About one out of every six men and one out of every four women will experience sexual abuse of some sort before age 16, according to one estimate.

In an interview set to broadcast Sunday, Massachusetts Sen. Scott Brown (R) told CBS that he had been sexually abused by a camp counselor when he was 10 years old. Such an experience is more common than most people believe, according to researchers who specialize in studying childhood sexual abuse. But victims, especially male victims, often feel silenced by shame, researchers say.

And while both male and female sexual abuse victims struggle with shame and stigma, stereotypes about masculinity often force men to wrestle with unique issues.

"Males, especially as children and youth, are less likely to disclose abuse," Elizabeth Saewyc, a professor of nursing at the University of British Columbia who helps create individualized programs for the treatment of abused children, told LiveScience. "Because a lot of our stories about men is that they're sort of in charge sexually, when there is sexual abuse it really undercuts all of our social scripts. It is not only a violation of a boy's boundaries and their most personal autonomy, that biggest right to privacy of the self, but it also contradicts their sense of masculinity."

Difficult numbers

There are no reliable estimates of how many people experience childhood sexual abuse. Many survivors keep their experiences secret, so law enforcement statistics don't provide good estimates, researchers say. Surveying the population turns up much higher levels of sexual abuse than law enforcement numbers, but even those studies have weaknesses: Survivors may not feel comfortable disclosing their experience even on a survey. Question wording may affect the responses. Even taking answers via telephone or in person can change people's willingness to answer.

Based on several large studies, a rough estimate is that one out of every six men and one out of every four women will experience sexual abuse of some sort before age 16, said David Lisak, a psychologist from the University of Massachusetts, Boston, who sits on the board of 1in6.org, an organization dedicated to helping men who have been abused.

"I can no longer get worked up about whether the number is one in four, one in three, whatever," Lisak told LiveScience. "It is an enormous number."

Because of underreporting, it's difficult to know whether there are many differences between the sexual abuse experiences of boys and girls. One study of 226 girls and 64 boys between the ages of 10 and 15 who disclosed sexual assault to the Midwest Children’s Resource Center at the St. Paul Children’s Hospital in Minnesota found that boys are less likely than girls to report the abuse within 72 hours (a critical time period that could have implications for gathering evidence to bring criminal charges).

Boys were also more likely to have been exposed to pornography during the abuse, and to have had pornography made of them. Girls were more likely to have multiple abusers, while boys typically had one perpetrator, often another minor who was older than them. Girls were more likely to say they'd tell a friend first about abuse, while boys listed their mother as their first contact.

And then the study turned up another troubling facet of male sexual abuse.

"The second most common person that boys said they would talk to about this was their perpetrator," Children's Hospital nurse practitioner Laurel Edinburgh, who co-authored the study with Saewyc, told LiveScience.

Response to abuse

For the most part, Lisak said, boys and girls who are sexually abused respond in the same ways. They feel fear, confusion and sometimes anger. Both genders are at higher risk for psychiatric conditions including anxiety and depression later in life. And both genders face stigma if they chose to report their abuse.

But for men, that stigma can take on a unique tone. Because guys aren't "supposed to be" sexual abuse victims, Saewyc said, they may have trouble understanding that they're being abused. Most perpetrators of abuse are male, so male victims also tend to struggle with issues of sexuality in ways many female victims do not. When perpetrators are women sexually abusing guys, Lisak said, it's just as harmful to the victims, but society is prone to shrug it off as a "Mrs. Robinson" thing.

"There's this view, 'Wasn't he lucky,'" Lisak said. "There is just this profound lack of empathy for what [the abuse] really means."

Lisak has interviewed perpetrators of childhood sexual abuse and said the motivations often vary, but there are some common themes. Abusers go for vulnerability, he said. Most are repeat offenders. Some assault both women and children, because they get their gratification from controlling another person, Lisak said. Contrary to "stranger danger" fears, most perpetrators know their abusers.

Breaking the silence around sexual abuse is key for both prevention and healing, researchers contacted by LiveScience said.

"Because it has impacted so many people, we need to be having these conversations," said Deborah Donovan Rice, the director of Stop It Now, an organization dedicated to child abuse prevention. Stop It Now runs a helpline for adults who are concerned about a child and unsure of how to intervene. People also need to be sensitive and alert when others disclose their experiences, Saewyc said.

"We shouldn't have such stigma around it, but it also shouldn't be happening," Saewyc said of childhood abuse. "As long as people dismiss it and disbelieve it and deny it, it does create space for this to keep happening."

You can follow LiveScience Senior Writer Stephanie Pappas on Twitter @sipappas.