The Butler Did It, of Course!

Here is a paper showing the potential practical utility of detecting and reducing biases: Confirmation bias in criminal investigations by O’Brien and Ellsworth. In an experiment subjects read a police file and were asked halfway through about their hypotheses of who the murderer was; practically everybody named the obvious suspect. On completing the entire file, where a second and stronger suspect emerges in the later half, they still tended to suspect the first guy. In a second experiment the subjects were asked to generate counter-hypotheses about why their suspect might be innocent, and this reduced the confirmation bias.

Another troubling source of bias is false confessions triggered by this confirmation bias and then strongly supporting the erroneous conclusion. The Psychology of Confessions by Kassin and Gudjonsson reviews this. During the preinterrogation review police, believing themselves to be better at detecting deception than they are, tend to confidently make false positive detections of deception in innocent people. Once they have convinced themselves they have caught a suspect a police interrogation then becomes guilt-presumptive and rather effective in generating false confessions, in particular in cognitively challenged people. And finally, juries and judges are easily convinced by the confessions.

Nothing of this may be total news to anybody on this blog, but it is still rather worrying how strong biases are accepted in police investigations and the legal system. Maybe the counter-hypothesis trick at least could be made part of police procedure: at certain points during investigations the investigators have to state possible disconfirming hypotheses for the record.

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  • pdf23ds

    I wonder about that paper–did people switch their suspicions more often if they weren’t asked in the middle who they were suspecting? (Articulating their suspicions early on might fix them more strongly in their minds.) What about if they were instructed not to form any theories before reading the complete report?

    I wrote here about a possible instance of confirmation bias in a person who has an extremely strong interest in eliminating it from their thinking (a chess player).

  • Robin Hanson

    These psychology studies are suggestive but most of them are not up to the usual economists’ standard to clearly show a bias, as they do not usually look at all of the relevant variations. Sure you can play “gotcha” by having the actual answer be something that surprised people. But how good are people if we average also over the other cases, where people aren’t surprised?

  • Anders Sandberg

    I think there is pretty good evidence for false confession bias given all the convictions that were overturned with DNA evidence. According to this
    (I have not dug up the real study) there appears to be a sizeable number of cases where it was relatively easy to show using DNA that the accused did not commit the crime. They represent a sample where it is at least in principle possible to find a lower bound of false confessions.

    The graph on is also rather interesting, although no doubt there might be bias in their presentation of biased witnessing (and the graph is badly conceived). Misinterpretation and statistical exaggeration was the most common kind of junk science, unsurprisingly.

  • Robin Hanson

    Jurors surely must consider the possibility that confessions are misleading. It can be reasonable to convict, even if you assign a substantial chance to a misleading confession. The fact that there are errors in conviction is not enough to show that jurors could have made a better choice, given what they knew.

  • Douglas Knight

    RH: “a substantial chance” sounds larger than “a reasonable doubt.”

    I’m more interested in hopeless questions about the police. Various biases all seem to point to them clearing too many cases, but what’s stronger, pressure to clear or overconfidence?

  • Doug S.

    Based on the stories I’ve read, I’m not going to give any weight whatsoever to confessions that the accused is not willing to repeat in court.

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