Being in the middle of a group of laughing friends is usually a happy experience, but for those with an unusual phobia it is anything but. They are people with gelotophobia - a fear of laughter.
Drummond* (all names have been changed), 18, from the US, told researchers: "I hear people laughing and I assume they are laughing at me. I tense up and get ready for a fight, I can feel the adrenaline.
"I hardly ever talk or do anything that could cause me to be laughed at. I remain stone-faced most of the day.
"I see other people having fun. Sometimes I want to change and be like them.
"But I don't want to get there and be made fun of because I am different."
His description of his condition was recorded by Dr Tracey Platt, at the University of Zurich, in Switzerland.
She is part of a body of scientists across the world, including Africa, Canada, India and Russia, trying to understand what causes the condition.
Gelotophobes either do not understand what laughter is, or they think it is directed at them in a negative, malicious way and feel scared when they hear it.
They often find being around people difficult, and may suffer stress headaches, dizziness and bouts of trembling in social situations.
Chukar, 37, is from Israel. He says he feels "ashamed" and "embarrassed" when he hears people laugh.
"When I hear laughter I feel above average to extreme anger which starts as a visceral reaction that would last for hours, and sometimes days. I also get extreme body tension, and headaches."
Chukar says he sidesteps social situations in favour of reading and playing solitary sports.
Avoided 'like plague'
He says: "When others sought to bait me and use me as the butt of their jokes to impress their friends then I would fight.
"I have got into few fights in my life, but when I did my target usually got badly hurt and the rest of the group avoided me like the plague."
Working in a busy office can seem an insurmountable challenge if every time someone laughs, it feels like a personal attack. People with the condition can be limited in the sorts of jobs they can get.
Long term, the phobia can lead to anxiety and low self-esteem. It is also associated with depression.
Gelotophobes can struggle to make friends, find love, or form lasting relationships.
"I am alone and cope alone," says Chukar.
Legacy of bullying
Academic research into people with gelotophobia started in 2008, so it is still largely a mystery exactly what lies behind it, meaning treatment is relatively limited.
Dr Platt says the causes are likely to be a child's environment, how their personality develops, their school life, social life and their own humour.
She says many gelotophobes report being bullied at school.
"The question is what came first? Does someone have a disposition that makes them sensitive and there is a mismatch, which means they feel they are being bullied in the first place?"
Or it could be a straightforward response to bullying, she says.
Dr Platt believes the phobia would not suddenly develop at the age of 30 and also that there is a link to Asperger syndrome.
Dr Platt works to understand what facial expressions are linked with feeling afraid of laughter.
After filling out online questionnaires, her volunteers come into the lab in Zurich.
She uses avatars to show them a range of facial expressions, to see exactly when a smile becomes upsetting.
"It may be that gelotophobes can be reprogrammed," she said.
But they are "not at the point where they can test treatment", as the field is so new, she adds.
Dr Platt says she wants the condition to be easily identified by therapists, who can then help to shape a patient's treatment in future.
Britain has the highest prevalence of people with the phobia, says Dr Platt, likely due to its culture of humour, with 13% of the population gelotophobic to some extent.
It leads the world for extreme gelotophobia, with 1% of the UK population categorised as having a pathological fear of laughter, which impacts greatly on their daily life.
There are also higher numbers in some parts of Asian culture, she says, where shame could be used as a form of control.
"But Denmark has the lowest gelotophobia in society, as laughing at somebody else - you just don't do it.
"It is seen as very wrong to laugh at another's misfortune," she added.
Just under 2% of Denmark's population have the condition.
'Index of problems'
Prof Sophie Scott at University College London is researching the neuroscience of voices, speech and laughter.
She said: "I do not think you can play down the importance of laughter. It is absolutely endemic."
Prof Scott said laughter had a key role in helping people to deal with negative emotions, and to feel calm and cheerful, that would not be available to gelotophobes.
She added: "You can imagine it would be highly unpleasant if you could not join in with laughter or react in a nice way."
Prof Scott said being gelotophobic could be an index of people's social problems or personality traits rather than laughter.
Work is ongoing to focus on what was happening in the brains of children with disorders which can result in gelotophobia, to understand its neurological basis, she said.