HOUSTON (AP) _ An expert defense witness described Andrea Yates as an isolated loner with a genetic predisposition for schizophrenia that began during childhood but had never been diagnosed.
Dr. George Ringholz, a neuropsychologist from Baylor College of Medicine, told prosecutors outside the jury's presence Monday that he is prepared to testify that Yates was insane when she admitted drowning her five children last June.
Yates is on trial for two counts of capital murder. Prosecutors are seeking the death penalty in the deaths of 7-year-old Noah, 5-year-old John and 6-month-old Mary. Charges eventually could be filed in the deaths of Paul, 3, and Luke, 2.
Jurors must decide whether they believe she was insane when her five children were submerged in a bathtub at the family's home. Prosecutors argue Yates was sane at the time of the killings.
Prosecutors will have the chance to respond to Yates' insanity claims when defense lawyers rest their case.
To prove insanity, defense lawyers must show Yates suffered from a severe mental disease and didn't know the difference between right and wrong at the time of the drownings.
Yates's schizophrenia began during childhood and surfaced initially after giving birth to her first son, Noah, in 1994 when she considered grabbing a knife and stabbing the child, Ringholz testified in front of the jury.
Yates told him she felt Satan's presence shortly after Noah's birth and ``heard Satan's voice tell her to pick up the knife and stab the child,'' Ringholz said.
Noah would have been 8-years old Tuesday.
The symptoms of the schizophrenia didn't resurface until Yates' fourth son, Luke, was born in 1999. Medical records show that Yates attempted suicide twice that year.
Jurors were told the mental illness is characterized by a significant impairment in functioning and symptoms like delusions, hallucinations, incoherence and isolation.
Ringholz said he made his determination based on Yates' medical and family history and a battery of tests he conducted on the 37-year-old former nurse turned stay-at-home mother. He will take the stand again Tuesday.
Ringholz said Yates' test score was well within the range of those who test positive for schizophrenia.
The doctor said he also discovered that Yates' father, sister and two brothers had a history of mental illness. One of the brothers is bipolar.
Defense attorney Wendell Odom asked Ringholz if Yates had ever been diagnosed with schizophrenia before.
``Not that I'm aware of,'' Ringholz testified.
Earlier Monday, Dr. Melissa Ferguson, who interviewed the Houston mother in jail the day after her children were drowned, said Yates considered stabbing her five children but decided it was too bloody and that drowning was a better way to end their lives.