What is the proper legal standard for assessing the prejudice resulting from deficient assistance of counsel in capital sentencing proceedings, particularly when at least one juror voted in favor of a life sentence without the omitted mitigating evidence and with identical aggravating evidence?

QUESTION PRESENTED This case concerns the prejudice resulting from ineffective assistance of counsel during the penalty phase of a capital case. For petitioner James Barber, it is undisputed that (1) the jury “never heard” new and noncumulative mitigation evidence about, for example, Barber’s serious mental health issues and suicide attempts, (2) there was no new aggravation evidence presented in post-conviction proceedings, and (3) at least one juror, presumed to follow the instructions and having heard much more limited mitigation evidence and the same aggravating evidence, had already voted in favor of a life sentence. Disregarding that split verdict and implicating widespread disarray in the prejudice standards that lower courts apply to capital sentencing cases, the court of appeals held that “[t]he aggravating circumstances in this case are simply too great to permit us to find a probability of a different outcome.” Pet. App. 17a. The question presented is: What is the proper legal standard for assessing the prejudice resulting from deficient assistance of counsel in capital sentencing proceedings, particularly when at least one juror voted in favor of a life sentence without the omitted mitigating evidence and with identical aggravating evidence. (i)