In In re Est. of Lopez, Guadalupe Lopez, Jr. applied for independent administration and heirship determination after his father’s death. 724 S.W.3d 847 (Tex. 2025). Gonzalez filed a petition claiming to be Lopez Senior’s common-law wife. At trial, the court allowed former judge Alicia York to testify as an expert that Gonzalez and Lopez Senior were informally married. York emphasized her judicial experience and stated she was using the same methodology she employed when presiding over informal marriage cases. The jury found that Lopez Senior and Gonzalez were married, and Lopez Junior appealed. The court of appeals affirmed, and Lopez Junior appealed to the Texas Supreme Court.

The Texas Supreme Court reversed the lower courts and the jury verdict due to the admission of York’s testimony. The Court reasoned that determining whether an informal marriage existed did not require specialized knowledge beyond an average juror’s understanding. The elements of informal marriage (agreement, cohabitation, and representation) are not technical or specialized concepts. Judge York’s testimony served only to provide an official endorsement of Gonzalez’s position, which was improper. The Court emphasized that former judges are not categorically prohibited from testifying as experts, but their judicial status should not be repeatedly emphasized to sway the jury. The Court’s summary of its opinion is:

“[T]he expert witness paints a powerful image on the litigation canvas,” possessing a “vast potential for influence.” In re Christus Spohn Hosp. Kleberg, 222 S.W.3d 434, 440 (Tex. 2007). The expert in this case is a former family court judge who opined on the central disputed issue in the case, testifying before the jury that, in her view, the parties were informally married. She expressed that opinion repeatedly and emphatically, though none of the relevant evidence was technical, specialized, or otherwise beyond an ordinary person’s understanding. Compounding the problem, she lent her opinion credibility by explaining that she formed it using the same methodology she employed when she presided over informal-marriage cases as a judge. Unsurprisingly, the jury was persuaded and found an informal marriage existed.

We hold that the trial court erred in admitting the expert’s testimony and that the error was harmful, warranting a new trial. The evidence was within the average juror’s common knowledge, so the expert’s opinions did not help the jury as Rule of Evidence 702 contemplates. See Tex. R. Evid. 702. The expert instead improperly opined on a legal presumption and explained that, if she were sitting as the judge, she would find that the parties were informally married. Mindful that the appearance of a judge as a witness carries the potential both to sway the jury improperly and to “threaten[] . . . public confidence in the integrity and impartiality of the judiciary,” Joachim v. Chambers, 815 S.W.2d 234, 238 (Tex. 1991) (quotation marks omitted), we conclude that admitting this testimony amounted to harmful error. Accordingly, we reverse the court of appeals’ judgment and remand the case to the trial court for a new trial.

Id. at 848-49.