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."