Answer:
Forensic psychologists, that is psychologists that specialize in psychological evaluation and treatment, especially in legal cases have become commonplace in death penalty cases ever since the Supreme Court ruled that capital sentencing must be based on the individual offense and the perpetrator of the crime. What this means, in essence, is that psychological factors among others must be considered as it could possibly influence jury decision.
The importance of forensic psychologists in the death penalty was greatly highlighted after the Supreme Court decided in 2002 that the use of the death penalty for mentally unstable people was unconstitutional. If an accused is deemed mentally unfit by a psychologist, such a person cannot face the death sentence. This goes on to underline the importance of the psychologists' assessments in death penalty cases.