“The incident when it happened was resolved quickly by the Chinese in a way that was drastic and made very clear that the Chinese would not tolerate trafficking,” said Ms. Cox, of Holt International. “I’m not saying there are not any other incidents, but people can be assured that the process in China is a good one.”

A 2010 State Department report said there were “no reliable estimates” of the number of children kidnapped for adoption in China, but cited Chinese news media reports that said the figure might be as high as 20,000 children a year, most of whom are adopted illegally within the country, especially boys.

But it is hard to know, said David Smolin, a professor at the Cumberland School of Law at Samford University in Birmingham, Ala., who has written extensively about international adoption and trafficking. Changes in China in the early 2000s — a rising standard of living, an easing of restrictions on adoption within the country, more sex-selective abortion — meant that fewer families abandoned healthy babies, Professor Smolin said.

“Orphanages had gotten used to getting money for international adoption,” he said, “and all of the sudden they didn’t have healthy baby girls unless they competed with traffickers for them.”

PROFESSOR SMOLIN has two daughters, whom he and his wife adopted from India as teenagers. Within six weeks the girls disclosed that they had been kidnapped from their birth parents. But when Professor Smolin and his wife tried to find the girls’ biological parents, he said, no one wanted to help.

When he started to speak publicly about his experience, he met other parents in the same situation — hundreds of them, he said. “They all said they felt abandoned by adoption agencies and by various governments,” he said. “There’s a sense that other people in the adoption community did not want to hear about these circumstances. People were told that it was not a good thing to talk about. So you’re left alone with these practical and moral dilemmas, and that is overwhelming.” In the end, it took more than six years for the couple to find their daughters’ birth parents, by which time the girls were young adults.

Susan Merkel, 48, who with her husband adopted their daughter, Maia, at 9 months old in August 2007, said that even within their own home, her husband did not like to talk about the possibility.

“My husband really feels like it’s something that we don’t know whether that’s the case and would rather not think about it,” she said at her home in Chesterfield, N.J.

But for Ms. Merkel, who is studying social work at Rutgers University, the uncertainty is haunting. Her daughter’s orphanage, in Hubei Province, which is immediately north of Hunan, is near an area known for strict enforcement of the one-child policy, and Ms. Merkel said she could not shake the possibility that a population-control official had seized her and turned her over to the orphanage.

Ms. Merkel was adopted as a child, and said that meeting her birth mother had helped her understand her past and herself. What, then, was her responsibility as a parent — to find Maia’s birth parents, who might make a valid claim for her return? How could Ms. Merkel, who got so much out of meeting her own birth mother, not want that for her child? “What I do know is that she’s my daughter and I love her,” she said. “We’re giving her the best family and life that we can. And if she has questions someday, we’ll do all we can to help her find the answers.”

Ms. Merkel said that she would support Maia’s meeting her birth parents if it was possible, but that she would not willingly return her to them, even if there was evidence that she had been taken.

An area in Hunan Province in China was said to be a source for stolen children. THE NEW YORK TIMES

“I would feel great empathy for that person,” she said. “I would completely understand the anger and the pain. But I would fight to keep my daughter. Not because she’s mine, but because for all purposes we’re the only family she’s ever known. How terrifying that would be for a child to be taken away from the only family she knows and the life that she knows. That’s not about doing what’s right for the child. That’s doing what’s right for the birth mother.”

BRIAN STUY, an adoptive father of three in Salt Lake City, runs a service called Research-China.org to help adoptive families learn about their children’s origins. When he has managed to contact birth parents, he said, most were content to learn that their children were alive, that they were healthy and in good homes. “Unfortunately, the reaction of most adoptive parents is to go into hiding,” Mr. Stuy said. “When they have suspicions, they don’t want to come forward.”

Many parents simply never have suspicions. Tony X. Tan, an associate professor of educational psychology at the University of South Florida whose research specialty is adoption, surveyed 342 adoptive parents of Chinese children last month. Two-thirds said they “never” suspected that their children might have been abducted, and one in nine said they thought about it “sometimes.” Several said the paperwork from the orphanages was inconsistent or suspicious.

One mother, who adopted two girls from different provinces, wrote, “My Guangxi daughter was adopted with a group of 11 other infants, all roughly the same age, and came home with an extremely detailed description of her first 11 months of life in her orphanage. Yet ‘her’ information was word-for-word the same as the info given the families of the other 11 children adopted at the same time — making it all too specific to be believable.”

Judy Larch, a Macy’s executive who lives in Pelham, N.Y., said she adopted two girls from China, in 2001 and 2007, because she had heard good things about the program, and because she could adopt as a single woman. Though she has read about trafficking, she said, “I’ve never had any doubts or concerns about their adoptions.” She said she had faith in the adoption agency, Holt International.

Such faith is small comfort to a woman named Ms. Chen, who said population-control officials in her hometown, Changle, in Fujian Province, took her daughter in 1999. Ms. Chen, who is in the United States illegally, applied for asylum as a dissident this year, but was denied. She declined to speak to The Times, but gave permission for a reporter to watch a videotaped interview conducted by a Christian group in Flushing, Queens, called All Girls Allowed, which works with women’s rights groups in China and maintains a database of photographs of missing children. Her story could not be corroborated.

In the interview, Ms. Chen said that her first child, born in 1997, was a girl, and that she was under great pressure from her in-laws to produce a son. She became pregnant soon afterward, but this child, too, was a girl. Ms. Chen was in violation of the one-child law, which in her area allowed parents to have a second child after six years. Officials came to her with a choice: give up the second child — then 5 months old — or undergo tubal ligation.

“I was holding my daughter and crying,” she said on the video. The official told her that if she gave up the child, in six years she could try again to have a son, she said. “I was afraid for my marriage,” she said. “Of course I didn’t want to give up the child. But I was afraid that without a boy my marriage wouldn’t last.”

She said, “I handed her over meekly.”

MR. MAYER, in Montclair, who also has an adopted son from Ethiopia, has accepted that he may never know the full truth about his daughter’s beginnings.

After absorbing the revelations about trafficking, he said, he took a step back. “O.K., what does this mean to my life today? And how does it change my life today?” he said he asked himself. “And today it changes absolutely nothing about my life with Keshi. If I want Keshi to be able to question and to come to terms with the issues of why she would have been put up for adoption in the way she was, she’s going to ask these questions. This is just another one of those questions to which I don’t have a concrete answer. That’s my role as a dad.”

In the future, families like his may have better answers. Parents or children may be able to search online databases of children whose birth parents say they were taken. For now, though, is it the parents’ duty to ask those questions? Or is it for children to decide, in time, how much they want to know?

“I can’t change the past or change whatever anybody has done in China,” Mr. Mayer said. “What’s most important to me is there are real significant issues for my daughter coming of age and understanding her birth story. And I’m committed to supporting her in that and making sure that it’s as honest and truthful and supportive as possible. And that’s a scary thing.”

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