More than half of schoolboys here have hit, punched, shoved or spat on another boy while in secondary school. More than four-fifths have taunted another for being "girly" or not being "manly" enough.
These were found in a survey of 809 teenagers aged 17 and 18 by women's rights group Aware.
The results, said Aware researcher Chong Ning Qian, are alarming. She found that those who were teased are more likely to go on to bully other boys.
Aware decided to survey boys after a campaign to end violence against women found that men, too, "were victims of violence because of gender stereotyping".
From October 2015 to March last year, it surveyed students from junior colleges, polytechnics and Institute of Technical Education colleges in a ratio representative of the population.
The boys were asked whether their peers told them to "man up", "stop being such a girl" or to not cry - a practice Aware calls gender policing -and whether they experienced or committed violence.
A whopping 84 per cent admitted to verbally bullying another, by insulting a boy for being feminine or weak and calling him a "sissy" or "gay".
Harmless fun - or bullying?
What looks like harmless fun to one, may be bullying to others.
For example, the schoolboy ritual called taupok - where a group of boys jump on top of a target, flattening him to the ground - is classified as physical violence by women's group Aware. But for some, it could be part of "an occasion where someone is celebrating something like a birthday", said Singapore Children's Society chief executive Alfred Tan. He said he would not classify a one-off incident as bullying.
But for child psychologist Carol Balhetchet, the main factor is whether the person feels victimised and had asked for the act to stop but was ignored.
"Children's sense of fun - especially for boys up to the age of 16 and some even older - is to see how far they can get away with what they see as poking fun at another," said Dr Balhetchet.
According to previous studies by the Singapore Children's Society, 20 per cent of primary school pupils were bullied. That rises to 25 per cent for secondary school students who were bullied.
While 24 per cent of bullied boys experienced physical bullying, only 5 per cent of girls were victims of physical bullying. Girls were more likely than boys to be excluded socially and to have rumours spread about them.
Percentage of teenage boys surveyed who had hit, punched, shoved, or spat on another boy while in secondary school.
Percentage of teenagers surveyed who had verbally abused another boy, by teasing or insulting him for being feminine, while in secondary school.
A lower - but still significant- proportion resorted to physical bullying: 29 per cent gave a schoolmate a wedgie by yanking his underwear; 54 per cent hit, punched, shoved or spat on another; and 69 per cent took part in a practice called taupok where boys pile on a target.
Aware then compared the experiences of the respondents and concluded that those told by peers to "man up" were four times as likely to then physically bully other boys.
Said Ms Chong: "In our survey, the vast majority have been gender-policed and they are more likely to commit violence. That's a very big group of people."
A 2006 survey of secondary school boys and girls on bullying found that 25 per cent had experienced bullying - defined in that study as happening at least twice every single month over a span of one school year.
However, Mr Alfred Tan, chief executive of Singapore Children's Society, questioned if the acts reported in the Aware survey were meant to cause harm.
He said: "If they're doing it because they are reacting to something, like if they were playing basketball and jostling, I don't classify that as violence. Boys tend to be a bit more physical. To be violent you have to have the intention to harm someone."
But for those at the receiving end of such treatment, the taunting was traumatic, said writer Daryl Yam, 26, who described himself as an "effeminate boy".
That led to name-calling, mean messages left in his locker, rumours of liking other boys and even molestation. "I was in the canteen and queueing for food. A bunch of rugby players taunted me and put their hands on my bum but I couldn't do anything about it.
"There is this incredible pressure to be a man, or to be masculine. Somehow, you have to have power over someone else," he said.
Aware is holding a dialogue on how boys use violence to enforce gender norms today at 7pm at its centre.