A reexamination of the evidence for the somatic marker hypothesis: what participants really know in the Iowa gambling task

Abstract

Bechara, Damasio, and coworkers [Bechara, A., Damasio, H., Tranel, D. & Damasio, A. R. (1997) Science 275, 1293-1295] have reported that normal participants decide advantageously before knowing the advantageous strategy in a simple card game designed to mimic real-life decision-making. Bechara et al. have used this result to support their view that nonconscious somatic markers can guide advantageous behavior. By using more sensitive methods, we show that participants have much more knowledge about the game than previously thought. In fact, participants report knowledge of the advantageous strategy more reliably than they behave advantageously. Furthermore, when they behave advantageously, their verbal reports nearly always reveal evidence of quantitative knowledge about the outcomes of the decks that would be sufficient to guide such advantageous behavior. In addition, there is evidence that participants also have access to more qualitative reportable knowledge. These results are compatible with the view that, in this task, both overt behavior and verbal reports reflect sampling from consciously accessible knowledge; there is no need to appeal to nonconscious somatic markers. We also discuss the findings of other studies that similarly suggest alternative interpretations of other evidence previously used to support a role for somatic markers in decision-making.

Fig. 1.

Questionnaire.

Fig. 2.

Participants' knowledge that one of the two best decks is the best deck, as reflected in several verbal report measures, compared with participants' tendency to behaviorally select from one of the two best decks. (As mentioned in the text, we define the two best decks to be the two decks with the highest observed mean net outcome, according to each individual participant's sequence of observations up until the trial under consideration.) The green line shows how many participants actually picked one of the two best decks behaviorally. The red and cyan markers correspond, respectively, to the number of participants who gave the highest rating to one of the two best decks and the number of participants who said that they would select from one of the two best decks if they could only select from one deck. The square markers correspond to Level 2 knowledge. The light-brown marker corresponds to the number of participants who gave the highest expected net to one of the two best decks, and the dark-blue marker corresponds to the number of participants who had the highest calculated net for one of the two best decks. (Note that on trial 70, the light-brown marker is covered by the dark-blue marker.)

Fig. 3.

Participants' knowledge that one of the two best decks is the best deck, as reflected in several verbal report measures, among participants who behave advantageously. The markers have the same meaning as in Fig. 2, but, rather than referring to the total number of participants, they refer to the percentage of participants, among those who behaved advantageously on the corresponding trial, who showed evidence of knowledge of the advantageous strategy in each of the verbal report measures. (Note that on trials 30 and 50, the light-brown marker is covered by the dark-blue marker.)