Prompted by Jared Loughner's crimes, Room For Debate asks who does, and who should, qualify for an insanity plea. Here's Dr. Beatriz Luna, "the director of the Laboratory of Neurocognitive Development, where she uses neuroimaging methods to understand the development of voluntary control":
Jared Loughner’s criminal act appears to have involved careful planning that required voluntary and well-thought out steps. However, the aim of this planned behavior may reflect a disordered, diseased state. Neuroimaging studies could show that such a criminal engages brain systems to support voluntary acts in a similar way as the normal population. However they could also show abnormalities in brain processes that support the ability to have empathy and control over anger, or show that reported hallucinations recruit brain processes that support real sensory experiences. Although such a person would be able to operate in a voluntary planned manner, their acts would reflect brain abnormalities that contribute to their urge to commit crimes.
Should such a disorder be an excuse for their voluntary behavior? Probably not, but it might provide extenuating circumstances that could influence sentencing.