The United States regularly executes mentally retarded people. More than 300 people on death row are mentally retarded. Since 1983, more than 60 people diagnosed as mentally ill or mentally retarded have been executed in the US, according to Amnesty.
An example NDCAP gives is of an inmate, Morris Mason, who on the way to his execution in 1985 shouted out that he was going to play baseball when he got back. The offender was not capable of understanding that he was about to be executed. But he stil was.
Christopher Burger was convicted of being an accomplice to murder at age 17. He had organic brain impairment and schizophrenia, although these facts were not presented at his sentencing hearing. He had the mental ability of a 12-year-old. In 1993, after being on death row for about 15 years, he was finally murdered by the State of Georgia.