Mitigating factor (United States)

When a person commits a capital crime, a mitigating factor is something that might help the person avoid getting the death penalty. ("To mitigate" means "to decrease." A "factor" is something that causes something else. So, in law, a mitigating factor is something that can cause punishment to decrease.)
Mitigating factors do not automatically result in decreased punishment. Judges and juries also think about aggravating factors things that are likely to cause a more severe punishment. 
Also, mitigating factors are not an excuse for committing a crime. They may help explain what caused a person to commit a crime, but they do not mean the person did nothing wrong. 
Importance.
In the United States, mitigating factors are very important in death penalty cases. Mitigating factors can help prosecutors decide whether to ask for the death penalty. Also, the United States Supreme Court has ruled several times that judges and juries must think about mitigating factors before deciding on a sentence.
Supreme Court rulings.
Since 1972, the Supreme Court has tried to make capital punishment in the United States fairer and more consistent. The Court has ruled that state laws cannot say a person "must" get the death penalty if they commit a certain crime, no matter what. To be fair, the Court ruled, each judge or jury has to think about whether an individual defendant deserves the death penalty. As Supreme Court Justice Harry Blackmun wrote in a 1994 case: 
As part of this process, the judge or jury has to think about mitigating factors that might make a less severe punishment fit better.
In "Lockett v. Ohio", , the Court ruled that state laws may not limit what mitigating factors judges and juries can consider. The Court added that mitigating factors should include:
In other words, as legal scholar Jeffrey Kirchmeier explains:
Each state has its own death penalty law. Each law gives a list of capital crimes in the state; aggravating factors that can make person more likely to get the death penalty; and specific mitigating factors. However, the mitigating factors listed in each state law are only examples of possible mitigating factors. Even if a state does not list a certain thing as a mitigating factor, a defendant can still bring up that factor in court.
Federal law.
The United States' federal courts can try people for federal crimes and can sentence them to death. Federal law includes a death penalty law, 18 U.S. Code § 3592. It says that when deciding whether or not to give a defendant the death penalty, the judge or jury must "consider any factor, including these:
State laws.
While each state law is a little different, most of them list the same mitigating factors as the federal law. Examples of other mitigating factors in some of the states' laws include:
Oklahoma's sentencing law, Code Section OUJI-CR 4-78, just says that mitigating factors are: