Managing Joint-Defense Vetting: Co-Retaining Experts in MDL

Joint-defense expert retention in an MDL rests on the common-interest doctrine, a court-created extension of the attorney work-product protection that Hickman v. Taylor recognized and that Federal Rule of Civil Procedure 26(b)(3) later codified for trial-preparation material. The common-interest doctrine itself is not codified in any federal statute or rule. It developed through case law, and both whether it is recognized and how far it reaches vary by jurisdiction. The doctrine is not a standalone privilege. It is a non-waiver rule. Where recognized, it lets separately represented parties who share a legal interest exchange otherwise privileged material without treating the disclosure to a co-defendant as a disclosure to an adversary.

When multiple defense firms co-retain one forensic expert, every communication that flows to and from that expert is a potential waiver event. The expert sits inside the tent, receiving privileged theories from several firms at once. The mechanism that keeps that traffic protected is a written common-interest agreement that predates the first substantive exchange, defines the shared legal interest narrowly, and names the expert as an agent of the group for the purpose of that shared interest.

• Shared legal interest, not shared commercial interest. Courts test whether the parties pursued a coordinated legal strategy against a common adversary. Co-defendants who are also cross-claiming against each other have a divided interest, and the doctrine can collapse on the exact issue where they diverge.
• The qualifying test varies by circuit. Some circuits require the shared interest to be identical and legal, not merely similar or commercial. Others accept a substantially similar legal interest, and at least one recognizes a common interest that is legal, factual, or strategic. Some circuits also require proof of an agreement to jointly strategize, written or implied by conduct, before the doctrine applies. Confirm the controlling standard in the transferee court and in each home circuit with counsel before relying on the doctrine.
• Agency framing. The expert must be retained as an agent assisting counsel in rendering legal advice and preparing for trial, not as a business consultant. The retention language controls how a court later characterizes the relationship.
• Oral agreements are fragile. A written instrument dated before information changes hands is the auditable artifact a special master will ask for.

The single most consequential procurement decision is which entity signs the expert engagement letter. It determines who controls the work, who owns the file, who pays, and whose privilege attaches. Three structures dominate.

1. Lead-counsel-of-record holds the paper. One firm, often designated liaison or lead defense counsel, executes the engagement and the other firms accede through the common-interest agreement. Cleanest privilege chain, single point of instruction, single invoice stream. The trade-off is control concentration. Non-signing firms depend on lead counsel to route instructions and cannot unilaterally task the expert.
2. Defense steering committee or coordinating entity holds the paper. In organized MDLs with a defense steering committee, a coordinating counsel or a special-purpose engagement holds the retainer on behalf of the group. This mirrors the plaintiff-side common-benefit model and scales to large defendant groups, but it requires a governance charter that specifies voting on scope changes and dispute resolution when defendants disagree on expert direction.
3. Parallel co-retention. Each firm signs its own engagement with the same expert. Avoids control concentration but multiplies conflict-check surface, creates ambiguity over which firm's privilege governs a given communication, and raises the risk that one defendant's later withdrawal drags the expert's whole file into a discovery fight.

For most defense groups the lead-counsel-holds-paper structure with a robust common-interest agreement is the defensible default. Name in the engagement letter exactly which firms are beneficiaries, and require the expert to route all substantive communication through a defined channel so the privilege log stays coherent.

A co-retained expert is a disclosure liability the moment they are designated as a testifying witness. Testifying-expert protection is governed by FRCP 26(b)(4), which shields two categories: draft reports under Rule 26(b)(4)(B) and most attorney-expert communications under Rule 26(b)(4)(C). Those protections have three statutory exceptions that survive: communications about the expert's compensation, facts or data the attorney provided that the expert considered, and assumptions the attorney provided that the expert relied on.

In a joint-defense setting those exceptions are the exposure. If four firms feed one expert, the facts, data, and assumptions that any of them supplied are discoverable, and the plaintiff can use them to argue that the expert's opinion was steered. The segregation mechanism is a disciplined intake and channel design.

• Separate the consulting hat from the testifying hat. Consider retaining a non-testifying consulting expert under Rule 26(b)(4)(D) for the group's strategy work and a distinct testifying expert for the record opinion. Consulting-expert protection is broader and harder to pierce. Do not blur the two roles on one person.
• Control what the testifying expert considers. Everything a testifying expert reviews becomes fair game for cross-examination and potentially discoverable. Route privileged strategy to the consulting track, and give the testifying expert a clean, documented evidentiary record.
• Maintain a single privilege log for the shared expert. Fragmented logs across firms produce inconsistent claims that a magistrate can exploit to order in-camera review or waiver.
• Segregate custodial data. If the expert performs forensic analysis on shared datasets, preserve chain of custody with hash verification such as MD5 or SHA-256 so no defendant can later claim the shared asset altered evidence attributed to them.

