杏吧原创

AT some point in human evolution, something of a Faustian pact was struck
over the nature of our emotional lives. Liberated from the tyranny of hormones
and basic drives for food and sex, we no longer had to rut at any particular
season, or in response to a whiff of sexual pheromone. At the same time,
interacting with other members of our species became a potentially pleasurable
experience in itself, on a par with having sex or consuming a tasty meal.

And it was this that turned out to be a mixed blessing. For the very changes
in brain structure that freed us from the sway of basic emotional drives also
made us sensitive to the pain of being slighted or marginalised by others. The
new tyrant became socially derived self-esteem.

It鈥檚 a trade-off that intrigues Barry Keverne, reader in the hybrid science
of 鈥渂ehavioural neuroscience鈥� at Cambridge, and director of the university鈥檚
subdepartment of animal behaviour. Under his microscope are the neural networks
and brain chemicals that enable us to take pain or pleasure in other human
beings. For decades he and his colleagues have been studying the hormones, brain
chemistry and social life of groups of captive monkeys and other animals. And he
knows that the worst thing anyone can do to a monkey is isolate it. The animal鈥檚
normal behaviour and physiology disintegrate as the desire to eat and the desire
to have sex die away. It鈥檚 no accident, Keverne believes, that repressive
political regimes condemn their victims to solitary confinement.

But what is it about the emotions of humans and primates that makes us all so
socially sensitive? Part of the answer, says Keverne, is simply brain size. As
the cortex expanded with evolution, primates acquired a new, social
goal鈥攇roup membership鈥攖o set alongside the basic drives for food and
sex. But something else happened too: a new neural pathway evolved enabling this
expanded cortex to talk to the inner 鈥渆motional鈥� limbic brain. And it was this
that made all the difference.

In lower mammals, most information from the senses flows into the limbic
brain in a raw and unprocessed state. So they are slaves to pheromones, hormones
and basic drives. But in primates, most sensory information is first processed
in the cortex, where cross-referencing to stored information and past
experiences takes place. This brain-wiring, explains Keverne, makes the
brain鈥檚 chemical 鈥渞eward鈥� systems sensitive to social encounters in ways that
can shape personality and emotional make-up. Monkeys with good early-life
experiences 鈥渃an love looking after infants and be very good at it, even if
they鈥檝e never been pregnant鈥�, says Keverne. But according to experiments done
decades ago, monkeys isolated since birth are 鈥渢errible mothers, really abusive
to the infants鈥�.

So, in ways that our post-Enlightenment culture, with its mind-body split,
has been slow to appreciate, happy or unhappy social experiences are written
into the biological fabric of our bodies. The question is who鈥攐r rather
what鈥攕upplies the ink.

Enter Keverne鈥檚 research into endorphins, natural chemicals that target the
same places in the brain as the drugs morphine and heroin. Known as the body鈥檚
own painkillers, endorphins seem to be released in the brain as a reward for
sexual and maternal behaviour. All mammals indulge in this chemical pleasure
seeking. And most also seem to benefit from the soothing effect of endorphins
released immediately after aggressive encounters and stressful experiences such
as separation from their mothers before weaning.

Primates are no exception, only in their case it seems endorphins are
released following a much wider range of social encounters.

Take primate grooming. Keverne鈥檚 team have found this social act taps into
the brain鈥檚 opioid system: levels of beta-endorphins in the cerebrospinal fluid
shoot up during grooming. But not as a reward for cleanliness. Primate grooming
is about building social bonds. It happens in a variety of social contexts:
during and after mating, between mothers and infants, and among peers. Indeed,
grooming seems to be the glue that holds monkey societies together. And that in
turn, says Keverne, puts the brain鈥檚 endorphin system at the heart of social
bonding.

The system probably evolved to bond mother and infant in lower mammals. A
ewe, for instance, surrounded by scores of other demanding lambs, must quickly
learn to recognise her own lamb. A rewarding shot of endorphin in the brain may
be just the incentive she needs. Humans and other primates don鈥檛 need such rigid
bonding systems. Yet this basic endorphin system has persisted, in a much
modified form. In fact, argues Keverne, evolution has expanded it to respond to
a wider range of relationships. Nobody has yet proved it, but when lovers or old
friends meet they could well experience a soothing surge of endorphins.

But there is a downside. Keverne and his colleagues have found that male
monkeys whose social experiences are consistently negative鈥攚ho fail to win
mates and end up at the bottom of the group pecking order鈥攆ail to respond
properly to endorphins. Paradoxically, this happens because over time their
brains produce too much endorphin in response to chronic stress. The normal
chemical reward system breaks down and the animals become lethargic. 鈥淣obody
grooms you, nobody huddles with you, all you get is hassle,鈥� comments Keverne.
The brains of these downtrodden males can no longer register the reward of
sexual activity even when mating with a receptive female becomes possible, so
they don鈥檛 even try.

Keverne suspects this could explain why drugs such as heroin and cocaine may
be especially appealing to people who feel worthless. If human brains can become
insensitive to their endorphins through social stress and low self-esteem, then
perhaps taking drugs artificially compensates for the deficit.

Female monkeys escape the worst of the downside. This is because they
establish social rank not by threats and fights but through grooming鈥攖he
highest-ranking female receives most attention. As a result, the brains of the
females aren鈥檛 awash with endorphins, with all the harmful knock-on effects that
entails. So when the top females are absent, subordinate females immediately
become interested in sex. All the same, being a low-status female does carry
some physiological penalties: the increased stress of being subordinate makes
them less likely to ovulate normally, and thus less likely to conceive after
mating. It is possible that some unexplained infertility in women could stem
from the physiological consequences of social stress and low self-esteem.

Indeed in biochemical terms, endorphins are uniquely placed to influence both
physical and emotional wellbeing. On the one hand, these 鈥渟ocially sensitive鈥�
chemicals can interact with other brain chemicals and systems that influence
mood and behaviour. On the other, they can act as a trigger for the body鈥檚
stress system, unleashing hormonal changes that put the body on alert鈥攂ut
only at the cost of long-term wear and tear to immunity, reproduction and much
more.

In most mammals, the experience of stress can be seen as an adaptive process.
Confronted with a frightening situation, the surge of stress hormones prepares
the body for confrontation or escape. 鈥淏ut once you start to live socially in
groups, that escape is not necessarily an option,鈥� says Keverne. 鈥淔or if an
animal leaves the social group, it runs serious risks.鈥� In a conflict situation,
your instinct is to 鈥済et the hell out of there鈥�, says Keverne. 鈥淏ut you simply
can鈥檛 do it. So you remain there and stress becomes chronic, which is what
produces all the harmful effects.鈥�

So here we have it鈥攖he uniquely human dilemma. As social creatures, our
emotional and physical wellbeing is utterly dependent on the quality of our
encounters with other people. With others, we experience our most intense
emotions: jealousy, shame and anger as well as happiness and joy. Jean-Paul
Sartre wrote that 鈥淗ell is . . . other people.鈥� But if it is then heaven is
other people too.