A sentencing court does not have to find that a juvenile is permanently incorrigible to sentence them to life in prison without parole, the U.S. Supreme Court held April 22 in Jones v. Mississippi.
While that holding received wide publicity, less attention was given to the opinion’s potential impact on the Court’s retroactivity case law.
And both the majority and dissent suggested further litigation over whether sentences of life without parole for juveniles are disproportionate.  
In 2012, the Court, in Miller v. Alabama, held that a juvenile who commits a  homicide offense may be sentenced to life without parole only if the sentence is not mandatory and the sentencer has discretion to impose a lesser punishment.
In 2016, the Court, in Montgomery v. Louisiana, held that Miller was retroactive on collateral review.   
Brett Jones was 15 years old when he killed his grandfather in 2004.  He was originally sentenced to mandatory life without parole, but was granted a new sentencing hearing after Miller and Montgomery.
Jones’ attorneys argued at resentencing that Jones’ young age and “its hallmark features” mitigated against life without parole.  They argued the court must make a finding that Jones was “permanently incorrigible” before being able to impose such a sentence.
The judge acknowledged he had discretion to impose a lesser sentence, but after considering factors “relevant to the child’s culpability,” determined the appropriate sentence remained life without parole.  The judge did not make a finding of permanent incorrigibility.
The Mississippi Court of Appeals affirmed.
The Supreme Court granted cert. to resolve a split amongst state and federal courts about how to interpret Miller and Montgomery.   
 In a 6-3 opinion, the Court held a finding of permanent incorrigibility is not required.
“Jones relies on language in Montgomery that described Miller as permitting life-without-parole sentences only for ‘those whose crimes reflect permanent incorrigibility,’ rather than ‘transient immaturity,” the Court said.  But “the Montgomery Court explicitly stated that ‘a finding of fact regarding a child’s incorrigibility … is not required.’”
Miller declined to characterize permanent incorrigibility as such an eligibility criterion,” the Court said.  “Rather, Miller repeatedly described youth as a sentencing factor akin to a mitigating circumstance.”
Miller held that a sentencer must have discretion to consider youth before imposing a life-without-parole sentence, just as a capital sentencer must have discretion to consider other mitigating factors before imposing a death sentence,” the Court said.
“Despite the procedural function of Miller’s rule, Montgomery held that the Miller rule was substantive for retroactivity purposes and therefore applied retroactively on collateral review,” the Court said.
But “the Court determines whether a rule is substantive or procedural for retroactivity purposes ‘by considering the function of the rule’ itself – not ‘by asking whether the constitutional right underlying the new rule is substantive or procedural,’” the Court said. 
“[T]o the extent that Montgomery’s application of the Teague standard is in tension with the Court’s retroactivity precedents that both pre-date and post-date Montgomery, those retroactivity precedents – and not Montgomery – must guide the determination of whether rules other than Miller are substantive,” the Court said.  “To be clear, however, our decision today does not disturb Montgomery’s holding that Miller applies retroactively on collateral review.”
The Court left open the door that juveniles might bring proportionality claims regarding life without parole sentences.
“[T]his case does not properly present – and thus we do not consider – any as-applied Eighth Amendment claim of disproportionality regarding Jones’ sentence,” the Court said.
Dissenting opinion
Justice Sotomayor dissented, joined by Justices Breyer and Kagan.
Sotomayor said the majority “guts” Miller and Montgomery.
“[T]he Court transforms Miller into a decision requiring only a ‘discretionary sentencing procedure,’” she said.  “At the same time, the Court insists that it “does not disturb’ Montgomery’s holding ‘that Miller applies retroactively on collateral review.’”
“In other words, the Court rewrites Miller into a procedural rule and, paradoxically, maintains that Miller was nevertheless ‘substantive for retroactivity purposes,’” she said.  
“That explanation undoes Teague’s distinction between substantive and procedural rules,” she said.  “If a rule that requires only a sentencing procedure is substantive for retroactivity purposes, then this Court has improperly classified numerous sentencing rules as procedural.’”   
Sotomayor listed as examples the Court’s holdings in Mills v. Maryland, which invalidated capital sentencing procedures requiring jurors to disregard mitigating circumstances that were not found unanimously, and Ring v. Arizona, which held that a jury, not a judge, must find aggravating circumstances necessary to impose death.
Such rules should have applied retroactively “under the Court’s logic today,” but the Court has held they do not, she said.  “If future litigants make such arguments, it will be because the Court’s contortion of Miller and Montgomery paves the way for them to do so.”
Sotomayor also noted that the Court “leaves open the possibility of an ‘as-applied Eighth Amendment claim of disproportionality.”  
“[S]entencers should hold this Court to its word:  Miller and Montgomery are still good law,”  Sotomayor said.  Thus, an as-applied disproportionality claim “should be controlled by this Court’s holding [in Montgomery] that sentencing ‘a child whose crime reflects transient immaturity to life without parole … is disproportionate under the Eighth Amendment.”