Gov. Bob Holden signed into law a bill that made Missouri the 16th state to ban the execution of mentally retarded inmates.
The bill defines mentally retarded people as having "substantial limitations" and "significantly subaverage" intelligence that harms their ability to communicate, work or take care of themselves. It does not specify an IQ.
To be spared the death penalty, an inmate must be diagnosed as retarded before the age of 18.
Attempts to pass similar laws had failed for a decade in Missouri but were successful this year because of the more precise language used in the definition.