evidence testimony
Status of ipse dixit statements
<p>Is <em>ipse dixit</em> ("he said it himself") specifically allowed or prohibited in law? I.e. a trusted witness (e.g. an expert) says something is true, but provides no evidence to back it up. It's presumed to be true solely because of who said it, not on any other merits.</p>

<p>I've seen some articles that say that in the US an expert must back their statements with other evidence, and demonstrate how they obtained their conclusions from the evidence before their testimony will be accepted.</p>

<p>I am interested mostly in English speaking and European countries.</p>

<h2>The Opinion Rule</h2>
<p>What you refer to as <em>ipse dixit</em> is dealt with by the <a href="https://definitions.uslegal.com/o/opinion-rule/" rel="nofollow noreferrer">opinion rule</a>. In summary, witnesses are required to testify to facts, not opinions - it is for the judge/jury to reason from facts to conclusions.</p>
<p>In New South Wales, this is stated in <a href="http://www7.austlii.edu.au/cgi-bin/viewdoc/au/legis/nsw/consol_act/ea199580/s76.html" rel="nofollow noreferrer">s76</a> of the Evidence Act 1995 which codified the common law rule applicable in most common law jurisdictions.</p>
<blockquote>
<p><strong>76 THE OPINION RULE</strong></p>
<p>(1) Evidence of an opinion is not admissible to prove the existence of a fact about the existence of which the opinion was expressed.</p>
</blockquote>
<p>Which is a little bit opaque to the non-legally trained so the act helpfully gives some examples:</p>
<blockquote>
<p>1 P sues D, her doctor, for the negligent performance of a surgical operation. Unless an exception to the opinion rule applies, P's neighbour, W, who had the same operation, cannot give evidence of his opinion that D had not performed the operation as well as his own.</p>
<p>2 P considers that electrical work that D, an electrician, has done for her is unsatisfactory. Unless an exception to the opinion rule applies, P cannot give evidence of her opinion that D does not have the necessary skills to do electrical work.</p>
</blockquote>
<h2>Exceptions</h2>
<p>There are a number of exceptions to the opinion rule; the relevant one for your question is the expert opinion exception. <a href="http://www7.austlii.edu.au/cgi-bin/viewdoc/au/legis/nsw/consol_act/ea199580/s79.html" rel="nofollow noreferrer">s79</a> deals with this:</p>
<blockquote>
<p><strong>79 EXCEPTION: OPINIONS BASED ON SPECIALISED KNOWLEDGE</strong></p>
<p>(1) If a person has specialised knowledge based on the person's training, study or experience, the opinion rule does not apply to evidence of an opinion of that person that is wholly or substantially based on that knowledge.</p>
</blockquote>
<p>If the court (and subsequent appeals courts) accept that a witness is an expert and the opinion is limited to the field of their expertise and the opinion is representative within the context of that expert community (i.e. the expert is not an iconoclast or is using unacceptable techniques), they are entitled to express it. Indeed, that's why courts use expert witnesses.</p>

<p>In Common Law Systems (i.e. the legal system of most countries that had some historical ties to the UK and the US) expert witnesses is allowed to offer opinion.  To validate their area of expertise, there is a process called <em>Voir Dire</em> and basically the lawyer can call into question their expertise.  I keep referencing the movie, but it really is a good movie that goes into how trials are handled, but the signature scene for the film <em>My Cousin Vinny</em> revolves around Vinny calling a last minute expert witness who the Prosecutor initially doubts is a credible expert witness (The witness doesn't work in the field and in fact, is out of work in the witness' chosen profession... However, the witness has an encyclopedic knowledge on the topic that is being discussed.).  The Voir Dire process is gone through and the witness is finally accepted, allowing the witness to testify to Vinny's line of questioning.  Notably, the witness is only allowed to testify to matters related to the subject.  I'd love to go into more detail, but the scene is such a massive spoiler, so I don't want to give too much away.</p>

<p>Wikipedia article for Voir Dire lists this as a rare display in media, as most court room dramas skip this because it tends to be boring.  <em>My Cousin Vinny</em> milks it for all it's worth.</p>

<p>Edit:  There are several conditions under which Hearsay testimony is allowed, the most likely of which are dying declarations (The victim of the murder told the witness who did it) and statements against interest (the witness heard the defendant plotting murder most foul while she was robbing his Chicken Farm.).</p>
