Limits on Former Judges as Expert Witnesses and the Rule 702 “Helpfulness” Requirement in Informal-Marriage Cases: Commentary on In re Estate of Guadalupe Lopez, Sr.

I. Introduction

The Supreme Court of Texas’s per curiam decision in In re Estate of Guadalupe Lopez, Sr., No. 24-0315 (Tex. Nov. 7, 2025), squarely addresses two recurring and increasingly important questions in Texas litigation:

When does expert testimony actually help the jury under Texas Rule of Evidence 702?
What limits exist on former judges testifying as expert witnesses, particularly when they opine on how they would decide the very issue the jury must resolve?

The Court holds that it was reversible error for the trial court to admit the testimony of a former family-law judge who told the jury that the elements of an informal (common-law) marriage were “clearly” satisfied and explained that she would find the parties to be married if she were the judge. Because the existence of an informal marriage is within the common understanding of lay jurors, and because the expert’s testimony strayed into legal instruction and an “official endorsement” of one side’s position, the testimony did not meet Rule 702’s “helpfulness” requirement and improperly invaded the province of both the judge and the jury.

The Court’s opinion does more than decide an evidentiary point in a probate dispute. It clarifies:

That proof of informal marriage generally does not call for expert testimony,
That former judges, while not categorically barred as experts, must not use their prior judicial status to tell a jury how to decide disputed fact questions or how to apply legal presumptions, and
That such error can be harmful and justify a new trial even where there is other evidence supporting the verdict.

This commentary analyzes the decision in detail, situating it within Texas precedent on expert testimony, judicial-witness concerns, and harmless-error review, and exploring its implications for probate, family, and civil trial practice.

II. Factual and Procedural Background

A. The underlying estate dispute

After the death of Guadalupe Lopez, Sr. (Lopez Sr.), his son, Guadalupe Lopez, Jr. (Lopez Jr.), applied in county court for:

Independent administration of his father’s estate; and
An heirship determination—a judicial determination of who the legal heirs are and what their shares should be.

The county court granted the application and declared that Lopez Jr. and his two siblings were the heirs of their father’s estate.

B. Gonzalez’s bill of review and informal-marriage claim

Some time later, Elvira Gonzalez filed a petition for bill of review, asserting that:

She had been the informal (common-law) wife of Lopez Sr., and
As a surviving spouse, she should be declared an heir of his estate.

A bill of review is an independent equitable proceeding that attacks a final judgment on limited grounds (e.g., lack of service, fraud, or other serious defects) when no ordinary remedy remains. Here, the trial court chose to defer ruling on the bill of review until a jury determined a threshold factual question: Were Gonzalez and Lopez Sr. informally married under Texas law?

C. The former judge as expert witness

To prove the existence of an informal marriage, Gonzalez offered the videotaped deposition testimony of Alicia York, a former district court judge who had presided over family-law cases.

Over Lopez Jr.’s timely objections, the trial court admitted this testimony. In it, Judge York:

Testified as an expert on the existence of an informal marriage,
Stated that Gonzalez “clearly” showed that all three elements of common-law marriage existed, and
Reinforced that she was speaking with the authority of her prior judicial experience, noting that she had presided over some 5,000 family-law trials and that she was effectively “wearing [her] judge’s hat” in reaching her conclusions.

The jury found that an informal marriage existed between Gonzalez and Lopez Sr. The trial court then granted the bill of review and awarded Gonzalez a share of the estate as a surviving spouse.

D. Appeal and court of appeals ruling

On appeal, Lopez Jr. challenged, among other things, the admission of Judge York’s expert testimony. The Fourth Court of Appeals in San Antonio affirmed the judgment. Importantly, it:

Did not decide whether admitting Judge York’s testimony was an abuse of discretion; instead,
Held that any error was harmless for four reasons:
1. The testimony was “cumulative,”
2. York did not articulate “improper legal concepts,”
3. Gonzalez did not “emphasize” the testimony, and
4. Other evidence supporting informal marriage “comfortably withstood” a factual sufficiency challenge.

Lopez Jr. petitioned for review to the Supreme Court of Texas.

