This is the captioned text from a 20-20 interview about SI (self mutilation). Due to possible triggering content, Please take care of you while reading.

We're about to tell you about something shocking. It's called self- mutilation. You may think you've never heard of it, but you'll be stunned to learn how many people are doing it in secret.

We know Princess Diana was one of them. In her now famous TV interview, she admitted that at one point in her misery, she deliberately cut herself on the legs. Unbelievably, as many as 2 million people in this country regularly hurt themselves on purpose, cutting, burning, jabbing their own bodies, anything to cause pain.

Why would anyone do it? Tom Jarriel found, like anorexia self-mutilation is rooted in a deep and overwhelming sense of worthlessness.

Reporter: she appears to be the girl next door. Heintz, a 16-year-old high school sophomore from Kansas, is pretty, sensitive and well-liked. She has loving parents, Vicki and Wayne Heintz, an older sister, Darla and younger brother, Ryan. Aside from the typical family struggles, life has always been good.

We're just an average typical family. We're about as average as you can get.

Reporter: it's hard to believe then that earlier this year, Talli Heintz came perilously close to taking her own life.

I didn't have a high self-esteem at all and I was really depressed. I didn't like the way I looked. I didn't like anything.

Reporter: her crisis began when Talli asked if she could get her belly button pierced, a popular fashion trend with girls her age. Her mother forbade it, so in defiance, Talli pierced it herself.

It just made me feel so much better when I was done. Piercing it, it hurts, but it -- everything was okay then. I had control. The feelings that I couldn't deal with inside of me become numb. It helped to distract me from the emotional pain. It was the only outlet that I had.

Reporter: for people who self-injure, the physical pain becomes the release for their underlying often unknown emotional. The decision to self-mutilate is often at the spur of the moment.

Reporter: this doctor routinely lectures about the treatment of self-mutilation. Why do you feel it is very important to share your information with your peers and with those who attend your lectures?

Well, because so many individuals are harming themselves and are not knowing what to do about it. They're afraid to tell people. They're afraid to come for treatment. And also there are many therapists who if they encounter these individuals, might not really know what to do.

This was a survival tool. This was something that enabled me to live.

Reporter: Shelley Goldberg, from New York City, has spent most of his life struggling with this behavior.

I was about 12 or 13, and I started to pick at my face. As the years progressed, I started using all kinds of tweezers and things to get at my face.

Reporter: is the scar tissue on your face today totally from self-injury?

Yes, yes, it is.

Reporter: have you discovered the emotional trigger that set off this problem for you?

I don't know. I guess it's just hatred of oneself.

Reporter: how frequently would you do this?

In the beginning, I only did it like maybe once every week, but like the near the end or the middle of it, I’d do it every day and usually more than once a day. Just whenever I’d have an emotion.

Reporter: the emotions triggered it and you'd cut?

Yeah.

Reporter: each episode became worse than the last, and over time, she had to make deeper cuts to get the same relief of her emotional pain. By now, Tallie began to fear for her life. She knew she had to stop cutting but couldn't. What was the moment that you realized that you really needed help?

I was really frustrated because I was in Spanish and I got my test back and I had failed it. So I just asked to go use the restroom. And when I went there and I cut myself, I mean, it just wouldn't stop bleeding. I mean, it just like reality hit. This was a big problem.

Reporter: in desperation, Tallie confided in a teacher who insisted that she confront her parents and ask for help.

I was dumfounded. I was totally confused and just couldn't understand why someone was going to cut themselves. When I saw that Tallie was cutting herself, I would ground her. I didn't realize the realm of danger she was in.

Reporter: became an outpatient at a nearby hospital and began intensive therapy.

It got better for a while, like right after I got out of the hospital, but then it just got so much worse. I mean, the hospital I went to first, it didn't help me at all, I think.

Reporter: Vicki Heintz searched for solutions and by chance learned about the rock creek center, a psychiatric hospital near Chicago, which had a treatment program specifically designed to help people like her, who self-injure. Titled S.A.F.E., self-abuse finally ends, it's the only in-patient program of its kind in the entire country. Dr. Wendy Friedman-Lader is a psychologist and clinical director of the program. She believes the behavior can be addictive.

If you're looking at addictions as an ability to run away from feeling and to numb, I think it is very much like that.

Reporter: I spoke with some patients about their self-injury. Emily, what happens? What do you do to hurt yourself?

I do a lot of different things.

Reporter: tell me specifically how you got that scar on your arm there.

This is from an iron.

Reporter: hot iron?

Right.

Reporter: Jan, I see scars, wounds on your arms. Tell me what you do to inflict injury.

These scars are from like household cleaning chemicals like oven cleaner or something that's used to unclog a drain.

Reporter: what's going through your mind as you prepare to go through self-injury?

I feel a pressure up inside of me and I self-injure and it's kind of like a release of the pressure.

Reporter: these injuries can be serious and life threatening, but suicide is not usually the intention. Cynthia, have you ever done this with the intention of killing yourself?

The majority of self-injury episodes that I’ve had have just been to block out the pain in my head.

Reporter: Karen Conterio is the program's founder. To sit across from the people you're dealing with and hear about acid burns, to hear about cigarette burns, to hear about ice pick stabs, to hear about razor slices, are these people kooks? Are they mentally unstable?

I think they're desperate. I think they're terribly desperate to find a way to cope and maybe to have people listen.

Reporter: in the S.A.F.E. program, patients are taught through intensive therapy to express their emotions in ways other than self-injury.

I'm looking for some information about cutting because I’ve been a cutter for many, many, many years and I can't seem to be able to stop this kind of behavior.

Reporter: the S.A.F.E. program also lists a toll-free information number, which enables them to help others who are in that desperate state. They receive calls from all over the country. To date, it is not understood exactly why some people, and not others, succumb to hurting themselves. But at the New York state psychiatric institute in New York City, psychologist Dr. Barbara Stanley and psychiatrist Dr. Kevin Malone are looking for answers. For the first time, they're trying to determine if self-mutilation may actually be a biological disorder involving abnormal levels of the brain chemical serotonin.

What we have found biochemically up to this point is that there are lower levels of serotonin in people who self-mutilate compared to people who do not self-mutilate.

Reporter: as a result, the doctors are now conducting numerous studies using anti-depressant medications and psychotherapy. Complete results are not expected for at least a year. As for her, she's been on anti-depressant medication since leaving the hospital and is no longer hurting herself. One of the things she learned at the hospital was to deal with her emotions by writing about them in a journal.

I don't think I’ll ever be completely recovered. I think I’ll always have thoughts about self-injury, but I don't have to act on them. And I know that I can control them.

You mentioned Princess Diana did this to herself. Why, in your opinion?

Princess Diana expressed herself in the television interview. She said that earlier in her marriage, she had such self-loathing. She was also bulimic. Part was to hurt herself. Part of it was a call for attention. With maturity, with some therapy she grew and got over that.

She came out of it. That was fortunate.