As used in Spain (and in this context) "motivada" refers to the legal grounds on which the expert's opinion is based

As an example of how "motivado/a" is used in Spain, an "auto" issued by a judge is a "resolución judicial motivada", that is, a decision requiring that the judge state the legal grounds on which his decision is based, while a "providencia" is a judge's order for which there is no requirement to state the legal grounds therefor.

--------------------------------------------------
Note added at 49 mins (2006-07-06 12:06:08 GMT)
--------------------------------------------------

The special legal meaning of "motivación" = "legal grounds" is reflected in the following definition:

Motivación- requisito fundamental que tienen que cumplir las sentencias y otras resoluciones judiciales, y que consiste en la expresión por parte del órgano juzgador de las razones en que fundamenta su decisión. (Diccionario jurídico Colex, Madrid, 2003)

Examples:

In general terms, AGROMEX sustains that there are no valid reasoning nor legal grounds (motivación and fundamentación) on which the Ministry of Commerce may ...
www.sice.oas.org/Dispute/nafta/english/M00041ce.asp

applicable law provisions and reasoning (fundamentación y motivación) requires that, in. this case, the Ministry of Commerce explain the legal grounds and ...
www.worldtradelaw.net/nafta19/urea-dumping-nafta19.pdf -

... support those statements, refer to the legal grounds that support the claim, ... de motivación suficiente para apartarse de lo que juzgaba orientación ...
biblioteca.uam.es/documentos/Derecho/practicum/carrillo_doctrina.pdf

"decisión motivada" is likewise often translated as "reasoned opinion" in English:

Infraction cases are taken to the European Court of Justice by the Commission for trial if their Reasoned Opinion is not adequately answered. ...
www.cabinetoffice.gov.uk/regulation/europe/glossary/index.a...

The UK was threatened with Infraction Proceedings at the European Court of Justice (ECJ) following the European Commission's 'Reasoned Opinion'; ...
www.mhra.gov.uk/home/idcplg?IdcService=SS_GET_PAGE&nodeId=9...

This is a judgment of the Court. This appeal arises out of three actions brought by ... a coherent reasoned opinion expressed by a suitably qualified expert ...
www.hmcourts-service.gov.uk/judgmentsfiles/j698/penney.htm