Existing studies cannot determine the percentage of premeditated versus impulsive murders. While this is in part a reflection of the difficulty in discerning the intentions of a given criminal, there are marked differences in brain function and intellectual ability demonstrated by the murderers committing these crimes.
While premeditated murderers do not have significant cognitive and intellectual impairment, they do show an increase in psychiatric disorders. Research shows that premeditated murderers are more than twice as likely as impulsive murderers to have a history of mood and psychotic disorders. Impulsive murderers are more likely to have a developmental disability, cognitive and intellectual impairment, reduced activity in the prefrontal cortex, a history of drug and alcohol abuse, and are more likely to have been intoxicated when committing the crime.