Andrea Yates didn't have history or statistics on her side when it came to pleading insanity for drowning her five children.

While some see the maneuver as a frequently successful attempt to escape justice, precedent actually shows Yates' lawyers faced an uphill battle convincing the eight-woman, four-man jury that the diagnosed psychotic should not be found guilty.

Despite popular perception, the insanity defense is rarely used in the U.S. And it is even more rarely effective.

Though statistics vary, experts concur that the insanity defense works in less than 1 percent of all cases — between 1/3 and 1/12 of 1 percent, by most estimates. And because the defense and prosecution agree the defendant is insane in 90 percent of those cases, juries accept the plea in an even smaller fraction of contested cases — about 1/120 of 1 percent.

"It's going to be very, very hard for her to be successful," Michael Perlin, a former public defender in Trenton, N.J., and author of The Hidden Prejudice: Mental Disability on Trial, said weeks before Tuesday's guilty verdict.

Even those who are acquitted on insanity face another harsh reality: Successful or not, the defense tactic usually means doing more, rather than less, time.

"Jurors don't like to find people not guilty by reason of insanity because they think they're getting off," Perlin said. "Even though with felonies, the person will probably be locked up 1 ½ to 2 times longer than a person convicted of the same charge."

Take John Hinckley Jr., found not guilty by reason of insanity for trying to assassinate former President Ronald Reagan in 1981. Though Hinckley is still confined to a Washington, D.C., mental hospital 21 years later, the overpowering national backlash his verdict incited caused a tightening of federal law in the Insanity Defense Reform Act of 1984.

Experts say the semantics of the verdict — not guilty by reason of insanity — and the inevitable controversy that accompanies it are troublesome for judges and jurors.

"There is always a public outcry when a jury finds a defendant not guilty by reason of insanity," said American University law professor Rita J. Simon, author of The Jury and the Defense of Insanity. "Most people have problems with it because a person is dead."

The current definition of insanity also makes it a challenging plea to prove and accept. Though it varies slightly from state to state, the general rule applied today — an English concept called the McNaghten Test — is whether mental illness impaired a person's ability to know right from wrong when committing the crime.

That test has been criticized as being too narrow, because it focuses on cognitive capacity, which can stay intact despite mental illness, rather than rational ability and behavior control. But an alternative, offered by the American Law Institute in 1962 and temporarily adopted by Texas and many other states, was also met with protest.

The ALI definition not only mentioned knowing right from wrong, but asked whether mental illness prevented the defendant from controlling behavior or conforming to the law.

"The insanity provision was quite influential but it did run into criticism," said Michael Greenwald, an ALI deputy director.

After the outrage over the Hinckley trial, most states went back to the McNaghten rule, though some use an expanded version of it. Montana, Wyoming and Idaho have eliminated the insanity defense completely, and several others are pondering doing the same.

Dr. Robert L. Sadoff, a forensic psychiatrist who has testified in hundreds of insanity-plea cases for both defense and prosecution lawyers, believes in refining the McNaghten definition to say mental illness prevented the person from "appreciating," rather than "knowing," right from wrong.

"'Appreciate' would include the mentally ill rather than only the mentally retarded," he said. "People might not fully understand or want to understand how mental illness impacts criminal behavior."

Yates' actions after the June 20, 2001, murders of her children likely hindered her lawyers at trial, experts said. Immediately after killing her children, ages 7, 5, 3, 2, and 6 months, she phoned her husband and the police, suggesting she did know she had committed a crime.

Other details point to the magnitude of Yates' illness. She had tried to commit suicide at least twice. She suffered from severe postpartum depression and psychosis, and was admitted to a mental institution on previous occasions.

Testimony also revealed that Yates believed she was possessed by the devil and could only be set free of the demons and save her children by bringing about her own execution. Interviews quoted her as saying she killed her children to punish herself and ensure that they'd go to heaven instead of hell because "I have not been a good mother to them."