When I was a child, I once had a falling-out with my two best friends.
I cannot remember what it was over.
But I can remember the misery of being excluded from their activities and games.
I have a particularly vivid memory of them taunting me by dancing Vaudeville-style across the school playground with big cheesy grins on their faces.
The falling-out was brief and normal relations were soon restored.
But the incident left a big impression on me.
I imagine that all readers will understand how I felt.
We all know what it is to be wounded by rejection, even if it is something as trivial as not getting invited to a party or being picked last for a sports team.
Such things tend to stick in our memories long after they happen.
We find ourselves reliving them years later.
In the early 2000s, researchers at the University of California in the United States conducted a study on the psychological effects of rejection.
They asked volunteers to play a computer game called Cyberball in which three players pass a ball around among themselves.
The volunteers were told that they were playing with real people who were sitting in separate rooms. But this was untrue.
The other "players" were, in fact, controlled by the computer.
At first, the computer played nicely and included the volunteers in the game.
But after a short time, the computer got mean and stopped passing to them.
At this point, most of the volunteers got upset, angry or sad. Some of them made rude gestures at the screen.
Meanwhile, their brain activities were monitored using a functional magnetic resonance imaging scanner, which does so by tracking blood flow in the brain.
This revealed a surge in activity of the parts of the brain that normally control our responses to physical injuries.
This was quite a surprising result.
After all, the volunteers were playing - or believed they were playing - with people whom they had never met and never would meet.
So why should they care whether they were included or not?
What was even more surprising was that in another similar experiment, volunteers reported comparable feelings of anger and unhappiness even when they were told that the passes were controlled by the computer.
Clearly, rejection is a very big deal to us humans.
Even the most trivial and unimportant slights can cause us real hurt.
But why is this so?
The answer seems to be that we have a primitive and automatic sensitivity to rejection genetically hard-wired into us.
For our ancestors, rejection by the tribe would have posed a very real threat to survival.
Those ostracised would lack protection and shelter and would have difficulty procuring food.
Clearly then, it would make sense for us to have evolved a strong negative reaction to rejection - a sharp twinge of psychological pain that prompts us to get back on good terms with our neighbours as quickly as possible.
Psychologist C. Nathan DeWall at the University of Kentucky put it like this: "Humans have a fundamental need to belong.
"This need is deeply rooted in our evolutionary history and has all sorts of consequences for modern psychological processes."
When we feel that we are being rejected, we experience an immediate, unpleasant reaction.
This is instinctive and makes itself felt even when, intellectually, we realise that the rejection is of little importance.
The 19th century German philosopher, Arthur Schopenhauer, had it right, it seems, when he wrote: "To be shocked at how deeply rejection hurts is to ignore what acceptance involves.
"We must never allow our suffering to be compounded by suggestions that there is something odd in suffering so deeply.
"There would be something amiss if we didn't."
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