The illusion of the perfect alibi: Establishing the base rate of non‐offenders' alibis

Abstract

The present study was designed to establish the base rate of alibis and supportive evidence for alibis of non-offenders. That is important because the presence and lack of an alibi are often seen as a clear indicator of innocence and guilt, respectively, of a suspect. A large sample of laypersons (N = 841) was randomly assigned to one of 32 conditions in which they were asked to generate a true alibi after they were falsely accused of being the perpetrator of a mock robbery. Each condition consisted of either a Tuesday or a Saturday and one of 16 timeframes. In general, the majority of the participants had an alibi (99.5%) and supportive evidence for their alibis (92.4%). The supportive evidence often consisted of a combination of supportive evidence rather than one distinct form of supportive evidence (33.3%). Although it is widely assumed that the alibi believability is determined based on the strength of the supportive evidence, our results show that the type of evidence that can be presented by laypeople depends upon the day and the timeframe wherein the crime has been committed. The results of the study therefore imply that determining alibi believability solely on the strength of the supportive evidence is not a fair measure.We suggest that the believability should also be based on the base rate of alibis and its supportive evidence. KEYWORDS alibi, base rate, supportive evidence, taxonomy

Where I was and how I will prove it

“…Police detectives also find alibis supported with physical evidence more believable than alibis supported with witness evidence . In contrast, we know from the base rate of alibis that only about 2% of people report having camera recordings to support their alibis (Nieuwkamp et al, 2017a). This finding adds to the assertion of Culhane (2008) that we expect strong evidence in order to believe an alibi, even though it is unrealistic to request such evidence from innocent people.…”

“…The supportive evidence can consist of three forms of supportive evidence: witness, physical, or knowledge evidence (Nieuwkamp, Horselenberg, & van Koppen, 2017b). From the results of alibi presentation and validation studies we know that about 95% of people can present an alibi and about 90% of them can report supportive evidence for it Nieuwkamp, Horselenberg, & van Koppen, 2017a;. The results of these studies show that witness evidence (about 80%) is much more common than physical evidence (about 25%).…”

“…Although the strength of the supportive evidence remains an important aspect of alibi believability, the base rate of alibis cannot be ignored. Whether an innocent suspect has supportive evidence for his or her alibi and what type of evidence can be expected depends on various factors; therefore the base rate needs to be taken into account when evaluating an alibi (Nieuwkamp et al, 2017a). It would thus be too simple to disbelieve an alibi simply due to the absence of strong supportive evidence.…”

“…The impact of the believability of physical evidence is larger than the impact of witness evidence , meaning that innocent suspects can more often present supportive alibi evidence that is less likely to be believed than evidence that is more likely to be believed. In fact, the presence of supportive alibi evidence depends upon the day, timeframe and presented alibi, so therefore the base rate of alibis and supportive evidence should also be taken into account to determine its believability (Nieuwkamp et al, 2017a).…”

True and false alibis among prisoners and their detection by police detectives

“…The assumption that non-offenders are able to present strong supportive evidence for their alibis has proven to be incorrect (Culhane, Hosch, & Kehn, 2008;Culhane et al, 2013;Olson & Charman, 2012). In fact, only 3% of the non-offenders can present strong evidence for their alibis (Nieuwkamp, Horselenberg, & Van Koppen, 2017).…”