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Supreme Court: scientific basis of expert opinion proves decisive - International Law Office

International Law Office

Litigation - Mexico

Supreme Court: scientific basis of expert opinion proves decisive

April 03 2012

Supreme Court decision

A recent Supreme Court ruling has established that when a judge is presented with contradictory expert opinions, the scientific grounds on which such opinions are based are essential in determining which opinion should prevail. The ruling is not binding on the Mexican courts; however, it is a persuasive precedent that could help to tip the balance in cases where the expert opinions submitted in proceedings reach different conclusions, but still appear accurate. Moreover, it provides guidance to parties on expert opinions, setting out significant requirements which are not established in law.


Except in rare cases, when a party produces an expert opinion during judicial proceedings, the process for submitting and hearing such evidence in court is as follows. One of the parties appoints an expert and prepares a questionnaire for the expert to answer. The opposing party is entitled to add to the questionnaire as it deems appropriate. The opposing party may also appoint an expert, to whom the same questionnaire process applies.

Moreover, the judge may appoint a third expert in cases where the two expert reports offer contradictory answers to the questions submitted by the parties. However, the third expert is not required to endorse the conclusions of one of the parties' experts and is free to reach an independent conclusion. Thus, the court may be faced with three expert opinions, each providing different views and conclusions on the same facts.

Where the parties' experts and the judge's expert reach divergent conclusions, the three opinions often provide insufficient grounds in support of their views. Expert reports frequently lack a methodical description of the scientific assumptions and rules on which their conclusions are based.

Supreme Court ruling

The court stated that the purpose of an expert opinion is to help the judge to render a decision on matters which are unfamiliar to him or her. Therefore, when faced with contradicting opinions, the judge should take into consideration the scientific assumptions and rules underlying the opinion and the manner in which they were applied to the case at issue in order to reach the conclusions provided in the opinion. If the judge understands the rationale behind the conclusions contained in each expert report, he or she will be able to make an informed decision and give grounds for upholding his or her decision in the event of a challenge.


Although the law does not require expert opinions to include the scientific assumptions and rules used in rendering them, and although the decision is persuasive rather than binding, the court has raised the possibility of a new requirement which may prove decisive to the outcome of future proceedings.

For further information on this topic please contact Luis Enrique Graham or Angelica Huacuja at Chadbourne & Parke by telephone (+52 55 3000 0600), fax (+52 55 3000 0698) or email (lgraham@chadbourne.com or ahuacuja@chadbourne.com).

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