III. Summary of the Supreme Court’s Opinion

A. Issues

The Supreme Court addressed two central questions:

Did the trial court abuse its discretion in admitting the former judge’s expert testimony that an informal marriage existed?
If so, was the error harmful—that is, did it probably cause the rendition of an improper judgment?

B. Holdings

The Court answered yes to both questions:

Abuse of discretion: The trial court erred by admitting the expert testimony because:
- The existence of an informal marriage is within the common knowledge of jurors; expert testimony was therefore not “helpful” as Rule 702 requires.
- The expert improperly opined on legal matters, including a Family Code presumption, and effectively told the jury how to apply the law.
- By repeatedly invoking her prior judicial role and explaining how she would rule, she supplied an “official endorsement” of Gonzalez’s case, undermining judicial impartiality and the jury’s independent fact-finding function.
Harmful error: The erroneous admission was harmful because:
- The testimony was crucial to the only contested issue in a case with sharply conflicting evidence.
- The testimony was not merely cumulative of other evidence; York was the only expert and she directly told the jury how to weigh the conflicting proof.
- Gonzalez’s insistence on presenting this evidence over objection showed it was a calculated and central component of her trial strategy.
- The testimony was repeatedly framed in terms of the witness’s judicial status, magnifying its persuasive (and prejudicial) impact.

C. Disposition

Without hearing oral argument, the Supreme Court:

Granted the petition for review,
Reversed the court of appeals’ judgment, and
Remanded the case to the trial court for a new trial.

IV. Legal Analysis

A. The Rule 702 “helpfulness” requirement and common knowledge

Texas Rule of Evidence 702 allows expert testimony if:

the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue.

The Court reemphasizes the principle, drawn from K-Mart Corp. v. Honeycutt, 24 S.W.3d 357 (Tex. 2000), that:

When the jury is equally competent to form an opinion about the ultimate fact issues or the expert’s testimony is within the common knowledge of the jury, the trial court should exclude the expert’s testimony.

Applying that standard, the Court concludes that the elements of an informal marriage under Texas Family Code § 2.401(a)(2) are not matters requiring specialized expertise:

Agreement to be married,
Cohabitation in Texas as husband and wife after the agreement, and
Holding out to others as a married couple.

These are ordinary factual questions about a couple’s conduct and representations—well within the experience and understanding of average jurors. The Court underscores that Gonzalez offered no argument that specialized knowledge was needed to evaluate the evidence of informal marriage.

By reaffirming that such issues fall within common knowledge, the Court effectively announces a strong presumption: informal-marriage disputes typically do not warrant expert witnesses, at least not for the core “agreement–cohabitation–holding-out” analysis.

B. Expert testimony that instructs on the law or applies legal presumptions

The Court notes that to the extent Judge York’s testimony involved anything arguably “specialized,” it was in explaining a Family Code legal presumption and how the jury should apply it to the evidence. That is precisely what experts are not allowed to do.

The Court cites In re Ayad, 655 S.W.3d 285, 288 n.1 (Tex. 2022), and relies on the general rule that:

The trial judge, not an expert witness, must define and explain the law to the jury.
Expert testimony on what the law is, or how to apply a statutory presumption, impermissibly usurps the court’s function.

The opinion quotes Greenberg Traurig of N.Y., P.C. v. Moody, 161 S.W.3d 56 (Tex. App.—Houston [14th Dist.] 2004, no pet.), which aptly states:

It is the trial court’s role—not that of the expert witness—“to define the particular legal principles applicable to a case.”

Texas Rule of Evidence 704(a) does allow experts to offer opinions that embrace an “ultimate issue” in the case, but there is a crucial distinction:

Experts may express fact-based opinions (e.g., “in my engineering opinion, this design is unreasonably dangerous”), but
They may not deliver pure legal conclusions or instruct the jury on how to interpret and apply law (e.g., “in my opinion, this meets the legal standard for strict liability”).

Judge York’s testimony went beyond factual synthesis. She:

Explained a Family Code presumption to the jury, and
Told them how that presumption should affect their evaluation of the evidence.

