Introduction

9.1 The common law rules of evidence generally render evidence of opinion inadmissible. Consistently with that position, s 76 of the uniform Evidence Acts provides a general exclusionary rule for opinion evidence:

(1) Evidence of an opinion is not admissible to prove the existence of a fact about the existence of which the opinion was expressed.

9.2 While the Acts do not attempt to define the term ‘opinion’, it has been held that an opinion under the uniform Evidence Acts is, variously, ‘an inference drawn or to be drawn from observed and communicable data’,^[1] an inference drawn from facts,^[2] or ‘a conclusion, usually judgmental or debatable, reasoned from facts’.^[3]

9.3 In theory there is no clear distinction between evidence of an opinion and evidence of fact because there is a ‘continuum between evidence in the form of fact and evidence in the form of opinion, the one at times passing imperceptibly into the other’.^[4] However, in its earlier inquiry into the laws of evidence, the ALRC decided to retain the distinction (such as it is) and a rule for excluding opinion evidence. Because of the need to exercise some control generally upon material at the opinion end of the continuum, and specifically to control the admission of expert opinion evidence, this was found to be ‘unavoidable’.^[5]

9.4 The uniform Evidence Acts provide a range of exceptions to the exclusionary opinion rule. These include exceptions in relation to lay opinion^[6] and opinion based on specialised knowledge (‘expert opinion evidence’).^[7]

9.5 Several concerns arise in relation to the operation of these exceptions. The chapter discusses but rejects reform of the lay opinion rule in s 78. Also discussed but rejected is reform of s 79 in response to Makita (Australia) Pty Ltd v Sprowles,^[8] a case concerning proof of the factual basis of expert opinion. The chapter also discusses the admissibility under the uniform Evidence Acts of expert opinion evidence on the behaviour and development of children and other categories of witness, such as victims of family violence or people with an intellectual disability. Such evidence can be relevant to the facts in issue in a case and to the assessment of the credibility of witnesses. The latter aspect is discussed further in the discussion of the credibility rule and its exceptions in Chapter 12.

9.6 Aspects of the opinion rule in specific contexts, including in relation to evidence of Aboriginal and Torres Strait Islander traditional laws and customs; evidence in family law proceedings; evidence in sexual offence cases; and evidence from child witnesses are discussed in Chapters 19 and 20.