spacing.

Its closest living relative is actually found on gorillas.

To Dr Reed, understanding how this new species colonised our bodies

is critical to understanding the evolution of human hair.

How would we have acquired a gorilla louse?

What must have happened in terms of changes in our body to allow that?

What's interesting to me is that

that move could not have happened

until the habitat was there and available.

First, we must have lost our body hair,

and second, we must have acquired pubic hair.

Even with the earlier species confined to our heads,

the new lice couldn't move in till we'd evolved a patch

of suitably coarse gorilla-like hair...

though exactly how they made the move

is open to question.

For lice to move among individuals,

there usually has to be direct physical contact.

If we're talking about a louse moving from gorillas

to the pubic region of humans, of course, the imagination can run wild.

Whatever the route,

the new arrivals separated from their gorilla ancestors

to create a new species, the human pubic louse.

Dr Reed realised that this branching of the louse family tree

provides the best evidence yet for when humans lost their body hair.

The genetic data that we studied from these lice tell us that the move

from gorillas to humans occurred roughly three million years ago.

Therefore we might assume that the body hair changes in humans happened

roughly three million years ago.

It's an astonishing conclusion,

placing the original loss of body hair

long before the evolution of modern humans.

Certainly this very old timeframe of three million years

for the loss of body hair flies against the general convention

that only modern humans much more recently lost their body hair.

To think about archaic hominids having no body hair

for millions of years is quite interesting, and quite controversial.

By Dr Reed's calculation,

nudity goes right back to Australopithecus.

From that early ancestor,

every branch of the human family tree inherited naked skin,

right up to Neanderthals,

and modern humans.

The answer to one final mystery of human evolution

lies in the genetic evidence of lice,

one that's essential to our concept of nudity.

The clothing louse is a direct descendant of the head louse

and, like the crab louse,

it could only evolve once its habitat existed.

What's remarkable about this one is, of all the lice,

it's the only one that lays its eggs

or lives in anything other than fur or feathers.

It lives entirely in the clothing.

By dating this branch in louse evolution,

Dr Reed could reveal something that fossils never could.

When humans first got dressed.

We can look at the molecular data for human head lice

and clothing lice, and deduce when those populations began diverging,

and when we do that, we see that they diverged about 650,000 years ago.

Dr Reed has re-written the history books on human nudity.

By his calculation, ancestral humans lived completely naked

for at least two million years.

Only then did they begin to cover up their nudity,

just over half a million years ago.

From the moment we covered up our bodies,

clothes began to shape our culture and our identity.

If you're clothed, then you don't have to posture yourself as much,

because the clothes you wear will do a lot of the...

the messages that needs to be said about you.

When you've got clothes on you can create an image for yourself.

Nudity to me is like a blank piece of canvas,

and when you put clothes on, you're kind of painting yourself.

But clothes also created a whole set of uniquely human problems.

For a start they concealed all the bits of the body

that were most important

for the essential business of sexual attraction.

At the University of California in Los Angeles,

Dr Kerri Johnson is investigating the secrets of human attraction.

She wants to find out how we still manage to attract each other

despite wearing clothes.

My research examines how people

make very fundamental social judgments about one another.

When you see them across the room, you know virtually instantly

whether it's a man or a woman,

how masculine or feminine the target is,

and consequently whether he or she is sexually attractive.

We use eye tracking methods that covertly measure the direction

of gaze of our participants,

allowing us to pinpoint precisely where our participants are looking.

In her experiments, Dr Johnson asks people

how attractive they find a series of computer-animated silhouettes.

They are neither nude nor are they clothed,

and this is really an important factor in our experiment.

If the target is naked, they're likely to look at

the areas that are the most informative -

the genitals, the breasts, presence or absence of those things.

So that's the judgment that you'll be making.

Is the image depicting a man or a woman?

The aim is to find out how we tune into each other's sexuality

when none of the obvious signals is visible.

Man.

My research has found that the body's shape and body's motion

are very important for judgments of attractiveness.

A woman's body is much more hourglass in shape,

and a man's body is much more tubular in shape.

It reliably differs between men and women,

and this is referred to as the waist-to-hip ratio.

A typical female walk would include a lateral hip sway.

This is the hips moving back and forth,

and actually a bit up and down as women walk.

