it certainly appears the Earth is flat.

The ground beneath you is stable and unmoving,

and stars and sun circle the Earth.

Hundreds of years ago,

elaborate theories were developed based on these common sense observations

to explain and predict the reach of the oceans

and the movement of celestial bodies.

When science demonstrated that these common sense observations were illusions,

and depicted the Earth and the Universe in a completely different way,

people slowly came to accept that the world was not as it seemed.

Scientific measurements and sophisticated calculations

have repeatedly demonstrated that

what we think is intuitive, obvious and common sense

cannot be trusted to be true.

For that reason, modern science is based on the denial of common sense

until apparently it comes to ourselves:

when science confirms a particular way of thinking about our mind and behaviour,

or depicts it in an unusual and a new way,

we tend to be skeptical that such a science is worthwhile

even if possible.

And instead, we fall back on intuition, prior beliefs, and yes, common sense.

For instance, if I told you,

scientific research's demonstrated that opposites attract,

wouldn't you tell me that we don't need a science to tell us something we already know?

But what if I told you that birds of a feather flock together according to scientific research,

wouldn't you say, we don't need a science to tell us something we already know?

Or you may have realised already,

of course, that these both may be self-evident truths,

but they can't both be true

since they are internally inconsistent.

The science of mind and behaviour is full of such examples:

self-evident truths that both can't be true.

We know, for instance,

that two heads are better than one

and we know that too many cooks spoil the broth.

The next time you hear a science report of some obvious result,

remember that the obvious result was equally obvious,

but it'd just been proven to be wrong.

It's obvious there we're rugged individualists.

True, true, true!

We're born to the most prolonged period of dependency,

but in our transition to adulthood, we achieve autonomy, independence,

to become kings of the mountain,

captains of our universe.

It's easily (easy) to think about our brain,

how's deep within a cranial vault, separated, isolated, protected from others,

when we look out onto the social world

other individuals certainly look distinct, independent, self vicinities

with no forces binding them together.

No wonder that we forget

that we are members of a social species,

born dependent on our parents, for our species to survive,

these infants must instantly engage their parents in protective behaviour

and the parents must care enough about these offspring to nurture and protect them.

Even once grown, we are not particularly splendid specimens.

Other animals can run faster

see and smell better,

and fight much more effectively than we can.

Our evolutionary advantage

is our brain and our ability to communicate, plan and reason and work together.

Our survival depends on our collective abilities,

not on our individual mind.

We are connected across our lifespan to one another,

through a myriad of invisible forces,

they're, like gravity, are ubiquitous and powerful.

After all, social species, by definition, create a merging structures

that extend beyond an organism,

structures that range from couples and families

to schools and nations and cultures.

These structures evolved hand in hand with neural, hormonal and genetic mechanisms to support them

because the consequent social behaviour

helps these organisms survive,

reproduce and leave a genetic legacy.

To grow to an adulthood for a social species, including humans,

is not to become autonomous and solitary,

it's to become the one on whom others can depend.

Whether we know it or not, our brain and biology

have been shaped to favour this outcome.

The evolutionary biologist, David Sloan Wilson,

notes that if you ask people:

"What are the traits of a good person?",

you'll hear traits such as kind, generous, compassionate and empathic.

If you ask people what are the traits of an evil person,

you'll hear traits such as

cruel, greedy, exploitative and selfish.

Said differently, the traits of a good person

depict someone who cares about themselves and others,

and an evil person cares about themselves

at the expense of others.

Across our biological heritage,

our brain and biology have been sculpted to incline us

to certain ways of feeling, thinking and behaving.

For instance,

we have a number of biological machineries

that capitalise on aversive signals to motivate us to act

in ways that are essential for our survival.

Hunger, for instance, is triggered by low blood sugar

and motivates you to eat,

an important early warning system for an organism

that'd require much more time and effort to find food

than going to the refrigerator door, kitchen cabinet

or fast food restaurants.

Thirst is an aversive signal,

that motivates us to search for drinkable water

prior to falling victim to dehydration.

And pain is an aversive system that notifies us of potential tissue damage

and motivates us to take care of our physical body.

You might think that the biological warning machinery stops there

but there's more.

Although not common sense, although not intuitive,

the pain and aversiveness of loneliness,

of feeling isolated from those around you,

is also a part of biological early warning machinery

to alert you to threats and damage to your social body,

which you also need to survive and prosper.

Just about all of us have felt physical pain

and nearly all of us have felt

the heartbreak of home sickness,

the agony of bereavement,

the torment of unrequited love

and the pain of being shunt.

