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How courts have viewed expert testimony based on psychological experiments

writer: YAMASAKI, YUKO (Ritsumeikan Global Innovation Research Organization, Senior Researcher) published: 2018-08

Around a hundred years ago, Terada Seiichi, who conducted an empirical investigation of the factors that influence witness testimony, became the first Japanese to engage in psychological research within the domain of the judicial system, and, in collaboration with criminal law scholars, tried to make the knowledge obtained from this research into something that could be used in a legal setting (Wakabayashi and Satō, 2012).
A hundred years has passed, but today it still cannot be said that psychological research concerning witness testimony is accepted by the judicial system.
Wells et al. (1998) analyzed cases in which a miscarriage of justice was brought to light by DNA evidence, and showed that 36 cases, which represented 90% of the total, involved mistaken witness testimony. DNA evidence proved to be the savior of those who had been falsely accused. But there are many cases in which there is no physical evidence that would allow for DNA analysis. When such cases involve witness testimony, finding the true guilty party or proving that this testimony is unreliable are the only routes to salvation for the falsely accused.
Psychologists sometimes give expert analysis regarding the reliability of witness statements at the request of those who claim to have been falsely accused or their lawyers. Even if the results of this analysis are that the reliability is extremely low, however, it is very rare for this to be accepted by the judicial system. In this paper, I examine two such cases – the 1992 Iizuka incident (two young girls went missing on the way to school, and their bodies were found the next day) and the 1974 Toyama incident (a man was killed on the street in broad daylight as part of an intra-gang dispute) – and examine the relationship between psychological research and these trials. In both cases the accused proclaimed their own innocence but were found guilty, and in both cases a psychologist was asked to conduct experiments to establish the reliability of the witness testimony as part of the petition for a retrial.

Witness testimony experiment concerning the Iizuka incident (Itsukushima, 2014)

Is it possible for someone to give a detailed statement about the place, car, and person they witnessed twelve days after having seen someone beside a stopped car as they drove past, approaching a curve at a speed of 25
30 km/h in a subcompact car? Witness T. in the Iizuka incident said that it was. T. described what happened as follows.
“The man was standing beside a station wagon, standing with his back pointing opposite the car, or in other words with his back to me. As for the man’s features, he seemed to be balding toward the front of his head, and I think he had long hair that was parted; as for his clothing, his outer garment was a light brown vest fastened with buttons at his chest that seemed to be made of wool, and below the vest he wore a white dress shirt. Based on this outfit I think he was around 30
40 years old.
“…I saw that the front tires were smaller than the rear tires, and the axle part of the rear wheels was indented, and I thought it was a double wheel. The axle part of the rear wheel was indented, and I even have a memory of blackish lines around the area where the central bolts are fastened, that is, around the edge of the hub.”
Itsukushima conducted a field experiment with conditions as close as possible to those T. actually witnessed to determine whether this kind of event could be recalled in such detail. There were 30 participants. The result of the experiment was that even if they were told ahead of time, “There will be a car parked in the opposite lane, so please pay attention to the car and the area around it as you drive by,” not a single participant was able to give as detailed a report as witness T. Facts that differed from reality were also reported.

Witness testimony experiment concerning the Toyama incident (Kondō and Hakoda, 2004)

Is it possible for someone with a visual acuity of 0.2 and 0.4 in the left and right eyes respectively to correctly select the photograph of a person they saw at a distance of 16.45m from a series of photographs shown to them by police after the fact? Witness I. in the Toyama incident said that it was.
In order to determine the reliability of Witness I.’s testimony, Kondō and Hakoda quantitatively examined the relationship between viewing distance and visual acuity regarding the recognition of someone having been seen for the first time. The results revealed that the greatest distance at which a person with a visual acuity of 0.4 could recognize a person’s face was 6.82, and it was impossible to distinguish a person 16.45m away. They also showed that when a person with a visual acuity of 0.4 sees someone positioned 16.45m away, it is impossible for them to identify the same person in a photograph, regardless of whether or not they are given the hint of information about the person’s hairstyle.

How courts have viewed expert opinions based on psychological experiments

In both of these cases, expert testimony based on psychological experiments was submitted with the petition for a retrial. In the case of the Iizuka incident, however, this expert testimony was not considered valuable because “the subjects in the experiment were different from the witness” (Itsukushima, 2014). Regarding the Toyama incident, too, the value of the expert testimony as evidence was denied on the grounds that “there are significant differences between the experiment and the circumstances of what was actually witnessed by those who testified regarding this incident” (Hakoda, 2009).
How should these results be viewed? Has the rejection of knowledge obtained from psychological experiments and adherence to the establishment of facts on a “rule of thumb” basis on the part of the courts become even stronger than it was a hundred years ago in Seiichi Terada’s era? Like Terada, who “when it came to education concerning science ancillary to criminal law, advocated the necessity of including instruction on experimental psychology in general, and including forensic psychology in legal education as part of this instruction” (Wakabayashi and Satō, 2012), I think it is important for people involved in the judicial system to receive education regarding psychological research as it pertains to the establishment of facts. Unless this is undertaken, the establishment of facts on the basis of scientific research may never be accepted.


  • Hakoda, Y. (2009). The Toyama Case : Expert Opinions Accompanying Supplementary Statements of Reason for Appeal to the Supreme Court, Japanese Journal of Law and Psychology, 8(1), 141-143.
  • Itsukushima, Y.(2014).Psychological expert opinion on Mr. T’s eyewitness statements in the Iizuka Case, Japanese Journal of Law and Psychology, 14(1). 17-28.
  • Kondo, M. and Hakoda, Y. (2004). The Effects of Visual Acuity on Eyewitness Face Identification, Japanese Journal of Law and Psychology, 1, 81-87.
  • Wakabayashi, K. and Sato, T. (2012). Historical connection between memory and testimony psychology by an experimental study of Seiichi Terada in Japan, The Japanese Journal of Psychology, Vol. 83, No. 3, pp. 174-181
  • Wells, G.L., Small, M., Penrod, S., Malpass, R.S., Fulero, S.M., and Brimacombe, C.A.E.(1998). Eyewitness identification procedures: Recommendations for lineups and Photospreads. Law and Human Behavior, 22(6),1-38.


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