HAMILTON — The slaying of Miami University professor Sherry Lee Corbett has the hallmarks of what prosecutors call an open-and-shut case.

        She was shot in broad daylight July 27, in front of multiple witnesses who say her assailant walked away and then came back to fire two more shots. Police say they caught the suspect, Tonda Lynn Ansley, 36, with a gun in her waistband.

Corbett

Ansley

        What's more, sources close to the investigation have said Ms. Ansley admitted the shooting to police.

        But her fate could turn on these questions: What was her mental state on that sunny Saturday afternoon? And what is her mental state now?

        Those issues are likely to surface today in Butler County Common Pleas Court. Ms. Ansley, held in the county jail without bond, is scheduled for her first appearance on an aggravated murder charge in a rare case of a woman accused of killing a woman.

        Assistant Prosecutor Craig Hedric declined to comment Wednesday. Ms. Ansley's court-appointed attorney, Melynda Cook-Reich, wouldn't say whether she would seek a mental competency evaluation of Ms. Ansley or would file a “not guilty by reason of insanity” plea on her behalf.

        Contrary to public perceptions, such a plea is rare and the success rate is low, some experts say.

        Only about 2 percent of all suspects pursue an insanity defense — and only about one-third of those defendants secure an acquittal for mental-health reasons, said Rita J. Simon, university professor who lectures on public affairs and law at American University in Washington, D.C.

        Ohio's law uses the M'Naghten Rule, which dates to 1843 in England. Simply put: “Can the defendant distinguish right from wrong?” Ms. Simon explained.

        And under the law, the defense must prove, by a “preponderance (majority) of the evidence,” that the defendant suffered from a mental problem that blocked understanding of right versus wrong.

        “It's not a popular defense. It very often does not evoke a great deal of sympathy for the perpetrator, especially if the person doesn't have a history of mental illness over a long period of time,” Ms. Simon said. If Ms. Ansley had any prior mental illness, no one has disclosed it so far.

        Still, Cindy Freytag, a tenant of Ms. Corbett and acquaintance of Ms. Ansley, said, “I don't think it would surprise anyone if she would plead insanity.”

        Nothing else seems to explain what led to the slaying that shocked the Dayton Lane Historic District, Ms. Freytag said.

        Both the victim and the suspect lived there; Ms. Corbett led preservation and renovation efforts and Ms. Ansley rented an apartment from Ms. Corbett. To date, no one has publicly recalled any animosity between the women.

        The only suggested motive to surface is a bizarre one. Law enforcement sources, who spoke on condition of anonymity, have said Ms. Ansley somehow connected a 1999 Keanu Reeves science-fiction movie, The Matrix, to the shooting of Ms. Corbett, 55. Channel 9, WCPO-TV, also reported that sources said Ms. Ansley planned to kill three other people, and Ms. Ansley believed Ms. Corbett and the others were trying to control her.

        Over the past 40 years, women have been accused in only about 10 percent of homicides. Female-on-female killings accounted for only 2.4 percent of homicides from 1976 to 1999, the U.S. Department of Justice says. In contrast, 40 percent of homicides committed by females were against children ages 5 and younger.

        A Houston jury rejected the insanity defense of Andrea Yates, 27, in the bathtub drownings of her five children. She avoided the death penalty but was sentenced to life in prison with parole eligibility after 40 years.

        But late last month, a three-judge panel found Bridget Stovall, 24, of Evanston, not guilty by reason of insanity in the bathtub drownings of her 4-year-old daughter and 20-month-old son. Ms. Stovall's lawyers said she was delusional and believed she was exorcising demons from the children. She is scheduled for a hearing today to determine treatment.

        Christo Lassiter, a University of Cincinnati law professor, was hard-pressed to recall other local cases where an insanity defense was successful. Still, he thinks it is a valid defense.

        Since John W. Hinkley Jr.'s 1981 assassination attempt on President Ronald Reagan and his subsequent acquittal by reason of insanity, “there has been a backlash” against that defense, Mr. Lassiter said.

        “I would say it's given a much more critical analysis by the court, perhaps received with a more jaundiced eye,” he said.

        Donald Hall, a professor of law at Vanderbilt University, said it's likely that Ms. Ansley's competency to stand trial might be addressed before any possible insanity plea is made.

        But, he said, “even if the client is found incompetent ... a victory of sorts ... that simply delays the inevitable.” Suspects who are found incompetent to stand trial are ordered to receive treatment. Then a judge may find they were “restored” to competency and ordered to stand trial, Mr. Hall said.

        Once a murder case is set for trial, the insanity plea “is a very unusual plea because it results in deprivation of freedom,” Mr. Hall said. Win or lose, the defendant typically lands in prison or a mental hospital for a very long time. Ms. Ansley faces 23 years to life in prison if convicted of aggravated murder with a gun specification.

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