That is classic legal-instruction territory, reserved exclusively for the judge through the jury charge.

C. “Official endorsement” and the dangers of former judges as expert witnesses

The Court’s concern is amplified by the witness’s prior judicial status. Relying heavily on Joachim v. Chambers, 815 S.W.2d 234 (Tex. 1991), the Court reiterates that when a judge becomes a witness:

There is serious risk that the judge’s testimony will act as an “official endorsement” for one side.
Such participation can undermine public confidence in judicial impartiality.

Here, Judge York:

Repeatedly referenced her experience as a family-law judge,
Indicated that she was forming her opinion by doing what she would have done as a judge in informal-marriage cases, and
Effectively told the jury: based on the evidence and my judicial methodology, Gonzalez should win.

This placed the jury in an untenable position: to disagree with Gonzalez’s evidence was, in effect, to disagree with a (former) judge who purported to apply judicial standards and experience.

The Supreme Court does not adopt a categorical rule barring former judges from testifying as experts. In a footnote, it explicitly recognizes that former judges may appropriately testify, for example, about the reasonableness of attorney’s fees. However, it cautions:

Court, counsel, and witnesses must avoid repeatedly touting or emphasizing the expert’s status as a former judge.
Any such testimony must still satisfy Rule 702’s helpfulness requirement and not intrude into the roles of judge and jury.

The core message: Former judges are not legal or factual “super-witnesses.” They are subject to the same evidentiary constraints as other experts, with additional sensitivity to how their prior role may unduly influence the jury.

D. Application to the facts: why expert testimony was improper

Synthesizing the above principles, the Court identifies two decisive reasons why admitting York’s testimony violated Rule 702:

No specialized knowledge was needed. The factual dispute was about whether the couple:
- Agreed to be married,
- Lived together as such, and
- Held themselves out as married.
Jurors can evaluate evidence such as invitations, tax documents, deeds, and testimony about how the parties described each other without any expert intervention.
The testimony improperly supplied legal guidance and a judicial conclusion. York did not merely summarize evidence or describe family-law practice in abstract terms. She:
- Explained a statutory presumption and its application to the facts,
- Repeatedly indicated that she was using her judicial methodology, and
- Gave the jury her bottom-line conclusion: all three elements of informal marriage were “clearly” established, and Gonzalez should prevail if she (York) were deciding the case.

Thus, the testimony failed the “helpfulness” test and improperly invaded judicial and juror roles.

V. Harmful-Error Analysis

Under Texas Rule of Appellate Procedure 61.1(a) and case law such as Reliance Steel & Aluminum Co. v. Sevcik, 267 S.W.3d 867 (Tex. 2008), erroneous admission of evidence is reversible only if the error “probably (though not necessarily) caused the rendition of an improper judgment.” Gunn v. McCoy, 554 S.W.3d 645, 668 (Tex. 2018), further refines this as whether the error “contributed in a substantial way” to the adverse judgment.

The Court’s harm analysis is multi-factor and case-specific, looking at:

The centrality of the evidence to key issues,
The strength or weakness of the remaining evidence,
Whether the evidence was cumulative,
Whether admission or use was calculated or inadvertent, and
Whether the evidence was emphasized at trial.

A. Centrality and contested nature of the issue

The expert’s testimony addressed the only contested issue: whether an informal marriage existed. The Court describes the evidence on this issue as “cut both ways”, not one-sided.

Evidence supporting Gonzalez’s claim included:

Photos of Lopez Sr. wearing a wedding ring,
Two quinceañera invitations for Gonzalez’s daughters identifying the couple as “Mr. & Mrs. Lupe Lopez,” and
A tax form and a warranty deed that identified them as married.

Contrary evidence included:

No joint bank account between them,
Various documents (tax forms, employment records, car insurance) in which they identified themselves as single, and
Testimony that during the same period Gonzalez claimed to be his wife, Lopez Sr. referred to another woman as his “fiancée.”

Given this conflicting record, whether there had been an agreement to be married was genuinely disputed. The Court finds it “far from clear” that the jury would have found an agreement without York’s testimony telling them how a judge would weigh that evidence and what the outcome should be.

