The nightmares stirring inside a mother's mind that would drive her to kill her children are beyond comprehension for most people.
But in some cases, psychologists say, extreme depression — possibly brought on after delivering a child — a psychotic breakdown and violence in the home have played a role when mothers have killed their children.
"This is a very rare type of crime," said Jeffrey Smalldon, a forensic psychologist from Columbus, Ohio, when asked to speculate on Andrea Yates' alleged confession that she had killed her five children in the family's Houston home. "There must have been many mental and emotional stress factors operating here."
Experts were careful not to draw any conclusions about the case, and noted that the crime was especially unusual because even when mothers do kill, they almost never kill all their children at one time.
Husband: Wife Had Postpartum Depression
Yates' husband, Russell Yates, told reporters today that his wife was a "kind, gentle person," but suffered from postpartum depression after the births of her last two children.
"What you saw yesterday, that wasn't her," he said, holding a framed photo of his wife and children in front of his chest.
On Wednesday, Andrea Yates, 36, called police to her home in Houston. There, they found the bodies of four boys, aged 2 years to 7 years old, and a 6-month-old girl. All had apparently been drowned, police said.
Mrs. Yates told the officers she had killed her children, police said. Police said she had been taking medication for postpartum depression, but that it remained unclear if her psychological condition may have contributed to the killings.
Russell Yates said his wife was taking two drugs for postpartum depression — efexor and remeron — but still showed symptoms of depression prior to the killings. He said after an earlier pregnancy, she had taken efexor, wellbutrin and haldol for postpartum depression.
Dr. Kim Pearson, a psychiatry instructor at Harvard Medical School and a doctor with the perinatal and reproductive psychiatry unit at Massachusetts General Hospital, said efexor, remeron and wellbutrin are anti-depressants. She added that haldol is an anti-psychotic medication used to treat symptoms such as hallucinations or delusional thinking.
"Certainly, that raises suspicion about whether she had a psychotic incident after the prior pregnancy," Pearson said.
She said 50 percent of women who have postpartum depression have a recurrence in subsequent pregnancies.
"If they've had true postpartum psychosis, than the risk is 75 to 80 percent that they'll have a recurrence," she added. "Certainly, if somebody is experiencing psychotic symptoms, it can lead them to psychotic behavior."
Mr. Yates said his wife had attempted suicide after the earlier pregnancy, but after being treated she had seemed to recover. After the most recent pregnancy, he said, she had seemed to recover to "65 percent" of her normal self and then "she plateaued."
‘A True Mental Break’
On ABCNEWS' Good Morning America today, ABCNEWS medical correspondent Dr. Nancy Snyderman said approximately 10 percent of mothers experience postpartum depression, and psychosis happens in about half of those cases.
It can be "so devastating the person feels like they're inside a bowl, [and] you can't even get up the sides," Snyderman said. "It's like pulling someone down, down, down, down, down — until they just don't feel like they can get out anymore. It's a true mental break."
Smalldon, the forensic psychologist, says there have been cases in which extremely depressed people may have difficulty separating their own identity from others'.
The sufferer believes murder is a way to deliver the victim to a better life than the current miserable life he or she is having, he said.
Such a person also may have been experiencing what is called "magical thinking," a mental state in which reality is distorted and the person believes an extreme act will relieve the pressure, he said.
"They come to believe that murder is the one thing to do to get out of the predicament," Smalldon said.
Domestic Violence Can Play a Role
A number of other factors have played roles in previous cases, though it is not clear what may have caused Wednesday's alleged killings.
Julie Blackman, a social psychologist who practices in New York City, said there is no simple answer as to why a mother might kill her children.
"There is no formula as to why a mother would kill her own children, especially so many," Blackman said, when asked to speculate on the deaths of the Yates children. "But there are risk factors."
Those factors have included violence in the home from an abusing husband, financial stress, multiple births in succession coupled with fathers not helping and psychotic breakdowns, Blackman said.
ABCNEWS' Michael S. James contributed to this report.