Police accusations this week that the woman charged with the murder of Sandra Cantu penetrated the Tracy, Calif., 8-year-old with a "foreign object" sent psychological chills up the spine of Americans.
The arrest of Melissa Huckaby, a 28-year-old mother and Sunday school teacher, on charges of kidnapping, rape and murder also sent up red flags for criminologists.
Women are rarely sexual predators -- only about 5 percent to 15 percent, according to experts -- and when they do molest children, they are more often accomplices.
Monday, San Joaquin County police accused Huckaby of murdering her daughter's playmate and stuffing her body in a suitcase that was found April 6 floating in a pond.
Huckaby, who is the granddaughter of the minister at Tracy's Clover Road Baptist Church and lived five doors down from Cantu in a mobile home park, could face the death penalty.
While rare, women do commit sexual acts against children. Research reported in a 2000 article in the Journal of Sex Research cites well-accepted studies by David Finkhor and Diana Russell that found women may account for up to 5 percent of the abuse of females and 20 percent of males.
These statistics include women either acting alone or with a partner. Six percent of sexual abuse against females and 14 percent against males is carried out by females alone.
"There are so few female pedophiles," according to Jack Levin, a criminologist who teachers the sociology of violence at Northeastern University in Boston. "There are some, but the ones I've seen who are sexually motivated have a partner."
"They go along with it for the love of the man," Levin told ABCNews.com. "So even when the crime is sexual, they may not be motivated by sex."
The author of "Serial Killers and Sadistic Murderers -- Up Close and Personal" said only one startling case comes to mind -- the Canadian couple Paul Bernardo and Karla Homolka.
The attractive, seemingly normal couple was convicted of the brutal 1995 kidnapping, rape and murder of two 15-year-old girls.
Homolka portrayed herself as the innocent victim and struck a deal and got off with a 12-year sentence for manslaughter. She was released in 2005, but soon after, videotapes of the crimes showed she was an active participant in the sexual assaults.
"She killed her own sister and one or two more," Levin said. "She was married to Bernardo, but it was very clear after the videotapes that she was enjoying it as much as Homolka.
"But because of the stigma, when people realized she was as guilty as her husband, she was a pariah."
In another case, in the 1980s, Sunset Strip Killers Doug Clark and Carol M. Bundy were convicted of a series of sexual murders in Los Angeles. Their victims were young prostitutes and runaways.
In a particularly gruesome murder, Clark took the severed head of one of his victims and had oral sex with it in the shower and stored it in an icebox. Bundy, who died in 2003, later admitted she had been present at some of the murders.
Today, Clark is on death row and has kept up a correspondence with Levin.
"He blames Bundy," Levin said. "Because of her name, Clark was a hero in her eyes. She timed the murders to coincide with the days Ted Bundy killed his victims."
Women are stereotypically nurturing and less threatening, and therefore images of women who are accused of molestation, such as the defendant in the Sandra Cantu case, jar our sensibilities, said crime experts.
When women are the perpetrators, they are often psychotic, according to Judie Alpert, a professor of applied psychology at New York University, who works with adults who were abused as children.
Often unreported, cases of female abusers show they are often victims of sexual molestation or emotional abuse themselves.
"Males are motivated by a lot of things, but they are usually not as unstable," she told ABCNews.com. "Men are more likely to abuse alone."
"Sometimes women participate with their husbands and abuse when the partner abuses," Alpert said. "Sometimes a married couple does it as part of ritual abuse."
"They are either repeating something that was done to them, or trying to understand it, or they are completely out of touch with reality," she said.
What makes the Sandra Cantu case more confounding to criminologists is the dual molestation and murder charges, according to Kenneth V. Lanning, a retired 30-year veteran of the FBI and police consultant on crimes against children.
Sexual molestation and murder rarely go hand-in-hand, he told ABCNews.com.
"The vast majority of child molesters do not abduct and kill their victims," said Lanning, who has no involvement in the case.
Strangers typically trick a child into sexual molestation -- by asking for help in finding a dog, "but they don't typically force or injure and kill the child," Lanning said.
"When it's an acquaintance, like a neighbor or baby sitter, the kid is groomed, seduced and manipulated," he said. "For a 6-year-old it might be a game, fooling around playing doctor. As a result, it decreases the likelihood that the child will tell anyone about it."
Lanning said police do best when they see molestation and murder as separate crimes, but sometimes the two overlap.
"What happens in some cases and what causes violence against the child is when the [predator] is grooming or seducing or manipulating and it didn't work out right," he told ABCNews.com. "They play games and the child goes along, then it gets carried away and they do something, and the child says, 'It hurts, I don't like it tell, I'll tell mommy.' That may cause the killing of the child."
Females can often disguise their sexual crimes because of the trust that is placed in them as caregivers, he said.
"Women are viewed as a lot less threatening, and they can get away with what men can never get away with," Lanning said.
Still, he said with the overwhelming evidence about the relative scarcity of female molesters, he often gives controversial advice.
"When women come up to me and ask what is the one thing I can do to protect my child from a baby sitter -- and this is a terrible thing to say and people are offended -- I say the simplest thing an average person can do is never hire a male to watch your children," he said.