In addition to eligibility and mandatory sentencing laws, another legal factor affecting juvenile LWOPs is whether a given state has a felony murder law. Felony murder is the legal doctrine that all parties involved in the commission of a felony in which a person is killed are held accountable for that death, regardless of how the death occurred or whether it was intentional. In Colorado, for example, felony murder is equivalent to first-degree murder and carries the same mandatory sentence of life without parole.

Unfortunately, tracking the effect of felony murder on juvenile LWOPs is not as simple as listing which states have felony murder laws on the books, because some states have exemptions for juveniles. While no organization has conducted a full study of the effect of felony murder on juvenile LWOPs, Human Rights Watch and Amnesty International estimated, based on a sample of cases, that 26 percent of juvenile LWOPs nationwide were convicted of felony murder.

In 2006 Colorado passed a law changing the mandatory sentence for juveniles from life without parole to 40 years before the possibility of parole. Previously, offenders as young as 12 were eligible for life without parole. The law was not made retroactive, however, so it does not affect the sentences of Colorado's 48 juvenile LWOPs.

California is also reconsidering its policy. In 2007, a state senate panel approved a bill that would lower the mandatory sentence for juveniles to 25 years to life -- meaning offenders would serve 25 years before the possibility of parole. The bill did not pass that year, but a similar measure was introduced in 2009.

There's movement on the federal level as well. The Juvenile Justice and Accountability Act of 2009, introduced in May, would end federal sentencing of juveniles to life in prison without parole and provide incentives to states to do the same. That same month, the Supreme Court agreed to hear two cases on the constitutionality of sentencing juveniles to life without parole.