summarizes the offers and demands made in the dictator and ultimatum games, out of 50 points. The data produced by comparison subjects were comparable to those in many other normal populations (Camerer, 2003
). Two patterns were significant: First, VMPFC patients gave less in the dictator games (only 4.7 out of 50, compared to 12 and 18 for the comparison groups), consistent with the hypothesis that they would exhibit less guilt. Second, in the ultimatum game we did not see significant differences in the offers or demands, but a striking difference between the groups was that the gap between ultimatum offers and demands was zero for all but one of the VMPFC patients (and only a -5 point difference for that one). By contrast, comparison subjects generally offered more than they demanded (as in most other studies (Camerer, 2003
)). This result is also consistent with our hypothesis about guilt, since the gap betweeen ultimatum offers and demands is an approximate measure for how much more a subject is willing to offer above what he/she believes is the minimum acceptable to the other player.
Figure 2 Behavioral results on dictator and ultimatum games (with standard error bars). A, Mean number of points (out of 50) given to the other player in the dictator game. VMPFC lesion patients gave significantly less than the control groups. One NC subject did (more ...)
shows the percentage of Trust and Repay choices in the trust games. Looking at the investor behavior, the VMPFC group chose to trust less often than the BDC group in all 4 combinations, and less than the NC group in 3 out of 4 combinations (trusting equally in one case). In the trustee role, the VMPFC group repaid less often than the control groups across the board but also seemed to generally reduce their Repay rates as the incentives to Betray increased.
Table 1 Percentages (raw proportions) of “Trust” and “Repay” choices in trust games with different temptation values T and different Repay payoffs R. The Avg column simply adds up the Trust and Repay choices across the four games, (more ...)
Because each subject played all four trust games, we were able to combine their choices to form a composite index of trust (the percentage of games in which they chose Trust) and trustworthiness (the percentage of games in which they chose Repay), denoted “Avg” in . Using this measure, the VMPFC patients were significantly less trustworthy than the NC group. The BDC group was not significantly different from the VMPFC or NC groups, reflecting the large variance in performance seen in this group. These results are consistent with our above findings that the VMPFC patients displayed less guilt, though they raise the possibility that impaired behavior on the Repay decision might result from damage to structures other than the VMPFC as well.
shows the results of the parametric analysis for guilt and envy. As suggested by the previous literature and our results from the individual game analyses, the VMPFC patients had significantly lower β values than the comparison groups, indicating that they were relatively insensitive to guilt. To make the β parameter more concrete, we can estimate a ‘betrayal point’, which is the value of the temptation payoff T at which, according to the estimated β, a person would be just indifferent between choosing Repay and Betray in the trust game (in the games where the Repay payoffs are R=(40,40), the betrayal point is T(β) = 40 + (β/4)). Pure self-interest (β=0) would yield an inferred betrayal point of 40 (due to complete indifference to the other player’s payoff). Converting the estimated β parameters into betrayal points we get T = 47 for the VMPFC patients, lower than either of the comparison groups (T=59 for BDC, and T=91 for NC).
Table 2 Parameter estimates derived from the pooled dataset. Median estimated parameters for guilt, envy, expected guilt and expected envy, as well as two unique λ values estimated for each group as a whole (guilt and envy analyses respectively). Superscripts (more ...)
A second finding from our analysis was that the α values for the VMPFC group were statistically indistinguishable from the comparison groups (possibly due to low statistical power and issues with the envy estimation procedure, see Supplementary Methods). This finding supports the idea that the VMPFC patients are not diminished across the board on all emotions, as also supported by other studies (Koenigs and Tranel, 2007
), but rather show selective impairments on only certain social emotions.
The parameter estimates for the subjects’ beliefs about the other players’ envy and guilt parameters, E(α) and E(β), yielded no significant differences between groups. This helps to rule out the possibility that VMPFC patients behave anti-socially because of unusual beliefs about the other players. Estimates of the responsiveness parameter λ are similar across the groups and reassure us that the different groups are not simply playing more or less noisily.