COGNITIVE confirmation bias - ask and granhag

researchers name(s)

ask

granhag

year

2005

aim of study

to investigate how pre-existing beliefs and expectations (specifically, guilt-presumptive reasoning) influence the way people evaluate evidence in criminal investigations

topic

confirmation bias

participants

50 criminal investigators

68 undergraduate students

procedure

read a set of facts from the preliminary investigation of a homicide case

all were given short background information explaining that a female psychiatrist was found dead in her apartment, and that a second woman was encountered inside the apartment

the second woman was established as the prime suspect

they were then given one of the following conditions:

‘Suspect motive’ condition: it was explained that the prime suspect had previously expressed suspicion about a sexual affair between her partner and the psychiatrist and that she was very jealous. This version thus provided a plausible motive for the suspect to kill the victim, and would supposedly suggest the hypothesis that the prime suspect is indeed guilty.

• read a set of facts from the preliminary investigation of a homicide case

• all were given short background information explaining that a female psychiatrist was found dead in her apartment, and that a second woman was encountered inside the apartment

• the second woman was established as the prime suspect

• they were then given one of the following conditions:

‘suspect motive’ condition:

• it was explained that the prime suspect had previously expressed suspicion about a sexual affair between her partner and the psychiatrist and that she was very jealous

• this version thus provided a plausible motive for the suspect to kill the victim, and would supposedly suggest the hypothesis that the prime suspect is indeed guilty

‘alternative culprit’ condition:

• it was revealed that the victim had recently received a number of

  phone calls from an anonymous male threatening to kill her

• the victim had inferred that the caller must have been a current or former patient of hers

• this version was intended to prompt the reader to entertain the hypothesis of an alternative, unknown offender

• all participants were then given the exact same 20-item list of observations made in the preliminary investigation of the murder

• an example of one of the observations on the list: the prime suspect’s

  fingerprints were found on the murder weapon

findings

• students displayed the confirmation bias and ascribed guilt to the prime suspect ONLY when a potential motive was presented (they chose the alternative culprit as guilty when told about him despite all evidence saying it was definitely the prime suspect)

• Investigators were not as influenced by the initial hypothesis, and ascribed guilt to the prime suspect regardless of the hypothesis, thus being less sensitive to alternative interpretations

• results indicate that the police officers and students were differently affected by the initial hypothesis manipulation

• specifically, the students were significantly swayed in their guilt judgements and ratings of observations, whereas the police officers were not

additional information?

• lab experiment

• students made judgements that conformed to the hypothesis that had initially been presented to them

• this tendency can be accounted for in terms of confirmation bias

• participants assimilated the available evidence with their prior opinion (the

  hypothesis they had read), creating a sense of having their position confirmed

• the findings support the notion that confirmation bias can manipulate decision making, but also indicate that the effect of the bias can be mitigated with training