• Bahador Bahrami, Daniele Didino, Chris Frith, Brian Butterworth, and Geraint Rees.
• Institute of Cognitive Neuroscience, University College London, 17 Queen Square, London WC1N 3AR, United Kingdom. bbahrami@gmail.com
• J Exp Psychol Hum Percept Perform. 2013 Apr 1;39(2):338-47.

AbstractMany joint decisions in everyday life (e.g., Which bar is less crowded?) depend on approximate enumeration, but very little is known about the psychological characteristics of counting together. Here we systematically investigated collective approximate enumeration. Pairs of participants made individual and collective enumeration judgments in a 2-alternative forced-choice task and when in disagreement, they negotiated joint decisions via verbal communication and received feedback about accuracy at the end of each trial. The results showed that two people could collectively count better than either one alone, but not as well as expected by previous models of collective sensory decision making in more basic perceptual domains (e.g., luminance contrast). Moreover, such collective enumeration benefited from prior, noninteractive practice showing that social learning of how to combine shared information about enumeration required substantial individual experience. Finally, the collective context had a positive but transient impact on an individual's enumeration sensitivity. This transient social influence may be explained as a motivational factor arising from the fact that members of a collective must take responsibility for their individual decisions and face the consequences of their judgments.

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