By ANDREW JACOBS


SHENZHEN, China - The thieves often strike at dusk, when children are playing outside and their parents are distracted by exhaustion.

Deng Huidong lost her 9-month-old son in the blink of an eye as a man yanked him from the grip of his 7-year-old sister near the doorway of their home. The car did not even stop as a pair of arms reached out the window and grabbed the boy.

Sun Zuo, a gregarious 3-year-old, was lured off by someone with a slice of mango and a toy car, an abduction that was captured by police surveillance cameras.

Peng Gaofeng was busy with customers when a man snatched his 4-year-old son from the plaza in front of his shop.“I turned away for a minute, and when I called out for him he was gone,”Mr.Peng said.

These and thousands of other children stolen from the teeming industrial hubs of China’s Pearl River Delta have never been recovered by their parents or by the police. But anecdotal evidence suggests the children do not travel far. Although some are sold to buyers in Singapore, Malaysia and Vietnam, most of the boys are purchased domestically by families desperate for a male heir, parents of abducted children and some law enforcement officials who have investigated the matter say.

The demand is especially strong in rural areas of south China, where a tradition of favoring boys over girls and the country’s strict family planning policies have turned the sale of stolen children into a thriving business.

Su Qingcai, a tea farmer from the mountainous coast of Fujian Province, explained why he spent $3,500 last year on a 5-year-old boy.“A girl is just not as good as a son,” said Mr.Su, 38, who has a 14-year-old daughter but whose biological son died at 3 months.“It doesn’t matter how much money you have. If you don’t have a son, you are not as good as other people who have one.”

The centuries-old tradition of cherishing boys - and a custom that dictates that a married woman moves in with her husband’s family - is reinforced by a modern reality: Without a real social safety net in China, many parents fear they will be left to fend for themselves in old age.

The Chinese government insists there are fewer than 2,500 cases of human trafficking each year, a figure that includes both women and children. But advocates for abducted children say there may be hundreds of thousands.

Sun Haiyang, whose son disappeared in 2007, has collected a list of 2,000 children in and around Shenzhen who have disappeared in the past two years. He said none of the children in his database had been recovered.“It’s like fishing a needle out of the sea,”he said.

Mr.Peng, who started an ad hoc group for parents of stolen children, said some of the girls were sold to orphanages. They are the lucky ones who often end up in the United States or Europe after adoptive parents pay fees to orphanages that average $5,000.

The unlucky ones, especially older children, who are not in demand by families, can end up as prostitutes or indentured laborers. Some of the children begging or selling flowers in major Chinese cities are in the employ of criminal gangs that abducted them.“I don’t even want to talk about what happens to these children,”Mr.Peng said, choking up.

Here in Shenzhen and the constellation of manufacturing towns packed with migrant workers, desperate families say they get almost no help from the local police. Several parents, through their own guile and persistence, have tracked down surveillance video images that clearly show the kidnappings in progress. Yet even that can fail to move the police, they say.“They told me a face isn’t enough, that they need a name,”said Cai Xinqian, who obtained tape from a store camera that showed a woman leading his 4-year-old away.“If I had a name, I could find him myself.”

The reluctance of the police to investigate such cases has a variety of explanations. Kidnappers often target the children of migrant workers because they are transients who may fear the police and whose grievances are not treated as priorities.

Moreover, the police in China’s authoritarian bureaucracy are rarely rewarded for responding to crimes affecting people who do not have much political clout. Mr.Peng said the police preferred not to even open a missing person’s inquiry because unsolved cases made them appear inefficient, reducing their annual bonuses.

There are exceptions. In a number of high-profile cases, the police have cracked down on trafficking rings and publicized the results. But such help remains rare, parents say.

Mr.Peng says that boys’abductions are a growing problem that only the central government can address. He and others have been agitating for the establishment of a DNA database for children and stronger antitrafficking laws that would penalize people who buy stolen children.“If the government can launch satellites and catch spies, they can figure out how to find stolen children,”said Mr.Peng, who helps run a Web site called Baby Come Home.

Mr.Su, the tea farmer, and his wife Zhou Xiuqin were still in mourning in October 2007 when they spotted a child at a Buddhist temple in their village, Dailai, a picturesque hamlet of 800 people.“The boy was eating candy like he was hungry,”Mr.Su recalled.“Everything he was wearing was too small for him.”

A man with the boy claimed to be his father. He said that he was from a nearby town and had three sons, but that he needed money to take his ill wife to the hospital.“I asked how much,”said Mr.Su, an earnest man who works long hours in a clothing factory when he is not tending his tea plants.

After some quick bargaining, the price was dropped to $3,500 from $4,100, and a few hours later, after borrowing money from friends and family members, they took the boy home. They named him Jiabao, which means “guarantor of the family.”

Their love for their new son was boundless. They did not think much of the fact that Jiabao did not understand the dialect spoken in that part of Fujian and seemed indifferent to the local cuisine. Mr.Su insisted that he never imagined that the boy had been stolen.

Last August, Mr.Su learned the truth after the police in Sichuan Province arrested the man who had sold them the child. Their son and the three other stolen children in their village were taken away by the police. The couple was inconsolable.“We were lied to, we were swindled,”Mr.Su said as his wife’s eyes welled up.

There was, however, a small consolation. A sympathetic policeman in Sichuan, the province where the boy was stolen, helped put them in touch with his birth parents. The two couples have since been in frequent contact; Mr.Su said the real parents held no grudge, acknowledging that the family had cared for their son well.

The father was so grateful, he told Mr.Su he would be on the lookout for local families who had two sons but were too poor to care for them.“He said that way I don’t need to deal with child traffickers anymore,”Mr.Su said.

Deng Huidong’s son was taken from her daughter’s grip. Su Qingcai and his wife, Zhou Xiuqin, left, paid $3,500 for a boy who had been stolen.