Prosecutors first involved Welner at the early stages after the arrest, before filing charges,[23] to consider defense claims from several psychologists that Hernandez was mentally ill and intellectually disabled and falsely confessed.
Welner testified for prosecutors at the original trial and then at the retrial, in which a jury unanimously rendered a guilty verdict to Hernandez in early 2017.
The case was contemplated for dismissal in state court [28] when federal prosecutors asked Welner to study the matter to a definitive end.
[27] His testimony drew particular attention to cognitive distortions as they differ from delusions,[29][32] and culture-specific beliefs of fundamentalist LDS adherents.
[47] [48] [49] [50] [51] [52] [53] [54] Harvey Updyke, a fanatic of Alabama Crimson Tide football, was charged with poisoning the iconic trees at Auburn University's Toomer's Corner in 2011.
[59] Welner also engaged mental health issues related to the culture of professional sports as the forensic psychiatry examiner in the manslaughter trial of former NBA star Jayson Williams,[60] and the death investigation of wrestling champion Chris Benoit.
In 2005, the conviction was overturned because the prosecution witness, Park Dietz, falsely testified that Yates' behavior and defense was identical to an earlier episode of Law & Order.
In the videotaped interview with Welner, Yates admitted that she had actually determined to kill the children two months earlier and at a time of relative stability, and was waiting for her first occasion to be alone with them.
[67] District Attorney Paul Connick asked Welner to review the available evidence, the circumstances of the interrogation and the setting in which it occurred, as well as Thibodeaux' vulnerabilities, to inform their decision-making about the case.
Welner has examined other cited disputed confession cases in North American courts,[69][70] including the killer of New York Socialite Linda Stein[71] and the Albuquerque murder investigation of Victoria Martens.
Welner's review of the case included a videotaped interview of the defendant,[73] in which they discussed the events of the crime, his movements before and during the shooting of Samuels, and what was influencing his decisions.
In the court's opinion, the error required a new trial because Welner's testimony included a detailed accounting of Cheever's actions in his own words, and was, "extensive and devastating.
[73] Khadr, a fifteen-year-old Canadian expatriate living in Afghanistan, was charged with the killing of U.S. Army medic Christopher Speer at an al-Qaeda safe house in Khost.
[75] The Department of Defense engaged Welner to examine claims by Khadr that his confessions were coerced, or alternatively, that he was too immature to withstand interrogation.
[75] Military prosecutors also asked Welner to assess Khadr's likelihood of recidivism into radical jihadism upon release, for presentation at a sentencing hearing.
[77] At the sentencing proceeding, Welner testified that Khadr was a high risk of recidivism into activities providing support for jihadist terrorism, although he did not expect him to be directly violent.
[77] Welner has consulted to the United States government on other terrorism cases as well, including the accused planners of 9-11[78] and the alleged mastermind of the USS Cole Bombing.
[80] The Forensic Panel includes over thirty professionals who offer consultation in fields such as psychiatry, psychology, neuroradiology, emergency and critical care medicine, nursing, toxicology, and pathology.
Welner has researched an evidence-based approach for courts and juries tasked with defining "heinous," "depraved," and "evil" crimes in sentencing determinations.
[82] This research emphasizes the importance of gathering objective evidence rather than relying on impressionistic arguments, aiming to establish a methodology that minimizes bias related to race, diagnosis, prognosis, socioeconomics, or other personal factors.
[82] The Depravity Standard, informed in part by this data, by higher court decisions, and by evidence from adjudicated cases,[86] is not a psychological evaluation or test.
[88] The CIEEO is inspired by the goals of vigilance for child abuse – namely, detection and identification being the first step toward intervention and treatment of the worst of behaviors.
[89] Welner was a contributor in a range of proposals within landmark mental health reform legislation that passed the United States Congress in 2016.
He was a forensic science consultant to ABC News' Law and Justice unit,[97] and familiar to many viewers in particular of Good Morning America [98] and 20/20.