If you think about the way a man moves his body,

we refer to that as shoulder swagger.

So when people are deciding whether a target is a man or a woman,

they look intently at a region of the body

that varies between men and women, the waist and hips.

The result is clear. With clothes covering up our naked bodies,

humans have perfected a new, more subtle code of sexual attraction.

Woman.

So we're talking about how the body is shaped,

the physical proportions of the body,

and how the body moves,

and both of those are available to observers

regardless of whether they're wearing clothing or not.

We're going to find out how well our subjects

can tune into these subtle sexual signals.

Foyez and Helen are being fitted with eye-tracking glasses.

We're going to watch where their eyes wander

as they scan the bodies of their fellow subjects,

clothed and naked.

First up is 25-year-old Rosie.

Helen's eyes are all over the place,

picking up details of Rosie's clothing,

but she's definitely lingering on her waist and hips,

exactly as predicted.

But Foyez seems to have locked his gaze on Rosie's head

and he's keeping his eyes unnaturally still.

How will they deal with a naked body?

Step forward Phil, our 39-year-old data analyst.

Helen's not fazed.

She's scanning his entire body again,

picking up the shape of Phil's waist and hips,

but not focusing on the obvious sexual features.

And Foyez...

..it seems nothing is going to shift his gaze...

and he's sticking to that strategy.

Something is stopping Foyez from following his instinct to look.

Helen seems at ease whether her targets are naked or not.

She's quite naturally drawn to the most sexually revealing parts

of the body - the waist and hips.

My intuition is that the eye movements demonstrated by Helen

were probably completely natural and spontaneous.

I suspect that Foyez may have been maybe embarrassed,

or for whatever reason, wishing to control his movements,

so that he looked very much at the person's face

rather than anywhere else.

Maybe, of course, it would be interesting to know

if he'd be looking in that way

if he hadn't known that his eyes were being monitored.

I didn't want to be labelled as a voyeur or a pervert.

I mean I was well aware that this footage

would be sort of reviewed afterwards.

I just treated it more like a video game.

A lot of self-control was involved even while all of this was going on,

so I was quite impressed with myself.

I think Foyez was very much controlling his natural response.

I just can't believe a male confronted with a nude female

is going to look, you know, at their, at their heads.

It was nice of him to show that degree of control.

Probably made me feel a bit better

because I actually found it more uncomfortable

watching myself on the screen.

Despite Foyez's self-control,

it's clear the signals of sexual identity carried in human shape

and movement are much more subtle than the ones most primates use.

Without the barrier of clothing, primates use striking visual signals

advertising exactly when they're ready for sex.

Females show that they're fertile by displaying a swollen red behind,

leaving no doubt in the minds of any amorous males.

Under our clothes, signals like these would be useless to humans,

so it's no surprise that women don't have such a display.

But does that mean that humans have to rely on guesswork

to pick the right moment to mate?

In another Los Angeles laboratory,

that's exactly what Dr Martie Haselton is trying to find out.

I think that for men detecting cues of fertility,

whether they be cycling fertility,

fertility over the course of the month,

or fertility associated with changes in age,

I think that those play a very dramatic role in mate preferences

and therefore in sexual selection.

Dr Haselton collects photographs of women

at various stages of their menstrual cycle,

including ones taken just before ovulation

when they are most fertile.

Then she asks men to choose which is most attractive.

We found that judges chose the high fertility photograph

as the one in which she was dressed up more.

About 60 per cent of the time that was well beyond chance.

That led us to wonder, "Well...

"are there detectable cues of ovulation

"that male partners are able to pick up on?"

This was quite a revelation.

For decades scientists had believed that people simply couldn't detect

human fertility levels.

We know now that other things are happening far off the radar

of conscious perception.

There are cues of cycling fertility, cues of ovulation

that are detectable even by complete strangers.

It seems that humans have developed some kind of sixth sense,

giving us incredible sensitivity to these secret signals.

There was a recent study done that looked at the amount of tips

that lap dancers earned on varying days of the cycle

and on high fertility days, men tipped them more generously.

No-one has yet worked out exactly what these fertility signals are,

but Dr Haselton has identified one likely candidate.

Women's body odours change.

They become more attractive on high as compared with low fertility days

of the cycle.

