BEIJING—On April 10, 2010, the Liu family was living the Chinese dream. The couple had moved to the city, rented an apartment, and were blessed with two beautiful children. They weren't rich, but they were getting by. Like many Chinese people, they felt their lives were getting better.

The next morning, strange men came to their house, grabbed their son Liu Jingjun, dragged him into a white van, and drove off. Since then, the Lius have been looking for him. They haven't found him, but they have discovered that there are an awful lot of people just like themselves.

Since at least the 1980s, kidnapping and human trafficking have become a problem in China, and most often, the victims are children. Estimates vary on just how bad things have gotten. The Chinese government reports that fewer than 10,000 children are kidnapped each year, but the U.S. State Department says it's closer to 20,000. Some independent estimates put the number as high as 70,000 (compared with 100 to 200 children kidnapped per year in the United States, for example).

The vast majority of kidnapped children will never see their families again. In China, kids are abducted not for ransom but for sale. Often, they come from poor and rural families -- the families least likely to be capable of tracking their kids down or fighting back. Some children are then sold to new "adoptive" families looking for children. Others are sold into slave labor, prostitution, or a life on the streets. In some cases, healthy children are brutally crippled by handlers on the theory that a child with broken legs or horrific boils looks sadder and can earn more money begging on the street.

Some children are even sold into adoption overseas. Chinese adoption agencies seeking the substantial donations foreign parents make when they adopt -- in some cases, as much as $5,000 -- have been known to purchase children from human traffickers, though these cases appear to be relatively rare.

Most instances of kidnapping are perpetrated by gangs that are large, national, and highly organized. Based on cases solved by Chinese police, it's not uncommon for some kidnapping rings to have dozens or even hundreds of members, and to be responsible for the kidnappings of hundreds of children.

The estimates vary so widely because official numbers are hard to come by and harder to trust. Pi Yijun, a professor at the Institute for Criminal Justice and an expert on crimes involving children, says, "Data about the dark side of society is extremely difficult to obtain, and even when it is made public, the Public Security Bureau [i.e., the police] only reports based on the number of cases they've uncovered." That means that China's official statistics on kidnapping are based only on cases that are proved to be crimes. Because most parents have no proof that their child was kidnapped (rather than running away on his or her own), many cases are filed as missing-person reports and thus go uncounted in official statistics.

On the morning of April 11, 2010, Mrs. Liu was in the apartment, but she and her husband's son -- just 2 at the time -- had wandered out the door and was playing with some other kids from the neighborhood. When she looked out the door and saw he was missing, she called her husband, and when the two of them still couldn't find the boy, they called the cops.

The police came. "They said nothing," said Mr. Liu. "They said, 'It's not urgent; just relax. Maybe he ran away by himself or he's at a neighbor's house. Just look around yourselves.'"

Other parents of kidnapped children say this is common; unless there's concrete, immediate proof of a kidnapping, police won't even accept a missing-person report until the subject has been missing for at least 24 hours.

Mrs. Zhu, the mother of kidnapped 12-year-old Lei Xiaoxia, who went missing on May 24 in Shanxi province, reported her daughter's case to anyone who would listen -- three different police authorities, her daughter's school, and even their city's education bureau -- but always got the same answer. "They said, 'All we can do is investigate for you; there's nothing we can really do [otherwise]."

These investigations leave much to be desired. Mrs. Zhu told me, "After we reported [the disappearance], they went out and patrolled for a bit, but after that we never saw them looking again." They also never went to the train station or the bus station to check the surveillance tapes, she said. Later, a reporter discovered that Zhu's daughter had been seen at school that day, but the police had also forgotten to check the school's security tapes, which had since been automatically deleted by the surveillance system. Lei Xiaoxia is still missing.

Even when police do investigate seriously, happy endings are rare. Trafficking gangs are highly organized. Children are moved over great distances and shuffled between handlers after they're kidnapped to ensure they are impossible to trace.

Li Yong, an adult who was kidnapped around 1988 and sold to another family when he was about 5 (he's not sure of his real age or birthday), remembers he was moved around quite a bit. "After I was kidnapped, I was taken into cars, a long-distance bus, and a train," he says. Years after his kidnapping, police finally tracked down one man involved with Li's kidnapping, but the trail ended there. The seller, the kidnapper, and the handlers who watched Li during various stages of his journey have yet to be found.

