Once a person has been convicted of a crime, the court proceedings move into the sentencing phase. The judge or a jury must decide the appropriate sentence for the crime, and there are legal parameters that must be followed. For example, a Class 4 Felony in Colorado carries a potential prison sentence of two to six years.

Sentences can then be lengthened based on aggravating factors, such as whether the person has committed an extraordinary risk crime or an aggravated crime. Sentences can also be lengthened if the person is deemed a habitual offender (a scenario many defendants refer to as looking at the bitch). Other aggravating factors might include a diagnosis of Antisocial Personality Disorder or psychopathy.

A judge or a jury may be inclined to lean more toward the minimum end of the sentencing range if there are mitigating factors. For example, a person who assaulted someone in a parking lot might get a more lenient sentence if the judge understands that person has a diagnosis of Bipolar Disorder and was extremely depressed and angry at the time of the assault because he needed to stop taking his medications due to side effect issues. These mitigating factors don't completely excuse the person from wrongdoing, but they help explain to the judge why the person may deserve a more lenient punishment.

In January of 1982, the Supreme Court of the United States issued a landmark decision regarding what type of mitigating evidence must be considered prior to sentencing a person to death. The case is Eddings v. Oklahoma.

Eddings was a 16 year-old "youth, raised without proper guidance, subjected to excessive physical punishment, and emotionally disturbed in general." He ran away from home with several friends and ended up shooting and killing a highway patrol officer. He was tried as an adult in Oklahoma and convicted of first-degree murder.

The trial court and another lower court both considered Eddings's age as the only mitigating factor prior to sentencing him to death, thus ignoring his emotional disturbance and violent family background. Eddings's attorneys argued ignoring that extended evidence violated his Eighth Amendment (Cruel and Unusual Punishment) and Fourteenth Amendment (Due Process) Rights.

The Supreme Court agreed with Eddings's legal argument. They concluded the lower courts made a mistake by not taking into account all potential mitigating evidence, including evidence regarding the defendant's background and mental health. The Court concluded, "While these [mitigating] circumstances did not suggest an absence of responsibility for the crime, they had to be considered in sentencing."

Because of the Eddings v. Oklahoma ruling, sentencing phases of death penalty cases can be very long, and often forensic psychologists are hired by the defense in an attempt to develop and explain mitigating evidence that might lower a sentence from Death to Life in Prison. The Court must consider all of that mitigating evidence prior to making its decision regarding the sentence.