And this discovery promises to solve another mystery

about the naked human body.

What pubic hair is for.

One explanation is that body hair is a conduit for scent, communication.

In the moist warmth of our pubic hair,

bacteria feed on hormones in our sweat

and produce distinctive aromas.

So for example underarm and pubic hair could be a way of transmitting

body odours out into the environment to enhance attraction amongst lovers.

Humans don't need flashy fertility displays.

Beneath our clothes, pubic hair has become our secret weapon

of sexual attraction.

After a day and a half naked,

four of our subjects finally get the news they're waiting for.

I'd like each of you to... get dressed again, please.

But as these four are reunited with their clothes,

the tables are about to be turned on the remaining subjects

who are left guessing what lies ahead.

Maybe there'll be wrestling. Oh, no! Can't bear to think about that, no.

Or maybe we'll do body artwork, you know they'll cover us in paint

and we'll just run into a canvas or something.

It's a scientific experiment in hairlessness.

Not painting us.

In fact we're going to ask them to do something more intimate

than any of them feared.

Undressing is a very common thing we all do every day,

but undressing in front of someone,

only usually done under very special circumstances

when a couple feel very safe with each other.

Here we've got something very different,

people are undressing or rather being undressed,

by someone they've only recently met in a room full of people.

Those of you standing on the floor,

we'd now like you to undress the person on the podium in front of you.

The subjects are paired,

so that they're not all undressed by someone of the opposite sex...

and they're clearly not finding this easy.

I felt quite relieved to be back in my clothes again.

I think that the whole thought of people taking their clothes off,

there's a sexual connotation to it.

There's that kind of, you only do it when certain things are gonna happen

and there's an uncomfortableness about that, I guess.

Unsurprisingly they found this one pretty stressful.

The tables were being turned. The people who had been naked previously

are now undressing the others.

It's also a reminder that...nudity is associated with sexuality

and being undressed is very much close to a sexual encounter.

Our subjects have reached a crucial point

in their exploration of nudity.

However hard they try, they can't escape the intense emotions

provoked by the simple act of undressing.

But does this sensitivity to nudity serve any purpose?

Scientists have long searched for an answer

and evolutionary psychologist, Professor Dan Fessler,

thinks he's found one.

Two emotions play an important role in sexual modesty.

At the less extreme end of the spectrum,

minor inappropriate exposure of the body results in embarrassment.

At the more extreme end of the spectrum,

grossly inappropriate exposure of the body

and exposure of one's sexuality results in shame.

Professor Fessler believes that, first of all,

the expression of shame is a simple self-defence mechanism.

All around the world individuals feel great shame

when they know that others know

that they have failed to be adequately modest.

Essentially they're signalling to those around them,

"I understand what the social norm is and I understand that you know

"that I have failed in this regard, so please don't hurt me."

But it's the fact that all humans are sensitive to sexual modesty,

even in largely naked cultures, that convinces Professor Fessler

there's a real biological reason for it.

He believes it's a direct result of our large brains.

Our very large brains in themselves create a problem.

We have a tight fit between the size of an infant skull

and the size of a mother's birth canal.

One solution to this is to take the bun out of the oven

before it's fully baked,

so our infants are born premature compared to those of many primates.

With their brains only partly developed at birth,

human babies are helpless for many years

and this has a major consequence for human sexual relationships.

What this means is that essentially human children require

a great deal of care.

Because of this, the human mating strategy,

if we look at humans around the world, is one in which often -

not always - but often,

men mate monogamously. At any one time they have a single partner,

and they raise offspring together.

Pairing for life ensures our babies get all the care

they need to survive and pass on our genes.

But it's a high-risk strategy.

Humans are considerably more social than the average primate.

We live in large populations

and we co-operate with large numbers of individuals.

This poses a challenge because those groups, of course,

provide a source of temptation.

Potentially both sexes can benefit...

by cheating on their partners.

The human body is a supreme sexual advertisement.

Flaunting it can send out a dangerous message.

Nudity is a threat to the basic social contract

because it is an invitation to defection.

They have exposed their person, their body and their sexual selves

in a way that presents an opportunity

for sexual behaviour outside of the principal union.

Professor Fessler believes the shame of nudity serves a real purpose.

