Kate St. Vincent Vogl first learned she should be ashamed when her sister got into a fight with a neighbor boy who retaliated, "Yeah, well, you're adopted!"
Her sister ran home and confronted their mother, who admitted it was true. The news that both were adopted stunned Vogl, now a 43-year-old writer from Minneapolis.
"I took one look at her blond hair and blue eyes and said, 'Maybe you are, but I'm not,'" she said. "I'd seen Sesame Street. I knew which one didn't belong. It was as if I knew already at age 8 that being adopted was not something you'd want to be."
Vogl is one of an estimated 6 to 8 million Americans who are adopted. Many now tell the Evan B. Donaldson Institute that they were stigmatized as children and struggled with their identity and self-esteem well into adulthood.
The study, "Beyond Culture Camp: Promoting Positive Identity Formation in Adoption," examined two adult groups -- Korean-born adoptees and white adoptees, but the findings have relevance to adoptees of all races, according to executive director Adam Pertman.
"This is the biggest and deepest study of its kind," he told ABCNews.com. "There are big universal truths and we are finally getting at some of them to help us be better parents, better professionals and people who grow up to be healthy human beings."
Many of the 468 adult respondents said they experienced discrimination. More white adoptees (35 percent) than Korean (21 percent) indicated teasing simply because they were adopted.
About 86 percent said they had taken steps to find their birth parents and that finding them was the single factor that helped them gain a positive adoptive identity.
"We would never deride people based on religion or a handicap or another piece of who somebody is," Pertman told ABCNews.com. "But that's what too often happens to people in the adoption world."
"There is institutional discrimination in our culture," Pertman said. "We watch TV shows and some kid is adopted and they'll say he's a bad seed and suspect from the get-go. Or when people ask, 'What happened to his real parents?' How do you think that makes kids feel?"
Pertman hopes the study will help promote laws, policies and practices to give adoptees greater access to information about their birth and to help erase stereotypes to improve their lives.
For Vogl, knowing she was adopted didn't change the love she felt for her family, but she also learned it was the "elephant in the corner, not to be talked about.
"I suppose you could say that adoption-related discrimination began from the moment adoption became a part of my life," she said. "My sister then dialed up all of her friends to confess she was adopted, like it was some sort of terminal disease."
The study also revealed that only 45 percent who sought out their birth parents were successful.
Adoptees told ABCNews.com that they were largely stymied by state laws that guard the secrecy of birth certificates and adoption records or the expense of hiring searchers.
Some were also rejected by their birth parents.
The secret surrounding her own adoption took a devastating toll on Joan Wheeler, a disabled social worker from Buffalo, N.Y. The last of five children born to a married couple, she was adopted as an infant when her mother died of cancer.