Unjust Rules for Insanity
In the 1970's, a more realistic standard was adopted by many states, including Texas. Even if the defendant knew the conduct was wrong, he would not be found guilty if he had been "incapable of conforming his conduct" to the requirements of the law. This standard recognized that as a result of mental illness an individual might know he or she is doing wrong but lack the ability to keep from doing it.

Then John Hinckley, trying to murder President Ronald Reagan, shot and wounded the president and three other men in 1981. Mr. Hinckley's acquittal by reason of insanity in 1982 shocked the nation. The next year Texas dropped the element of "conforming conduct" and reverted to a strict "knowledge-based" standard almost like the old M'Naghten rule.

In the case of Andrea Yates, the prosecution focused narrowly on the question of what she knew at the time of the killings. Her call to 911 and her admission to police officers that she expected to be punished by the criminal justice system were pivotal evidence for the prosecutors. The details of her knowledge were important; her mental illness at the time was not.
http://www.nytimes.com/2002/03/13/opinion/13BARD.html