At George Porter's 1988 trial for a double murder of his former girlfriend and her new boyfriend, Porter's appointed counsel for the penalty phase made no investigation of Porter's background and presented essentially no mitigating evidence. Porter was convicted and sentenced to death, and the conviction and sentence were affirmed by the Florida Supreme Court. Porter obtained new counsel, whose thorough investigation produced extensive mitigating evidence about Porter's abusive childhood, his service in the Korean War, his apparent post-traumatic stress disorder as a result of that service, his long-term substance abuse, and his impaired mental health and mental capacity. Despite this evidence, the Florida state courts denied post-conviction relief, although two members of the Florida Supreme Court dissented. The federal district court granted a writ of habeas corpus, finding that Porter had been denied effective assistance of counsel in the penalty phase of his trial and that this failure had prejudiced him, but the Court of Appeals for the Eleventh Circuit reversed, holding that the state courts' analysis of the evidence was not unreasonable.

In a unanimous per curiam opinion, the Supreme Court granted Porter's petition for writ of certiorari and, without argument, reversed the Eleventh Circuit's judgment. The Court agreed with the lower federal courts that the performance of Porter's trial counsel was constitutionally deficient, finding that the lawyer's complete failure to investigate his client's background did not satisfy the prevailing professional standards at the time of the trial. It disagreed, however, with the court of appeals' acceptance of the Florida courts' conclusion that Porter was not prejudiced by his lawyer's deficient performance. To obtain relief, Porter was required under Strickland v. Washington, 466 U.S. 668 (1984), to show that, but for counsel's deficiency, there was "a reasonable probability he would have received a different sentence." Here, if the jury and the trial judge had known of the mitigating evidence that Porter's lawyer failed to present, and had they been aware that the state courts had subsequently discounted several of the factors that allegedly aggravated the severity of Porter's crime, there was clearly "a reasonable probability" that they "would have struck a different balance," and it was unreasonable for the Florida courts and the Eleventh Circuit to conclude otherwise. As the Court observed, quoting the dissent in the Florida Supreme Court, "there exists too much mitigating evidence that was not presented to now be ignored."

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