Hot tubbing (known formally as concurrent evidence) is a different approach to testing expert evidence. This article explains what it is, compares it to traditional methods, and discusses its benefits and pitfalls.

What is hot tubbing?

In common law jurisdictions such as England, Singapore and Australia, expert evidence is traditionally tested in an adversarial context. The expert is cross-examined by the opposing party’s lawyers who decide the scope and tone of the questioning. Experts are examined individually with the decision-maker listening to and evaluating the evidence. Although the decision-maker can intervene to clarify matters, such interventions are limited. The decision-maker’s role is to decide the dispute based on the information as presented.

Hot tubbing breaks from that traditional approach. First, the expert no longer gives evidence alone. Instead, experts from like disciplines give their evidence together. Second, it is the decision-maker that takes the lead in questioning – similar to the inquisitorial approach adopted in civil jurisdictions, such as France. Whilst the lawyers may also ask questions, that opportunity is more limited than would normally be the case.

Does hot tubbing save costs?

Hot tubbing is commonly associated with cost saving, the theory being that by limiting the parties’ ability to cross-examine, the length of the hearing will reduce, and so too the cost. In reality however, significant cost savings are unlikely to result. In most cases the costs associated with cross-examining experts will represent only a small portion of the total cost. More significantly, parties are often reluctant to adopt a pure hot tubbing process, instead opting for a hybrid approach where hot tubbing is used in addition to normal cross-examination.

Keeping control

Adopting a hybrid approach to hot tubbing eliminates a major risk – loss of control. Handing control of expert questioning to a decision-maker makes little sense when it is the parties who are more familiar with the issues being addressed. The adversarial system gives the parties full control over their fates – they can question an expert on technical details and background facts that a decision-maker coming fresh to a dispute may not fully appreciate. Whilst hot tubbing questions are often agreed in advance, those questions are more high level and are unlikely to delve into the detail as would normally be the case.

The true benefit – expert debate

The opportunity for the experts to discuss and question each other is where hot tubbing distinguishes itself. This interactive process is simply not provided for in either the traditional adversarial or inquisitorial approaches. While an expert may find it easy to criticise his opponent’s views in a report, it is another thing to have to justify a position when sat face to face and being questioned by a peer. Allowing inter-expert discussion is likely to expose weak arguments and elicit concessions which might otherwise not occur.

In some cases this benefit might be lost. In a recent Scottish case1 the hot tub contained seven experts. Whilst order was maintained by providing only one microphone, and allocating each expert a portion of the time available to speak, the number of experts did not allow a proper exchange of views to take place. In that same case two experts speaking to a separate issue later sat together in the hot tub. That discussion was free flowing, concessions were more forthcoming, and the issues were narrowed as a result.


Hot tubbing is a useful addition to the expert examination process. Although unlikely to save significant costs, having experts give evidence together is likely to lead to more reasonable positions being advanced. Whether this is in a party’s interest will very much depend on the case being advanced and the strength of the expert.