Duke Lacrosse and Confirmatory Bias

A very interesting and thorough commentary on the Duke Lacrosse case recently at the “Durham in Wonderland” blog extensively details the utter failings of the Durham PD in properly investigating the Duke Lacrosse case.  Some highlights:

First, what the police didn'?t investigate:

April 6, in her first and (as far as we know) only written statement to
police, the accuser described a crime in which Kim Roberts played a
central role. Roberts had already given her statement, on March 22, the
first time she met with police. So too did the captains. So too did the
accuser?s ?drivers.? So too, to my knowledge, did every witness in this
case?except for the accuser. For reasons never revealed, the police
waited 21 days after first interviewing the accuser to take her

In the event, the accuser claimed that:

  • Roberts was crying, in the house, after racial epithets occurred in the dance;
  • she
    was separated from Roberts at the bathroom door, with three other
    lacrosse players dragging Roberts off into parts unknown;
  • Roberts and one of the people who attacked her dressed the accuser after the attack.

statement contained nothing even resembling these items. Moreover, if
true, the accuser?s statement meant that no fewer than six people at
the party committed a crime that evening?the three people who attacked
her, and their three accomplices who dragged Roberts away from her at
the bathroom door….

If Roberts was never re-interviewed, how did
Nifong determine that the accuser?s version of events was more credible
than that of Roberts? And on what basis did Nifong?as he did in
May?clear the other 43 white members of the lacrosse team of all
suspicion, since according to the accuser, at least three more of them
committed a crime?

It would seem as if Nifong employed the following investigative technique:

  • He believed only the elements of the accuser?s story that aided his desire to get indictments before the election;
  • He
    disbelieved the elements of the accuser?s story that he hadn?t
    described as elements of the crime in his March 27-April 3 publicity
    barrage (i.e., that three players tore Roberts away from the accuser at
    the bathroom door, an allegation the accuser first made on April 6);
  • He
    declined to re-interview Roberts so he could avoid basing arrests on an
    accuser?s tale that a police investigation had proven false.

Basically, the police and Nifong were quite guilty of what psychologists refer to as “confirmatory bias.”  As defined nicely at wikipedia, “Confirmation bias is a phenomenon wherein decision makers have been
shown to actively seek out and assign more weight to evidence that
confirms their hypothesis, and ignore or underweigh evidence that could
disconfirm their hypothesis. As such, it can be thought of as a form of
selection bias in collecting evidence.” 

What strikes me is that time after time when I read about the wrong people being arrested, this is exactly what happened.  The police “find their man,” stop investigating, and discount all further evidence that does not support their original (and premature) conclusion.  The destructive power of this bias in policework is made no more clear than in one of the most brilliant documentaries I have ever seen, “Murder on a Sunday Morning,” about the mis-arrest and police abuse of a teenage Black male in Florida.  You really, really should watch it. And as for the Duke Lacrosse case, this is just further evidence of a miscarriage of justice.

About Steve Greene
Professor of Political Science at NC State http://faculty.chass.ncsu.edu/shgreene

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