Troubled kids are more vulnerable," said Lanning. "There is a greater need for attention and affection."
When they are teens, boys are more "compliant," as well, he said. "They have raging hormones and are at their peak of sexual arousal -- that's what [molesters] take advantage of."
Anderson said the sexual attention from his piano teacher was "especially confusing," and for that very reason he struggled with knowing how to embark on relationships later in life.
So-called "nice guy" molesters also have a great love of children and often set up programs where they can access them.
"The primary reason most of these guys do this, is because they are trying to convince themselves that they are not evil, disgusting perverts and to rationalize and justify what they are doing," said Lanning.
Experts don't know why some men develop a sexual interest in young children and whether it is learned or inherited behavior.
Although much of the research says they are often victims of abuse themselves, Lanning said new research refutes that.
"Why are some men aroused by a 5-year-old?" he asked. "Certain events could have taken place in early development for reasons that we don't understand."
Lanning said often those around the molester see signs, not overt sexual activity -- but "boundary violations" like horsing around in the shower or rubbing a child's back.
Too often such signs are not reported. Camp and school officials should keep records and evaluate the "big picture" when there are suspicions, he said.
Youth organizations have an obligation to keep good records, he said.
"Sometimes it's covered up and sometimes it's damage control and some of it is the good-old-boy network and some of it is ignorance," said Lanning. "But at some point people don't process this information totally."
These molestations can go on for 10 years or more until the molester is either tired of the victim or finds a newer, younger one.
"[The molester] may have a range of boys, like a pipeline," said Lanning. "He is recruiting and looking at new kids who are 10, 11 or 12, then the next stage is grooming and seducing them. The third step is having sex with a boy and the fourth and hardest step is dumping the boy."
Victims often report that the abuser ends the relationship and they feel used when they are no longer getting the attention.
"These cases are difficult to investigate, not only because society doesn't understand, but police don't understand," he said. "Boys who are pushed out are most likely to come forward and tell what is going on."
As for Anderson, he spent years in therapy and is now in a good relationship with a woman. He works as a garden artist and also runs a support group for male abuse victims sponsored by the Rape, Abuse and Incest National Network.
The years of abuse affected his self-worth and relationships with others.
"Our society doesn't see men very easily as victims," he said. "Or if we do, we are ambivalent about helping them."
By delaying helping men, they can develop severe psychiatric symptoms.
When Anderson eventually told his brother about the incident with the camp doctor, he told him: "Everybody knew Doc liked boys."
"All the adults in my life sided with the perpetrator and no one sided with me," said Anderson. "That is the lesson I learned. I am on my own in the world. You cannot trust anyone."