Dateline: June, 2021, Issue 1
Judges often limit the number and kinds of experts who can testify at trial, paying particular attention to repetition of information across experts. Nonetheless, attorneys often still can choose to "diffuse" needed expert testimony across multiple expert witnesses or "concentrate" needed expertise within one (usually multidisciplinary) expert witness. The choice matters.
Harkins and Petty (1981, 1987) conducted numerous experiments focusing on the persuasiveness of (a) multiple versus single sources of information, (b) making either one or multiple arguments, (c) in favor of proposals participants strongly oppose.
In one of their experiments, Harkins and Petty (1981) focus on the persuasiveness of the number of sources (1 or 3) and the number of arguments (repeated or distinct). Participants watch one of four videos:
Participants were least persuaded when hearing either single sources or repeated arguments. Participants were most persuaded when hearing three sources each give a distinctly different argument. Said differently, only the combination of (a) multiple experts where (b) each makes distinctly different arguments is likely to generate a persuasive "boost".
In a follow-up experiment, Harkins and Petty (1981) focus again on the number of sources (1 or 3) and add an additional focus on the quality of the arguments that are made (weak or strong). Participants receive one of four written statements:
As expected, strong arguments are more persuasive than weak arguments. However, it is more persuasive for multiple sources to make distinctly different strong arguments than for one source to make those very same strong arguments. Further, the opposite is true for weak arguments: Multiple sources making distinctly different weak arguments are less persuasive than one source making those very same weak arguments. Said differently, a persuasive "boost" occurs only when multiple experts make distinctly different strong arguments; a persuasive "landmine" occurs when multiple experts make distinctly different weak arguments.
In a series of three further experiments, Harkins and Petty (1987) report that multiple sources enhance persuasiveness because of recipients' perceptions that information from multiple sources is more likely to be based on different perspectives and independent pools of knowledge and, thus, is more worthy of diligent consideration. Independence is key for a persuasive "boost" to occur from using multiple sources. However, multiple sources must not only be independent of each other, they also must be independent of a third party (Moore, Reardon & Mowen, 1989).
In conclusion, multiple independent expert witnesses each forwarding different points can help the persuasiveness of a case only if those points are strong. Multiple expert witnesses forwarding different unconvincing points hurt a case. Multiple expert witnesses who repeat the same point or are perceived not to be independent offer no persuasive benefit, and potentially open the door to hurting a case if the point(s) being forwarded are weak.
Source Harkins, S.G. & Petty, R.E. (1981). Effects of source magnification of cognitive effort on attitudes: An information-processing view. Journal of Personality and Social Psychology, 40(3), pp. 401-413.
Source Harkins, S.G. & Petty, R.E. (1987). Information utility and the multiple source effect. Journal of Personality and Social Psychology, 52(2), pp. 260-268.
Source Moore, D.J., Reardon, R. & Mowen, J.C. (1989). Source independence in multiple source advertising appeals: The confederate effect. In T.K. Srull (Ed.), Advances in Consumer Research, vol. 16, pp. 719-722.