Cite as: 539 U. S. 510 (2003)
(a) A federal writ can be granted only if a state court decision "was contrary to, or involved an unreasonable application of, clearly established" precedents of this Court. 28 U. S. C. § 2254(d)(1). This "unreasonable application" prong permits the writ to be granted when a state court identifies the correct governing legal principle but unreasonably applies it to the facts of a petitioner's case. Williams v. Taylor, 529 U. S. 362, 413. For this standard to be satisfied, the state court decision must have been "objectively unreasonable," id., at 409, not just incorrect or erroneous. An ineffective assistance claim has two components: A petitioner must show that counsel's performance was deficient, and that the deficiency prejudiced the defense. Strickland v. Washington, 466 U. S. 668, 687. Performance is deficient if it falls below an objective standard of reasonableness, which is defined in terms of prevailing professional norms. Id., at 688. Here, as in Strickland, counsel claim that their limited investigation into petitioner's background reflected a tactical judgment not to present mitigating evidence and to pursue an alternative strategy instead. In evaluating petitioner's claim, this Court's principal concern is not whether counsel should have presented a mitigation case, but whether the investigation supporting their decision not to introduce mitigating evidence of Wiggins' background was itself reasonable. The Court thus conducts an objective review of their performance, measured for reasonableness under prevailing professional norms, including a context-dependent consideration of the challenged conduct as seen from counsel's perspective at the time of that conduct. Id., at 688, 689. Pp. 519-523.
(b) Counsel did not conduct a reasonable investigation. Their decision not to expand their investigation beyond a presentence investigation (PSI) report and Baltimore City Department of Social Services (DSS) records fell short of the professional standards prevailing in Maryland in 1989. Standard practice in Maryland capital cases at that time included the preparation of a social history report. Although there were funds to retain a forensic social worker, counsel chose not to commission a report. Their conduct similarly fell short of the American Bar Association's capital defense work standards. Moreover, in light of the facts counsel discovered in the DSS records concerning Wiggins' alcoholic mother and his problems in foster care, counsel's decision to cease investigating when they did was unreasonable. Any reasonably competent attorney would have realized that pursuing such leads was necessary to making an informed choice among possible defenses, particularly given the apparent absence of aggravating factors from Wiggins' background. Indeed, counsel discovered no evidence to suggest that a mitigation case would have been counterproductive or that further investigation would have been fruitless, thus distinguishing this case
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