Not a reliable source. In a death penalty prosecution, this is what mitigation means (cut & pasted from mitigation treatise):
1. Defects in the capital defendants intelligence, psychological and emotional makeup, maturity and development as an adult, and character which may explain why he or she engaged in the behavior that led to the homicide and help the jury understand why it happened and why a life sentence is appropriate.
2. Positive aspects of the capital defendants character and life that may give the jury a reason to vote for life because the defendant hasnt always behaved the way he or she did at the time of the homicide.
The ABA Guidelines for Appointments and Performance in Death Penalty cases 11.3.19(c), p. 93 (1989) and ABA Standards for Criminal Justice 4-4.1.1, Commentary, p.4-55, were relied upon heavily by the United States Supreme Court in Wiggins v. Smith, 123 S. Ct. 2527 (2003). In that opinion the Supreme Court made it absolutely clear that trial counsel representing an accused person facing the death penalty was responsible to investigate and prepare a rigorously developed social history of the accused. The Supreme Court mandated that a social history be prepared as a basic component of investigation and should include medical history, educational history, employment and training history, family and social history, prior adult and juvenile correctional experience, and religious and cultural influences.