Thursday, 31 January 2013

Drunk eyewitnesses are more reliable than expected

Imagine you are on a jury: would you trust the testimony of a drunk eyewitness? In a surprising new study, Angelica Hagsand and her colleagues report that drunk witnesses performed just as reliably as sober witnesses at recognising a criminal in a line-up.

One hundred and twenty-three students (60 per cent were women; average age 25) were split into three groups - one third drank orange juice for 15 minutes; another group spent the same time drinking enough orange juice mixed with vodka to reach a blood alcohol concentration (BAC) level of .04 per cent; the final group drank enough vodka and orange to reach a BAC level of .07 per cent. This last value is just below the legal drink driving limit in the UK and USA, and is approximately equivalent to an average-sized man drinking two or three shots of vodka in that time.

Five minutes after they'd finished drinking, the participants watched a five-minute video of a man kidnapping two women at a bus stop, shot from the perspective of a witness. Close views of the man's face were available for a total 31 seconds during the film.

A week later, the participants were invited back and completed a surprise identification task. In a sober state, they saw an 8-man line-up on a computer screen that either did, or did not, feature the kidnapper who they'd seen in the film. The test administrator didn't know which condition participants were in, nor whether the culprit was present. Each participant had to say whether the culprit was in the line-up, answering either "yes", "no the culprit is not present" or "do not remember".

Although better than chance, overall performance was poor, consistent with a great deal of past research showing the limited accuracy of eyewitness memory. Crucially, for both the culprit-present and culprit-absent conditions, there was no difference in accuracy across the different participant groups. This result held even after excluding participants who answered that they could not remember.

In fact, although not a statistically significant difference, the most intoxicated (.07 per cent BAC) participants actually achieved a higher accuracy percentage than the controls in both the culprit-present (47.1 per cent vs. 38.5 per cent) and culprit-absent (56.3 per cent vs. 41.7 per cent) line-up conditions. These results contradicted the researchers' expectations. Based on alcoholic myopia theory (a loss of memory for peripheral details), they predicted that the intoxicated participants would match the controls when the culprit was present, but would make more incorrect identifications when he was absent.

The results also clash with the common sense beliefs of the general public that drunk witnesses will be less reliable than sober witnesses. Given how common it is for witnesses to crimes to be intoxicated, there's been surprisingly little research on how alcohol affects eyewitness performance. Sure, this study has its limitations - the alcohol levels used were only moderate and the crime wasn't a real event - but it makes a welcome contribution to a neglected research area.

_________________________________ ResearchBlogging.org

Angelica Hagsand, Emma Roos-af-Hjelmsäter, Pär Anders Granhag, Claudia Fahlke, and Anna Söderpalm-Gordh (2013). DO SOBER EYEWITNESSES OUTPERFORM ALCOHOL INTOXICATED EYEWITNESSES IN A LINEUP? The European Journal of Psychology Applied to Legal Context: http://www.usc.es/sepjf/images/documentos/Volumen_5/hagsand%20et%20al.pdf

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

13 comments:

  1. The headline and the research contradict one another. Someone who has a blood alcohol level below the legal driving limit is not someone who most of us would describe as "drunk". Drunk conjures up images of someone who has consumed several times that much or more.

    A more accurate headline would be "mild alcohol consumption does not affect eyewitness accuracy".

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    1. hi Jack - the headline and research do not contradict each other. Although you obviously have your own barometer of what counts as drunk, at .07 per cent BAC the effects of alcohol on behaviour and judgment are significant - there is no doubt that these participants were intoxicated. Although .07 per cent is just below the drink driving limit in the UK and USA, it is well over the limit in many other countries.

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  2. Anonymous12:17 pm

    I have to agree with Jack on this one. Whilst it would depend on body size and alcohol tolerance, 2-3 shots of vodka (the 0.07 equivalent in a medium sized male) would leave a lot of people feeling completely sober. It is only just over one pint of 5% beer.

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    1. That's incorrect - according to online BAC calculators, 15 minutes after a pint of strong 5 per cent beer, an average sized man would be .04 per cent BAC, more akin to the low intoxication group in this study. Also, this is not about "feeling" sober or drunk, which is obviously an unreliable indicator. At 0.07 per cent BAC, the effects of alcohol on behavioural and cognitive measures are significant. I appreciate different people mean different things by their use of the word "drunk" - one person's drunk is another person's tipsy - but given that a principle meaning of the word is "under the influence of alcohol", it seems to me apposite to use the word to describe the high-intoxication group in this study.

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  3. Personally I think that people are starting to research this is a great idea. Although a naturalistic observation would be preferred because, it would be unethical to create a situation in which someone would be harmed. So naturalistic observation would allow the researchers to study human behaviors that cannot ethically be manipulated or recreated in an experiment. Also, the naturalistic method can also be generalized more confidently do to the fact that the situation is not staged. However, to have these intoxicated people out and about in dangerous conditions is not preferable. So without a doubt these scientists are doing a good job and making great strides in the field of the reliability of eyewitnesses.

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  5. Anonymous9:39 am

    I'd have to agree with Jack as well. I can see what you guys tried to achieve with this study, but I think people usually have more alcohol in their bodies than just below the legal limit. Legally drunk and drunk are two completely different things, and should be observed as such. I mean, please don't get me wrong, but just watch TV mystery series a little - every time they bring in a drunk to question him, he usually has more alcohol than it is legal.

    Great study, and great information. I wish you well in all your future endeavors.

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    1. Funny how people have such definite ideas about what "drunk" means and how they draw such a clear distinction between "legally drunk" and the other kind.

      The reality is far more messy: "Among the 124 countries that allowed drivers to have alcohol in their blood, there was a ten-fold variation between the least (e.g., Panama) and most generous (e.g., United Arab Emirates)."

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  6. Anonymous11:30 am

    In France, driving with a BAC over 0,05 is illegal.

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  7. I agree. Where these subjects tolerance limits also controlled for? Did they perform worse on a motor / reaction times task than the sober group for example. The brain's tolerance is not subjective, it is a neural adaptation response to the chemical. Therefore, the people in the study may have had high tolerance levels and therefore could be considered sober. If you took 12 people who had never touched alcohol and gave them 3 shots of vodka I doubt they would be able to see straight. On the other hand if someone spiked my drink with 3 shots of vodka then I doubt I, or anyone else would be able to detect it if they did not perform a blood alcohol test.

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    1. Intake was adjusted to account for body weight. The research also involved taking subjective reports from the participants and they reported experiencing effects of the alcohol in line with their BAC. However, there were no objective measures taken of the cognitive effects of alcohol (besides memory performance).

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  8. Anonymous8:05 pm

    Anyone who says 0.07% is not drunk hasn't participated in a wet lab. Though some allowance must be made for tollerance, most people are too impaired to drive at 0.04%, the lower level used in this study. 0.08% is simply the per se DUI limit, and does not encompass a legal definition of "drunk."

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  9. Anonymous8:39 pm

    I can understand the point this research is trying to examine not that the levels of alcohol, is to the limit but the person ability to reconcile sensory details while under the influence and after intoxication has left their system. Obviously its society way of stigmatizing every thing without considering the physiological and biological implication at work. I agree that some people given (07% BAC) or (04%) levels may not necessarily be drunk, some people tolerate differently for instance my friend drinks for hours brandy strait sometimes three bottles and can recollect how much money she spent and who was out during the night days after events. I on the other hand have two drinks and am drifting on three i stop my heads all different. So this study is bit bias but something to think about.

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