John V. Caffaro is a distinguished professor at the California School of Professional Psychology, Los Angeles. He is the author of Sibling Abuse Trauma.

Lena Dunham holds her memoir, “Not That Kind Of Girl.” Critics have accused her of sibling sexual abuse based on certain passages about interactions with her sister. (Joel Ryan/Invision/AP, File)

Passages in Lena Dunham’s new memoir, “Not That Kind of Girl,” have ignited national debate over a topic our society rarely discusses — sibling sexual abuse. The actress recounted using candy to bribe her younger sister into giving her long kisses and noted an instance when she touched and looked into her infant sibling’s vagina. Some have condemned Dunham’s actions as abusive, while others – including Dunham herself — have dismissed it as normal sexual exploration.

Drawing a conclusion about Dunham’s interactions with her sister is impossible without much more contextual information about her family.  But, in general, the topic of sibling sexual abuse is more common than many realize and deserves much more discussion than it has received. Sibling sexual abuse is the most closely kept secret in the field of family violence. More than one in three cases of sexual assault against children in the U.S. are committed by other minors. Siblings often are the perpetrators. In fact, estimates suggest that sibling sexual abuse is far more common than parent-child abuse. A 2002 study by the U.S. Department of Health and Human Services found that at least 2.3 percent of children have been sexually victimized by a sibling. By comparison, 0.12 percent are sexually abused by an adult family member.

To be clear, sexual curiosity in children is normal. All children explore their bodies and may engage in visual or even manual exploration of a sibling at times. This is one way that children discover sexual differences between boys’ and girls’ anatomies. Even siblings of the same gender become curious about variations in shapes and sizes of their sex organs. Two small children exploring each other’s bodies does not predestine them to a life of emotional suffering.

But the fact that some sexual contact between siblings is normal has allowed society to ignore a lot of unhealthy behavior. Sexual activity between children long has been regarded as harmless. Some evidence also suggests that parents are significantly more likely to blame and doubt their child when she or he was sexually abused by a minor than by an adult. Because children rarely report sibling sexual assault and because parents frequently overlook it, research is scarce and estimates of its prevalence likely are conservative. This underreporting has hindered psychologists in developing universal criteria to differentiate abusive sexual contact from normal sexual exploration.

I began treating adult survivors of sibling sexual abuse in 1990 and, based on my initial research, I published an early definition of sibling sexual abuse that has been widely adopted by other psychologists. I believe sibling sexual behavior becomes abusive when:

  • the victim is not developmentally prepared for it,
  • the behavior is repeated,
  • or the interaction doesn’t reflect normal curiosity for the perpetrator’s age.

Other researchers have developed different criteria. Some draw the line when the interaction includes oral-genital contact or intercourse. Others believe coercion is the difference between natural curiosity and abuse. Sexual abuse frequently includes coercion by older or more powerful brothers or sisters. For instance, one of my adult male clients was introduced to sex by his adolescent sister when he was 9. Intensely stimulated by the sexual contact, he begged for sex while his sister would hold out contingent on certain behavior, vaguely defined as “being a good boy.” He felt implicated as a co-conspirator because he remembered desiring the sexual contact.

Whether both siblings consented to the behavior may not matter, as sibling sexual abuse can be based on fear, as well. That was the case with another client, whose older brother began sexually abusing her when she was 6. Initially, she looked up to him because he taught her how to ride a bike and tie her shoes, and he served as her companion and protector. But he also would become fiercely angry and cruel. After one argument, he pulled the heads off her dolls and put them in her school backpack. After the abuse began, he would tell her she was “only good for one thing.” Sometimes he used physical force and the threat of violence to emphasize his dominance and keep her quiet.

Some psychologists believe sexual contact between siblings is abusive when there is a large age difference between the children. Victims of sibling abuse are an average of 9 years old when the assaults begin, while offenders average 15 years old. Age differences can be compounded by gender. Girls are less likely to be sexually abused by a sister than by a brother. For example, my initial research included qualitative interviews with 29 adults who had experienced sibling sexual abuse as children. Eighteen were women who had been sexually abused by their brothers. Only two women had experienced sister–sister sexual abuse. But sexual interaction between siblings can be harmful even when they are very close in age or of the same gender. Differences in their sizes, strengths, intelligence, and developmental stages can influence the power dynamic.

While our society widely condemns sexual abuse of children as a devastating crime, it does not appreciate that much of that abuse is perpetrated by other children. Too many victims are made to suffer alone, convinced by siblings that they were complicit in the behavior or that their parents won’t believe them if they tell. Most victims don’t reveal the abuse until they are adults and have endured serious, long-term effects. To protect them, adults must start talking about sibling sexual abuse, recognizing it when it occurs and taking action to end it. Sexualized activity kept secret because of fear, coercion, or threat should not be considered harmless sex play.