Three friends walk into a bar.
One instantly turns to a group of strangers and launches into boisterous conversation. Another sits back, momentarily, before joining in (while longing for something more intimate).
The third, in an attempt to mask a spiralling mind, has their head down and appears disengaged.
It's a vicious cycle.
"Someone with social anxiety disorder will go into that social situation already being primed by thinking 'no one's going to talk to me,' or 'they're going to think I'm boring.' So when they get into that situation, they are already in an anxious state," Professor in Clinical Psychology Kim Felmingham told The Huffington Post Australia.
"They are more aware of their anxiety and therefore they are less engaged with the actual conversation. Inadvertently, they can appear disengaged to others who then may actually remove themselves from that interaction.
"It's a vicious cycle."
The distinction between introversion, shyness and social anxiety is one that frequently comes into question in clinical psychology -- for they can easily be misconstrued.
"People will vary on the spectrum. There is certainly a dimensional nature to how much shyness or introversion a person may have and at what point that may become a clinical disorder," Felmingham said.
And there's no simple answer.
Social Anxiety Disorder: what is it?
Social anxiety disorder (or SAD) is characterised by a fear of criticism or rejection by others. It is diagnosed when this fear becomes chronic and debilitating.
'People with SAD develop very negative cognitions about themselves in social situations," Felmingham said. "That is the driving factor that leads them to fearing negative evaluations from others."
And it goes beyond feeling shy or introverted.
This can go on chronically for many years where people are really socially isolated and may turn down any interaction that they come across. It can cause heightened distress and functional impairment.
"The fundamental line of difference is that shyness and introversion are normal personality styles or traits," Felmingham said.
"A person who is shy may feel uncomfortable if they are in the limelight, or an introvert may not particularly like loud conversation. But that doesn't necessarily cause them significant stress and can often be viewed positively."
SAD can be characterised by severe levels of anxiety that can lead to avoidance of social situations.
"This can go on chronically for many years where people are really socially isolated and may turn down any interaction that they come across. It can cause heightened distress and functional impairment."
You may be a non-anxious introvert -- whereby your perception of time spent in solitude is fulfilling and stimulating -- or you may be an anxious introvert.
"Not everyone who is an introvert has social anxiety. Shyness and introversion are really quite common. They are not always equatable," Felmingham said.
And when it comes to monitoring shyness, the line becomes blurred. "SAD most commonly emerges in late teens to early 20's and many people who do present in clinics report that they have always been shy," Flemingham said.
How much stress is this causing the person? If it is really intense anxiety with a lot of avoidance, then it requires treatment.
"If you are reaching the high end of shyness that might be associated with some anxiety and more negative emotions, that can be a risk factor that contributes to the development of SAD."
According to Flemingham, it comes back to functional impairment.
"How much stress is this causing the person? If it is really intense anxiety with a lot of avoidance, then it requires treatment."
How can it be treated?
Due to the role of cognitive processes that maintain social anxiety, cognitive behavioural therapy (CBT) is a proven treatment.
"We train people to identify their negative thoughts about themselves and to challenge them, as well as their pre and post- event processing of social situations," Flemingham said.
"We perform behavioural experiments, whereby in a gradual way, we get people to overcome their avoidance and go into social situations to really test out their beliefs."
"These treatments may take longer if patterns are entrenched, but they are effective."
If you need help in a crisis, call Lifeline on 13 11 14. For further information about social anxiety disorder contact beyondblue on 1300224636 or talk to your GP, local health professional or someone you trust.
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