It encourages us to stay faithful to our partners

and share the responsibility of bringing up our children.

It was an interesting experience.

I'd say, up until now the only people I've really undressed,

probably are ex-partners and my wife when they've been a little inebriated

and I had to sort of put them to bed.

I thought I would feel like I would be...the vengeful thing, "Ha-ha!

"I'm in control now," but as I said when it came to it,

I...I just wanted to make sure she was OK.

I felt almost quite empowered going through that exercise,

sort of being in control of the whole situation.

And I can, I can sympathise with the fact that the other guys

might have been feeling quite uncomfortable

and possibly quite vulnerable with us going through that motion.

After two days looking at each other naked,

we're pushing the subjects way beyond the normal limits

of social acceptance.

In the final test, they will touch.

They're painting the body into comfort zones.

Green is fine to touch.

Yellow is not and red is a no-go zone.

But despite the potential for embarrassment,

our subjects seem to be enjoying themselves.

If they'd done the body painting exercise right at the beginning,

it might not have been all the fun and games

that it seemed to turn out to be at the end.

It might have been a pretty stressful and aversive exercise.

I think as the weekend's gone on and they've gone through these exercises,

they've somewhat habituated to the stress.

They're more relaxed and they know each other,

so it's become a more comfortable and congenial setting overall.

Lucy, a 42-year-old mother, clearly still has some reservations.

But Brummie Phil, 27-year-old Alex and especially Kath from Dorset

have become surprisingly matter-of-fact about nudity.

Their attitudes and inhibitions have changed,

and this is the crucial thing about our relationship with nudity.

We're not born with sexual modesty,

so we're free to shift the boundaries of what's acceptable...

and what is not.

So long as everyone agrees, we can create new rules

and avoid the risk of offence just like at a nudist camp.

Within this house, our subjects have created their own set of rules.

After two days of social nudity, it's mostly OK.

It's mostly green.

I feel really quite happy, you know,

and...I feel somewhat more confident now,

also, that I didn't make a woman

feel sort of uncomfortable round me, you know.

I think if I'd been asked to do the task any other day,

there'd have been a lot more, sort of like, red,

red and sort of yellow painting.

I was quite impressed at his complete cool...

approach to it and the fact that he was just, you know,

quite nonchalant about the fact that he didn't mind touching me anywhere,

so that was quite a surprise to me.

I think once you've been painted by

or you've been painting someone else's body,

that's a pretty big bonding experience.

At the end of their naked weekend,

our volunteers are finally comfortable to be nude together.

For scientists, the extreme emotions that nudity can cause

will always be a paradox.

The irony of human nudity and hairlessness

is that really it's the apex of human evolution.

Only humans have moral emotions such as shame

that enforce cultural standards.

One can think of nudity and sexual modesty as exemplifying

our uniquely human emotional morality.

The state of nudity is the state of being human.

Essentially having a naked skin and understanding the evolution

of that naked skin is understanding everything about being human.

For our volunteers, there's a last chance to reflect

on their naked experience.

Hmm...I'm relieved that it's all over, firstly.

I was more relieved when he said, "Put your clothes back on."

The whole sort of terror of the first experiment

and as that developed and I realised I was taking my clothes off

in front of what I thought was just a normal mirror

and the awful realisation that it was actually a two-way mirror

and there was someone sat the other side of it,

that was just really terrifying.

This is the first time we've done something normal

and we're all naked!

To be honest the whole painting thing, I wouldn't have imagined

something that extreme and the fact that somebody else had to undress me.

So, yeah, in a way I sort of exceeded my expectations

because it was more...

bizarre than I imagined it would be, definitely.

It's just how...how comfortable we became around each other gradually,

you know, in that, I mean when we were all nude and everything,

it didn't even occur to me that they were naked at all.

One thing I think I'll take away from this weekend

is how actually easy it was to bond with complete strangers

in what should really be an artificial environment

and one that should, by all society's standards,

we should've been uncomfortable with.

Our volunteers ended the weekend naked together,

but how many will accept the final challenge -

to leave the privacy of the house?

Six out of eight step out into the street.

For Lucy and Helen, even after a weekend of nudity,

this is a step too far.

Subtitles by Red Bee Media Ltd

E-mail subtitling@bbc.co.uk

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