Because the expert testimony was crucial to a central, hotly contested issue, the presumption tilts strongly toward harmfulness.

B. Was the expert’s testimony cumulative?

The court of appeals reasoned that York’s testimony was harmless because it was “cumulative”—essentially duplicative—of other evidence. The Supreme Court disagrees:

York’s testimony did not merely restate facts already in the record; it interpreted, weighed, and prioritized them.
She provided a normative conclusion—that the elements of informal marriage were “clearly” met and Gonzalez should win.
She was the only expert on the informal-marriage question; there were no other similar expert opinions that might render her views cumulative.

The Court distinguishes situations like:

State v. Dawmar Partners, Ltd., 267 S.W.3d 875 (Tex. 2008), and Mancorp, Inc. v. Culpepper, 802 S.W.2d 226 (Tex. 1990), where multiple experts offered similar testimony; and
GTE Southwest, Inc. v. Bruce, 998 S.W.2d 605 (Tex. 1999), where an “abundance of evidence” overwhelmingly supported the verdict.

By contrast, this case involved:

One expert,
Conflicting fact evidence, and
An opinion that likely “tipped the balance” (as in Bagheri v. State, 119 S.W.3d 755 (Tex. Crim. App. 2003), where an expert’s intoxication opinion was found non-cumulative and harmful despite other evidence).

C. Calculated use versus inadvertent admission

The Court also emphasizes whether the admission and use of the evidence were:

Inadvertent—e.g., a stray comment or unobjected statement, or
Calculated—deliberately sought and defended as a strategic component of the case.

Here, Gonzalez:

Offered the testimony affirmatively,
Insisted on its admission over objection, and
Relied on it as part of her evidentiary package to prove informal marriage.

Citing U-Haul Int’l, Inc. v. Waldrip, 380 S.W.3d 118 (Tex. 2012), and Sevcik, the Court notes that such insistence “indicates how important [the party] thought it was to [her] case” (quoting Alvarado v. Farah Mfg. Co., 830 S.W.2d 911, 917 (Tex. 1992)). That calculated use weighs in favor of finding harm.

D. Emphasis during trial

The court of appeals downplayed the error on the ground that Gonzalez supposedly did not “emphasize” York’s testimony. The Supreme Court rejects this view, citing Sevcik for the proposition that even evidence mentioned only once can be prejudicial:

“[I]f that were the only rule, there would be little use for the rules of evidence as everyone could ignore them once with impunity.”

In fact, York’s judicial status was highlighted at multiple points. She was introduced to the jury as a former family-law judge, and references to her judicial experience, methodology, and prior cases pervaded her testimony. The Court finds that this repeated emphasis enhanced the weight and potential prejudice of her statements.

E. Overall conclusion on harm

Considering all these factors—the centrality of the issue, the conflicting evidence, the non-cumulative and authoritative nature of the expert opinion, and the calculated, emphasized use—the Court concludes that the error:

Probably caused the rendition of an improper judgment; and
Contributed in a substantial way to the adverse judgment against Lopez Jr.

The error therefore cannot be considered harmless; a new trial is required.

VI. Precedents and Authorities Cited

The opinion is tightly woven into a fabric of prior Texas decisions on expert testimony, judicial neutrality, and harmful-error review. The principal authorities and their roles include:

1. In re Christus Spohn Hospital Kleberg, 222 S.W.3d 434 (Tex. 2007)

The Court opens by quoting Christus Spohn on the powerful impact of expert witnesses, who “paint[] a powerful image on the litigation canvas” and possess “a vast potential for influence.” This sets the tone: expert testimony is inherently potent and must be carefully controlled. The Court uses this to justify a strict enforcement of Rule 702’s “helpfulness” requirement and cautions particularly when the expert is perceived as an objective authority figure.

2. Joachim v. Chambers, 815 S.W.2d 234 (Tex. 1991)

Joachim directly addresses the dangers of a judge testifying as a witness. The Court cites it for two main propositions:

Such participation can provide an “official endorsement” for a party’s position, and
It can “threaten[] public confidence in the integrity and impartiality of the judiciary.”

