In China earlier this year a spate of violent attacks by intruders who targeted schoolchildren shocked the country.
China's response was to step up security around schools.
But one academic in Shanghai believes the attacks may have been a symptom of a more serious issue.
The upheaval of the Cultural Revolution in the 1960s and 70s, he says, could have created a generation who see violence as the only answer to their problems.
If he is right it could be that as that generation hits middle age, and many of them start to feel left behind in China's rapidly changing society, they are hitting out.
The violence can come suddenly.
In September in a Shanghai suburb, the Yangpu district, an argument between an apartment's security guard and a visitor over the fee for storing his bicycle turned into a fight.
The visitor called other relatives for help. In the struggle the security guard stabbed and killed three of them.
Some of those in the crowd then turned on the guard and killed him.
"If one of them had backed down a little none of this would have happened," says a resident of a nearby building, who declined to give his name.
"If only one of them would have let it go when they were still shouting. If only one of them would have conceded a little. It escalated because none of them would show a bit of tolerance," he says.
But how does rage turn people with no history of violence into murderers?
The series of attacks on schools in different parts of China that started in March left 21 dead and more than 90 injured.
Just like the security guard, most of the attackers were people who hadn't been thought of as violent - they had just snapped.
At Shanghai's Xiang Yin kindergarten, they have a high metal gate, a security guard armed with a hook to tackle a knife attack, and a police car parked outside as the children are dropped off by their parents and grandparents.
It is a model kindergarten. The Shanghai government arranged for us to visit, to demonstrate what they are doing to ensure pupils here are protected.
It has smart, bright and airy facilities that would be the envy of many head teachers in the West.
But the head of this kindergarten has a monitor on her desk showing feeds from 16 CCTV cameras. Any intruder would be spotted in an instant.
In the infants' classroom a teacher is going through the safety drill.
"If a bad man comes into our kindergarten what should we do?"
"Go to the bedroom, then the bad man won't be able to go inside," one of the children replies.
"After we go in we can lock the door," another one adds.
Head teacher Zhu Guo Fang says after this year's attacks the school has little choice but to teach the children to try to protect themselves.
"The reason schools and kindergartens are easily targeted is because children are small and weak," she says.
"It's easier to attack them, they're more vulnerable. I cannot say I am 100% confident we will be safe with all these security measures but they help."
Security measures like those at this school are expensive.
Pattern to attacks
Many schools across the country can't afford them, so the authorities need to find out why some Chinese people resort to sudden and extreme violence.
At Fudan University in Shanghai, Professor Sun Shi Jin, a psychologist has noticed a pattern in the violent incidents.
Nearly all the attackers, he says, were born before or during the Cultural Revolution, the violent mass movement that started in 1966 and lasted a decade.
"My belief is that those who lived through the 60s, who lived under an autocracy, who had no way to rebel at the time, they feel today the only choice they have when things go wrong is to turn on the vulnerable," Prof Sun says.
In contrast, he says, those born later when they lose their dignity or they start to wonder why life matters, choose to kill themselves.
The professor admits his theory needs more work, but argues that other explanations for the causes of violence here, such as alcohol or increasing inequality in society, don't really provide all the answers.
"Even a drunk man's behaviour has something to do with his or her character when he's sober," he says.
"If he or she is decent there's little possibility they will carry out violent attacks."
There have been fewer attacks on schools in recent months, or at least the state-run media have covered fewer of them, but pick up any newspaper here and you'll find reports of violence, within families and in the street.
It seems individuals with problems are not spotted before they cause crises.
If the professor is right there could be many people who grew up in more violent times and were damaged, who have the potential to explode into violence at the smallest provocation.