This is procurement education, not legal advice. Confirm the current text and local interpretation of Rule 26 with counsel of record before designing the channel.

Co-retention exists largely to spread the cost of an expensive forensic asset across a defense group. The budget mechanism has to allocate cost fairly, insulate the expert from any appearance of a defendant-specific financial incentive, and survive audit if a defendant exits or a court reviews fees.

• Pro-rata or exposure-weighted assessment. Defendants contribute either equally or in proportion to their exposure. Exposure-weighting is more defensible commercially but requires an agreed allocation formula in the governance charter to prevent later disputes when the expert's work benefits one defendant more than others.
• Escrow or trust account administration. Contributions flow into a jointly administered account, and the expert invoices that account rather than individual firms. This keeps the compensation record clean, which matters because Rule 26(b)(4)(C)(i) makes compensation discoverable. A single, transparent payment trail is easier to defend than a web of side payments.
• Common-benefit framing. The plaintiff-side common-benefit fund model described in the Manual for Complex Litigation, Fourth is instructive for structuring defense-side shared cost, including holdback percentages and court-supervised accounting where the MDL court oversees fees.
• Compensation neutrality. The expert should be paid the same regardless of which defendant benefits from a given opinion. Any bonus, success fee, or contingency tied to outcome is a Daubert and credibility hazard and should be prohibited in the engagement letter.

Vetting a co-retained expert is harder than vetting a solo expert because the conflict surface is the union of every retaining firm's client roster and every defendant's adverse relationships. A conflict invisible to one firm can disqualify the asset for the whole group.

• Run the conflict check against the full defendant group, not one client. Collect each participating firm's relevant party list and screen the expert against all of them, including prior engagements adverse to any co-defendant.
• Screen prior and concurrent testimony. Pull the expert's deposition and trial history for positions inconsistent with the group's theory. In an MDL, plaintiff counsel will have that transcript history indexed. A prior opinion that contradicts the shared theory is a cross-examination trap that lands on every defendant at once.
• Audit concurrent retentions. Confirm the expert is not simultaneously retained by a party adverse to any group member in a related matter, which can breach the common-interest premise and threaten the shared privilege.
• Verify the methodology will clear the admissibility bar before you commit the group's budget. Under Daubert v. Merrell Dow and Federal Rule of Evidence 702, and Frye in general-acceptance jurisdictions, the method must be reliably applied to the facts. Where a recognized standard governs the discipline, such as NFPA 921 for fire origin-and-cause, ASTM E1618 for ignitable-liquid analysis, or SWGDE guidance and ACE-V for forensic examination, confirm the expert follows it. A single expert speaking for many defendants concentrates admissibility risk, so a successful challenge removes the group's entire technical position at once.

Defense groups in an MDL rarely stay intact. Defendants settle, sever, or develop cross-claims, and when they do the co-retained expert becomes contested property. The exit mechanism has to be written into the retention and common-interest documents at inception, because negotiating it mid-dispute favors whoever holds the file.

• Define file ownership and post-exit access. Specify who retains the expert's work product when a defendant leaves, whether a departing defendant keeps a copy, and whether they may use it. Silence here produces satellite litigation over the file.
• Address common-interest survival. State that a defendant who becomes adverse to the group forfeits access to further privileged expert communications and cannot compel the expert to disclose the group's shared work by virtue of the prior common interest. The doctrine generally does not protect communications between former allies once their interests diverge on the disputed issue, so the boundary must be explicit.
• Handle expert re-designation. Decide in advance whether the remaining group can continue with the expert if one defendant who helped shape the opinion departs, and whether the departing defendant can designate the same expert against the group. A no-adverse-use covenant reduces that risk.
• Reconcile the budget on exit. The escrow agreement should state whether a settling defendant recovers unspent contributions or forfeits them to the common benefit.

The efficiency of co-retention is also its single largest strategic liability. One expert, one report, one methodology, and one credibility profile now carries the defense of every participating defendant. A successful Daubert exclusion, an impeachment on a prior inconsistent opinion, or a methodology gap under the governing standard does not damage one defense. It removes the technical foundation for the whole group.

Procurement leads and lead counsel should price that concentration into the decision.

• Model the single-point-of-failure exposure. Ask what the group's position looks like if the expert is excluded on the eve of trial. If the answer is catastrophic for every defendant, consider a backstop consulting expert who can be elevated, retained under Rule 26(b)(4)(D) so their existence stays protected until needed.
• Stress-test the methodology early. Commission an internal reliability review against the applicable standard before designation, not after the plaintiff files a motion to exclude.
• Do not overreach the opinion. An expert stretched to cover facts specific to defendants they did not analyze invites both exclusion and a waiver argument that the opinion was authored by counsel. Keep the shared opinion to the genuinely common technical questions and let defendant-specific issues sit with defendant-specific experts.

None of this guarantees admissibility or any outcome. It structures the procurement so the group is buying a defensible, auditable asset rather than a shared liability.