At the time we did this research there weren’t any studies comparing the showup, where the police present one suspect to the witness and ask “is that him”, to the lineup, where the police present multiple individuals with one being the suspect and ask the witness “which one”. This paper attracted quite a bit of attention because the findings were not completely as the field had expected. It leads to some interesting hypotheses about decision making and choice strategies.

Gonzalez, R., Ellsworth, P., & Pembroke, M. (1993). Response biases in lineups and showups. Journal of Personality and Social Psychology, 64, 525-537.

doi:10.1037/0022-3514.64.4.525  PDF

Abstract

The showup, or presentation of a single suspect to an eyewitness, is widely believed to be a more biased and suggestive identification procedure than the lineup even though there has been no empirical work on this issue. Results suggest, however, that witnesses at a lineup are less likely to say “not there” than are witnesses at a showup. This tendency is seen in both live and photographic identification procedures, in both laboratory studies and real-world identifications. Showups in the lab resulted in no more mistaken identifications than lineups. Results also suggest that a showup is not equivalent to a poor lineup (i.e., a lineup with a functional size of!). It is hypothesized that a showup leads to an absolute judgment, whereas a lineup leads to a relative judgment, and that the police pressures on witnesses are unlikely to be any greater for showup than for lineup identifications.