Sex crimes by minors are increasing because of pornography and a breakdown in family values, a judge has said.

The number of juvenile cases involving prostitution, sexual abuse and rape has grown since 2002, records from Beijing's Haidian District People's Court showed.

Court Judge Wang Lijuan said 70 percent of the teenaged criminals, most 16 or 17, had not graduated from high school. About 60 percent came from broken homes.

In 2005 there were five cases of rape, compared with 11 last year, she said.

Wang cited the case of one 16-year-old who constantly watched porn films after his mother went abroad and his father was too busy to take care of him.

In time he wanted to put into practice what he had watched and raped a woman, Wang said the youngster told her. He was imprisoned for the offense.

"More than 90 percent of the young criminals involved in rape cases admitted pornographic websites, books and films had influenced them," Wang said.

"Teenagers have plenty of curiosity but little self-control when exposed to obscene material."

Another factor behind the increase in sex crimes by minors is the earlier onset of puberty, Li Bian, deputy director of the AIDS Prevention Education Project for Chinese Youth, said.

A recent survey suggested the average age of puberty has dropped from 13 to 11.

Both Wang and Li called for better care at home and more education at school.

"Parents should provide not only material security for their children, they also need to care and be more enlightened when it comes to the proper transmission of sex knowledge," Wang said.

Li also called for more sex education classes and instruction in the law.

"Young people's knowledge of sex in China lags behind their physical growth," Li said. "Schools are the most important place for the prevention of sex crimes."

Tong Lihua, director of the minor protection law committee of the All China Lawyer's Association, called on the government to control the flow of pornography.

"Porn magazines and related publications should be destroyed and movies (with adult content) should be managed according to ratings," Tong said.

"All video shops renting out porn movies and illegal Internet cafes should be shut down."