534
Opinion of the Court
Counsel's investigation into Wiggins' background did not reflect reasonable professional judgment. Their decision to end their investigation when they did was neither consistent with the professional standards that prevailed in 1989, nor reasonable in light of the evidence counsel uncovered in the social services records—evidence that would have led a reasonably competent attorney to investigate further. Counsel's pursuit of bifurcation until the eve of sentencing and their partial presentation of a mitigation case suggest that their incomplete investigation was the result of inattention, not reasoned strategic judgment. In deferring to counsel's decision not to pursue a mitigation case despite their unreasonable investigation, the Maryland Court of Appeals unreasonably applied Strickland. Furthermore, the court partially relied on an erroneous factual assumption. The requirements for habeas relief established by 28 U. S. C. § 2254(d) are thus satisfied.
III
In order for counsel's inadequate performance to constitute a Sixth Amendment violation, petitioner must show that counsel's failures prejudiced his defense. Strickland, 466 U. S., at 692. In Strickland, we made clear that, to establish prejudice, a "defendant must show that there is a reasonable probability that, but for counsel's unprofessional errors, the result of the proceeding would have been different. A reasonable probability is a probability sufficient to undermine confidence in the outcome." Id., at 694. In assessing prejudice, we reweigh the evidence in aggravation against the totality of available mitigating evidence. In this case, our review is not circumscribed by a state court conclusion with respect to prejudice, as neither of the state courts below reached this prong of the Strickland analysis.
The mitigating evidence counsel failed to discover and present in this case is powerful. As Selvog reported based on his conversations with Wiggins and members of his fam-
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