The opinion also echoes Joachim’s warning that “the entrance of a judge into the litigation arena in aid of a combatant” affects not only the outcome of the case but the broader concept of judicial impartiality. This backdrop is central to the Court’s disapproval of York’s repeated references to her judicial role.

3. Service Corp. Int’l v. Guerra, 348 S.W.3d 221 (Tex. 2011)

Guerra is cited for the standard of review—abuse of discretion—governing evidentiary rulings and for its discussion of harmful error. It provides the framework that:

Appellate courts must evaluate the entire record, “from voir dire to closing argument,”
Considering how the evidence fit into the whole trial, whether it was emphasized, and how it compared to other proof.

4. In re J.P.B., 180 S.W.3d 570 (Tex. 2005)

Cited for the abuse-of-discretion standard of review, reinforcing that trial courts have wide latitude in evidentiary matters but that this discretion is not unfettered.

5. K-Mart Corp. v. Honeycutt, 24 S.W.3d 357 (Tex. 2000)

Honeycutt provides the foundational rule for this case:

“[W]hen the jury is equally competent to form an opinion about the ultimate fact issues or the expert’s testimony is within the common knowledge of the jury, the trial court should exclude the expert’s testimony.”

The Court uses Honeycutt to conclude that no expert assistance was needed to evaluate the informal-marriage evidence.

6. Guevara v. Ferrer, 247 S.W.3d 662 (Tex. 2007)

Guevara is cited for contrast: in medical-causation cases involving conditions “outside the common knowledge and experience of jurors,” expert testimony is required. By juxtaposing Guevara with the present case, the Court emphasizes that informal-marriage determinations are the opposite—inside common knowledge, and thus do not call for experts.

7. Dallas Morning News, Inc. v. Hall, 579 S.W.3d 370 (Tex. 2019)

Hall is cited for the proposition that if expert testimony is unhelpful, it is inadmissible and its admission is error. It reinforces that “helpfulness” is not a trivial threshold but a substantive filter.

8. Reliance Steel & Aluminum Co. v. Sevcik, 267 S.W.3d 867 (Tex. 2008)

Sevcik anchors the harmful-error standard: erroneous admission is reversible only if it probably caused an improper judgment. It also supplies factors:

Role of the evidence in the trial,
Whether it was cumulative, and
Whether the rest of the evidence was so one-sided that the error likely made no difference.

The Court uses these factors to reject the court of appeals’ harmlessness holding.

9. Gunn v. McCoy, 554 S.W.3d 645 (Tex. 2018)

Gunn refines the harmfulness inquiry: an error is harmful if it “contributed in a substantial way to bring about the adverse judgment,” quoting Lorusso v. Members Mut. Ins. Co., 603 S.W.2d 818 (Tex. 1980). The Court uses this formulation as the operative standard in Lopez.

10. Kia Motors Corp. v. Ruiz, 432 S.W.3d 865 (Tex. 2014)

Kia Motors underscores that improper evidence can be harmful even if there is other legally sufficient evidence to support the verdict. This rebuts any suggestion that the existence of supporting evidence, by itself, makes the error harmless. The Court relies on this to reject the court of appeals’ focus on factual sufficiency.

11. U-Haul Int’l, Inc. v. Waldrip, 380 S.W.3d 118 (Tex. 2012)

Waldrip is cited for the factor that a party’s insistence on introducing challenged evidence over objection suggests that it was a “significant element” of the case, supporting a finding of harm.

12. Jackson v. Takara, 675 S.W.3d 1 (Tex. 2023); In re Estate of Poe, 648 S.W.3d 277 (Tex. 2022); Caffe Ribs, Inc. v. State, 487 S.W.3d 137 (Tex. 2016)

These decisions are cited collectively as recent examples of the Court’s modern, fact-intensive harmful-error jurisprudence, illustrating that the Court routinely undertakes a thorough review of how improper evidence fits into the entire trial.

13. In re Ayad, 655 S.W.3d 285 (Tex. 2022)

Ayad is invoked (via a footnote) for the principle that experts may not testify about what the law is or opine on legal standards, reaffirming the boundary between legal instruction (the judge’s role) and factual opinion (the expert’s domain).