All of these are variations on the experience of loneliness.

When I started to study the effects of loneliness

and brain and biology a couple of decades ago,

loneliness has been characterized as a non-chronic disease

without redeeming features.

It was even equated with shyness and depression

with being a loner, a person with marginal social skills.

Scientific measurements and sophisticated calculations,

to our surprise, revealed that these were myths.

Science and common sense had again produced

two very different depictions of a phenomenon.

And yet if you look at the way we are increasingly living our lives,

it shows the extent to which we still buy into

those myths of loneliness and values of autonomy and independence.

For instance, if you look at

the percentage of one-person households in 1940 across the United States

it was largely less than 15% of the households by state.

Fast-forward to 1970,

and it's grown to be between 15 and 20%.

Fastforward to 2000

and it now exceeds 25% in most states in America.

And that light blue state, Utah

in 2010 census has gone darker blue.

The prevalence of loneliness is also on the rise.

In the 1980s, scholars have estimated that about 20% of Americans

felt lonelier than at any given point of time.

Two recent nationally representative surveys indicate

that this number has doubled,

but you don't hear people talking about feeling lonely,

and that's because loneliness is stigmatised.

The psychological equivalent to being a loser in life or a weak person.

And this is truly unfortunate,

because it means we are more likely to deny feeling lonely,

which makes no more sense than denying we feel hunger, thirst or pain.

For living with loneliness we now know is the major risk factor

for broad-based morbidity and mortality.

Consider a couple of the conditions we know about premature death.

Living with air pollution increases your odds of an early death by 5%,

Living with obesity, we know, a national health problem,

increases your odds of an early death by 20%.

Excessive alcohol consumption: 30%.

A recent med analysis of around a hundred thousand participants

shows that living with loneliness increases your odds

of an early death by 45%.

We're not the only social species and the experimental investigation

of non-human social animals who were isolated shows

they too suffer deleterious physiological consequences

and an abbreviated lifespan.

Across our history, as a species, we have survived and prospered

by banding together,

couples, families and tribes, for mutual protection and assistance.

We think of loneliness as a sad condition,

but for social species, being on the social perimeters,

not only sad, it is dangerous.

The brains of social species including our own have evolved

to respond to being on the social perimeter

by going into a self-preservation mode.

If you isolate a rodent and then put it in an open field

such as these dots in the bottom of the image,

it engages into what's called predator revision,

it walks around the outside and doesn't venture into the middle

where escape from a flying predator would be more more difficult.

When humans feel isolated,

they're too, and not only in an unhappy circumstance,

but in a dangerous circumstance.

Their brains too snap into a self-preservation mode.

In a brain-imaging study that we conducted,

we showed people negative images

that had nothing to do with other people

or negative social images,

while they were sitting in a scanner and we were scanning.

What we found was

the lonelier the brain,

when a negative social image was presented,

that is in a person's environment,

when something negative socially happened,

the brain allocated more attention,

greater visual cortical activity depicted in yellow here, to that image.

Now, as you follow that image forward,

you come to those two blue areas:

that's a temporoparietal junction.

This is a piece of brain tissue that's involved in theory of mind,

in mind reading and mentalizing,

in taking another person's perspective and empathy.

It's responsible for the attentional control required to step out of your head

and put yourself, at least figuratively, inside the head of someone else

so you can take their point of view.

The lonelier the brain,

when something negative in the social context was depicted,

the less the activation in this region.

It's dangerous on the social perimeter.

When something happens negative in the social environment,

that brain is focused on self-preservation,

not a concern of the other person.

The similarity in neural and behavioral effects across phylogeny

is a testimony to the importance of the social environment for social species.

And these deep evolutionary roots tilting our brain and biology

towards our self-preservation

also suggest that much of what's triggered by social isolation is non-conscious.

For instance, when you feel isolated

you feel this motive, this desire, this intention

to connect with other people again.

What you don't feel,

is that your brain has gone into a hypervigilance for social threats

and this hypervigilance means you introduce

intentional, confirmatory and even memory biases in terms of those social interactions.

And if you're looking for dangers,

you more like to see dangers

whether they exist or not,

meaning that you more likely

to have negative interactions.

And that threat surveillance of always looking for the next foe

activates neuro-biological mechanisms

that can degrade your health and lead to early mortality.

Loneliness increases defensiveness

because you're focused on your own welfare

rather than taking the position or perspective of people with whom you interact.

Loneliness increases depressive symptoms

which has the odd effect of decreasing your likelihood

of having social conflict

and through the acoustic and postural

and facial expressions of sadness,