Investigating kidnapping cases effectively requires sustained effort, ongoing cooperation between numerous local precincts, and high-tech methods of tracking and identifying both kidnappers and children. When a particular gang gets onto the police's radar, higher authorities may help organize this sort of sustained effort, and when caught, human traffickers face stiff sentences and even the death penalty. But many kidnapping cases never make it past the local precinct, where they're filed as missing-person cases and, generally, forgotten.

Some parents accept their fates and wait quietly for a phone call from the police that will probably never come. But more and more parents are taking to the Internet and to the streets to search for their children.

"We look every day," Mr. Liu, a contract laborer, said. "Before my son was taken, I didn't know how to use the Internet, but now I go to an Internet cafe every day. I can't afford my own computer, so I go there to look for my son, making posts about him and searching through the Net."

An entire ecosystem of Internet services has sprung up for Chinese parents like Liu. Sites like "Baby Come Home" collect information, photos, and other data from tens of thousands of parents and help them publicize it all. They also collect photos and reports of street children for parents of kidnapped kids to browse, looking for their children.

Many parents also take to the street. Mr. Liu connected online with other parents of kidnapped children in his area, and now they organize events together. One of the parents, whose son was also kidnapped, has decorated his truck with photos of his son and dozens of other missing children. The parents pick a busy street corner, park the truck there, surround it with large posters about their children, and hand out fliers and cards to passers-by.

When he can, Liu brings his young daughter along to these events, where she helps her parents pass out flyers. She's too young to understand what happened to her little brother, but she hasn't forgotten him. She dreams about him, her father says. "When we hear her talk about him, it's devastating."

Some have placed the blame for China's child-trafficking problem squarely at the feet of the one-child policy, but that's an oversimplification according to Pi Yijun, of the Institute for Criminal Justice. Part of the problem is that compared with other things one might steal, such as cars or computers, children are easy to get ahold of and difficult to track, he says. "Additionally, if [the kidnapper] has got a buyer already, they can reap the rewards quickly, and I think that's an important reason" that kidnapping is so common in China.

Of course, without buyers there would be no sellers, and there are still buyers aplenty in China. True, the one-child policy has made children scarcer, but because families with more than one child -- regardless of whether the children are adopted or birthed -- must pay fines, there's no real reason for healthy parents to choose to purchase a kidnapped child rather than just having another one of their own. Often, the buyers of kidnapped kids are married couples who can't conceive or who have given birth to only daughters and want to be sure their next child is a son. Some families also buy older girls as brides for their sons if the son can't attract a wife through traditional means (often because of some mental or physical disability).

China's culture of silence also plays a role. "My son will never know he was kidnapped and purchased," Mr. Liu says. "In our hometown, when people buy wives, no one says anything. No one talks. Our child was too young to understand what happened to him; when he grows up he won't understand that it's all fake."

This is not an uncommon phenomenon. After Li Yong was kidnapped and sold to his new family in Jiangsu, he walked around telling neighbors his original name and asking to go home, speaking in a dialect foreign to that province. But no one reported anything to the authorities until more than a decade later, and by then, it was way too late. Many Chinese believe that getting involved in someone else's business is asking for trouble, and in some rural areas where education levels remain low, purchasing children is still considered an acceptable alternative for couples who are infertile or too old to conceive safely.

For his part, Mr. Liu doesn't blame the men who kidnapped his son. "We parents, the parents of lost children, hate these people, and society hates them too, but sometimes you can't blame human traffickers. Sometimes you have to blame our society. What I mean is, [in China] we still don't have a strong rule of law. If it were stronger, could this kind of thing happen?"

Mr. Liu and his wife are still searching for their son. Mrs. Zhu and her husband are still searching for their daughter. They work when they need to, but their lives are on hold until they get some news, just like the tens of thousands of other parents nationwide who are searching. "It's like we're living with dead hearts," Mrs. Zhu told me between sobs. "If we can't find our child, life is meaningless."