Images of child sex abuse have reached a crisis point on the internet, spreading at unprecedented rates in part because tech platforms and law enforcement agencies have failed to keep pace with the problem. But less is understood about the issue underlying it all: What drives people to sexually abuse children?

Science in recent years has begun to provide some answers. One thing most pedophiles have in common: They discover, usually as teenagers, that their sexual preferences have not matured like everyone else’s. Most get stuck on the same-age boys or girls who first attracted them at the start of puberty, though some retain interest in far younger children.

“People don’t choose what arouses them — they discover it,” said Dr. Fred Berlin, director of the Johns Hopkins Sex and Gender Clinic. “No one grows up wanting to be a pedophile.”

[Read The New York Times’s investigation into the spread of online child sex abuse.]

Over the past generation, psychologists, forensic specialists and others have studied pedophilia, a disorder characterized by “recurrent, intense arousing fantasies, urges or behaviors involving sexual activity with a prepubescent child,” according to psychiatry’s diagnostic manual. These experts have interviewed patients in depth, piecing together life histories and performing a variety of psychological and anatomical measures.

While no study offers a complete picture, a portrait is emerging — one that helps elucidate the mental dynamic behind the surge in abuse images and the deepening depravity they depict. These findings also defy common stereotypes about what pedophilia is, and what the risks are for engaging in physical abuse.

A majority of convicted offenders are men who prey on children ages 6 to 17. But women also commit hands-on offenses; rough estimates put the rate of pedophilic attraction at 1 to 4 percent in both men and women. Studies suggest that a small subset of male and female pedophiles have an interest in toddlers, or even infants.

As scientists seek to understand how the disorder develops, there is growing consensus that the origin is largely biological. This view is based in part on studies pointing to subtle physical traits that have a higher incidence among pedophiles.

“The biological clues attached to pedophilia demonstrate that its roots are prenatal,” said James Cantor, director of the Toronto Sexuality Center. “These are not genetic; they can be traced to specific periods of development in the womb.”

Psychological and environmental factors may also contribute, though it is not yet clear what those are or how they interact with developmental conditions.

By contrast, the common presumption that pedophiles were themselves abused as children now has less support. Child victims are at far greater risk of future substance abuse, depression, persistent traumatic stress or criminal aggression than of becoming molesters. The vast majority of offenders deny any sex abuse in their childhood, even though they could garner sympathy in court by doing so, experts say. “A chaotic childhood increases the likelihood of a chaotic adulthood, of any kind,” Dr. Cantor said.

The relationship between viewing or collecting images and committing hands-on abuse is a matter of continuing debate among some experts, and one that is critical to evaluating the risk an offender poses. Until recently, the prevailing view was that only a minority of people caught viewing such images, between 5 and 20 percent, also committed physical abuse.

That perception began to change in 2007, when a pair of psychologists at the Federal Bureau of Prisons reported that 85 percent of convicted online offenders acknowledged in therapy that they had raped or otherwise sexually abused children.

That finding circulated widely before the study was formally published, creating an uproar among therapists, researchers and law enforcement specialists. The prisons bureau balked at publishing it at all, and withdrew it from a peer-reviewed journal close to its release date.

Many cited concerns that the study sample was biased: It was based on the confessions of 155 convicts who had sought out therapy in prison, not on a representative sample of pedophiles, a much broader group with diverse habits.

“It was what we call a convenience sample — that was a legitimate criticism,” said Michael L. Bourke, a co-author of the study with Andres E. Hernandez, in a telephone interview. Dr. Bourke is now chief of the behavioral analysis unit of the United States Marshals.

Since then, several other studies have supported the prison finding, if not precisely the 85 percent number. In one, inspectors from an array of government agencies interviewed 127 online offenders shortly after their arrests. Less than 5 percent admitted to previously molesting at least one child.

When agents followed up with more in-depth, polygraph-assisted methods, another 53 percent admitted to hands-on offenses, for a total of nearly 60 percent.

“This was not a convenience sample; these were offenders, some of whom had downloaded just a single image, with no known history, from all over the country, interviewed by people from different agencies,” Dr. Bourke said. “They had zero incentive to admit to a previous offense — very much the opposite.”

The high rate of previous, hands-on offending undermines another common assumption about pedophiles. “We shouldn’t assume that viewing online images leads to abuse of a child victim in person,” said Joe Sullivan, a specialist in sex crimes against children in Ireland and Britain. “In my clinical experience, it’s the other way around. Most of these men have already committed hands-on offenses.”

From this point of view, downloading abuse images — and especially connecting with groups of like-minded pedophiles online — does not awaken latent desires. The desires are very much awake and, in many cases, have already been acted on. But the images and online communities can help erode inhibitions further, drawing pedophiles into more frequent or more aggressive acts, Dr. Bourke said.

“What you see, in their search histories,” he said, “is that they learn how to evade law enforcement, they become more confident and they begin to use cognitive distortions to overcome their moral inhibitions.”

Some therapists and researchers say these findings from law enforcement threaten to unfairly tar people who never act on their desires. This group certainly exists — they’re sometimes called “virtuous pedophiles” — but in an era of increasing alarm over the proliferation of online abuse, they are going only further underground.

“That is a shame, a tragedy,” Dr. Cantor said. “That is the group we need to learn about. That’s the kind of person we’d like our clients to become, a person who’s aware of the urges and learns to effectively manage them.”

Learning to manage a drive as visceral, and often consuming, as sexual desire is possible, therapists say, but it cannot be shut off; nor can it be replaced, the way heroin can be swapped for methadone. Treatment can require drugs that reduce circulating testosterone and software that limits online browsing habits.

Often, therapy addresses substance abuse as well. Studies suggest that at least 40 percent of sex offenders were using drugs or alcohol when they committed their crimes.

“The important thing, I think, is that people know that treatment is possible,” Dr. Berlin said. “There’s a subgroup out there, they refer themselves here, and they are quite convinced that they do not want real-life sex with children.”