14. Greenberg Traurig of N.Y., P.C. v. Moody, 161 S.W.3d 56 (Tex. App.—Houston [14th Dist.] 2004, no pet.)

Moody is quoted for two key points:

It is the trial court’s job to define applicable law, and
Allowing lawyers (or, by extension, legally trained experts) to opine on what the law is “usurps or diminishes” the trial judge’s role as the court’s legal expert.

15. State v. Dawmar Partners, Ltd., 267 S.W.3d 875 (Tex. 2008); Mancorp, Inc. v. Culpepper, 802 S.W.2d 226 (Tex. 1990)

These cases are used to distinguish situations where improper expert evidence was held harmless because it was cumulative of substantially similar testimony from multiple other experts. In Lopez, by contrast, there was only one expert, and her testimony was not duplicative of any other expert evidence.

16. Bagheri v. State, 119 S.W.3d 755 (Tex. Crim. App. 2003)

Bagheri is a criminal case in which expert testimony on a defendant’s intoxication “almost certainly tipped the balance” in favor of the State. The Court analogizes this to York’s testimony, which likely tipped the balance on the informal-marriage question, reinforcing the conclusion that the error was harmful and not cumulative.

17. GTE Southwest, Inc. v. Bruce, 998 S.W.2d 605 (Tex. 1999)

GTE stands for the idea that improper evidence may be harmless when there is an “abundance of evidence” independently supporting the challenged finding. The Court distinguishes Lopez because the evidence on informal marriage was not one-sided; thus, York’s testimony cannot be written off as inconsequential.

18. Alvarado v. Farah Mfg. Co., 830 S.W.2d 911 (Tex. 1992)

Cited via Sevcik, Alvarado contributes the insight that a party’s persistence in offering inadmissible evidence indicates its perceived importance and bolsters a finding of harmfulness when that evidence is wrongly admitted.

VII. Complex Concepts Explained in Plain Terms

A. Informal (common-law) marriage in Texas

Texas recognizes informal marriages (often called “common-law marriages”) under Texas Family Code § 2.401(a)(2). To prove an informal marriage, a party must show:

Agreement to be married: Both people had a present, mutual intention to be married to each other (not a future promise to marry).
Cohabitation in Texas as husband and wife: They lived together in Texas, in a manner consistent with a marital relationship.
Holding out: They represented to others that they were married (e.g., by using the same last name, introducing each other as spouses, filing documents as married).

This case turns almost entirely on the first element—whether Lopez Sr. and Gonzalez had an actual agreement to be married—evaluated through their conduct and representations.

B. Independent administration and heirship determination

An independent administration is a relatively streamlined form of probate in which the executor or administrator can act with minimal court supervision.

An heirship determination is a formal proceeding, usually necessary when there is no will (intestacy) or when there is uncertainty about who the legal heirs are. The court issues a judgment identifying the heirs and their respective shares under Texas intestacy law.

C. Bill of review

A bill of review is an equitable, post-judgment remedy. It allows a party to challenge a final judgment after the usual period for appeal has expired, but only on limited grounds (such as lack of proper service, fraud, or other extraordinary circumstances). In this case, Gonzalez used a bill of review to attack the existing heirship judgment, arguing that she had been wrongly excluded as a surviving spouse.

D. Expert testimony and “helpfulness”

Under Rule 702, an expert may testify if:

The subject matter requires scientific, technical, or other specialized knowledge, and
The expert’s testimony will help the jury understand the evidence or decide a disputed fact.

If jurors can understand the subject using common sense and ordinary experience, expert testimony is not helpful and should be excluded. Likewise, if the expert is simply telling the jury what result to reach under the law, the testimony is improper.

E. Harmless error and harmful error

On appeal, not every mistake leads to reversal. An error is considered:

Harmless if it probably did not influence the outcome of the case; the judgment stands.
Harmful if it probably did influence the outcome in a meaningful way; the judgment is reversed and (usually) a new trial ordered.

The appellate court asks: looking at the whole trial, is it reasonably likely that this error affected the verdict? If yes, it is harmful.

F. Per curiam opinion and nonparticipation

A per curiam opinion is issued “by the court” without a named author, often signaling that the Court views the legal issues as relatively straightforward or as reaffirming existing law. Here, Justice Lehrmann did not participate in the decision, which is noted in the opening paragraph but does not affect the opinion’s precedential force.

VIII. Impact and Implications

A. For probate and informal-marriage litigation

This decision has immediate consequences for probate and family law practitioners:

Reduced role for experts in informal-marriage cases: Parties seeking to prove or disprove an informal marriage should focus on lay testimony, documentary evidence, and circumstantial proof, rather than expert opinions on whether the statutory elements are met.
Sharper focus on objective evidence: Evidence like tax returns, deeds, insurance applications, and how the parties identified themselves socially will likely be more decisive than any expert gloss.
Heightened scrutiny on “legal experts”: Attempts to call attorneys or former judges to interpret the Family Code or opine that a “marriage exists” will face strong resistance and likely exclusion under this precedent.

B. For use of former judges as expert witnesses

The opinion sends a clear signal about the limits on former judges as experts:

There is no blanket prohibition, but:
Their testimony must be tightly confined to areas where they have specialized knowledge that lay jurors do not (e.g., prevailing rates and reasonableness of attorney’s fees, typical court procedures).
They must avoid:
- Explaining what the law is,
- Describing legal presumptions and how to apply them, and
- Opining on how a court should rule on disputed issues in the case.
Counsel and trial courts should take steps to minimize references to the witness’s prior judicial status, to reduce the risk of undue influence on the jury.

Practically, litigants should think twice before using a former judge as a “legal expert” on the merits of their case; the safer path is to use such witnesses only in areas that are genuinely technical and do not overlap with the jury’s fact-finding or the judge’s role in stating the law.

C. For trial judges managing expert testimony

The decision reinforces trial judges’ gatekeeping responsibilities:

Scrutinize “helpfulness”: Even a well-credentialed expert must be excluded if the subject is within common experience, or if the testimony is simply a legal opinion in disguise.
Guard the jury’s role: Do not allow experts—especially those with judicial experience—to tell the jury what result to reach or how the law applies to specific facts.
Be cautious with former judges and lawyers as experts: Their legal training may tempt them to opine on legal standards, but that is not permissible expert testimony under Texas law.

D. For appellate practice and harmful-error arguments

From an appellate perspective, Lopez illustrates that:

Courts will not accept generic “cumulative evidence” or “enough other evidence” arguments without a careful, issue-by-issue analysis.
Improper expert testimony, especially on a central and contested issue, is a strong candidate for a finding of harmful error.
Advocates should make a record of:
- How the evidence was used in argument,
- Objections and rulings, and
- The relative strength and weakness of the other evidence.

The decision also confirms that the presence of some supporting evidence does not automatically render an error harmless. The key is whether the error could well have been outcome-determinative in a closely contested case.

IX. Conclusion

In re Estate of Guadalupe Lopez, Sr. is a significant clarification of Texas law at the intersection of expert testimony, judicial impartiality, and informal-marriage disputes. The Supreme Court holds that:

The existence of an informal marriage is a question within the common knowledge of jurors and ordinarily does not warrant expert opinion under Rule 702.
Experts—including former judges—may not explain legal presumptions, instruct jurors on how to apply the law, or offer what amounts to a judicial ruling on disputed factual issues.
Improper expert testimony by a former judge, especially when presented as an official judicial endorsement of one party’s position, can be harmful error requiring a new trial, even when other evidence supports the verdict.

Beyond its immediate impact on this probate dispute, the decision strengthens the boundaries between:

The judge, who declares and explains the law,
The jury, which finds the facts, and
The expert witness, who may assist only when specialized knowledge is truly needed and then only on factual, not legal, questions.

By insisting that experts—especially former judges—stay in their lane, the Court reaffirms fundamental trial principles: juries, not experts, decide disputed facts; judges, not witnesses, state the law; and public confidence in the judiciary requires that its members, current or former, not lend the mantle of judicial authority to one side’s cause in the courtroom.