We tested whether depressed people would make more deceptive or altruistic decisions in the modified trust game. The results support our hypotheses that people with depression would in fact make fewer altruistic and fewer deceptive responses.
In this study, executing deceptive or altruistic responses required cognitive affective processing far more complex than that required for simply repaying the suggested amount. For deceptive or altruistic responses, participants needed to consider the risk and payment conjunction and then calculate the difference between the amount of actual repayment and the requested amount before making a decision. Therefore, cognitive load would be much higher if they chose to cheat the investor or to repay an amount different from that of those recruited as reference. People with depression have been widely reported to have compromised cognitive and affective processing (Harvey et al. 2005
; Ritchey et al. 2011
). Thus, it is logical to reason that these people would simply adhere to the requested payment when preferring to be honest, choose the least repayment when wanting to deceive, or repay as much as possible when deciding to respond altruistically, since other choices would tax their limited cognitive and affective resources. But if this were the case, we should have found a larger ratio of either altruistic or deceptive choices in depressed patients. Instead, compared with healthy participants, people with depression made a smaller ratio of choices on both deceptive and altruistic decisions. The special behavioral patterns of the depressed patients in this study should therefore not have resulted from their limited cognitive or affective resources.
Since the between-group difference was significant in some but not all conditions, this implies that depressed patients were responsive to the varying level of repayment proportion involved in the experiment. Compared with the healthy volunteers, the depressed patients made deceptive responses less frequently and by a smaller ratio only when the repayment proportion was high; they also made altruistic responses less frequently and by a smaller ratio only when the repayment proportion was medium or low. These observations suggest that the behavioral pattern of depressed patients was indeed modulated by the task factor of repayment proportion. The different levels of repayment proportion reflected how benevolent or malevolent the investor was to the participant; in other words, the higher the repayment proportion the investor requested, the less money the participant could retain, and vice versa. In this study, the controls tended to respond altruistically to the investor's benevolent request (low or medium repayment proportion) but deceptively to the investor's malevolent request (high repayment proportion). This is consistent with previous findings that decisions on interpersonal interaction are based on how individuals have treated each other previously (Juliusson et al. 2005
; Rilling et al. 2008
; Krach et al. 2009
). Perceiving a partner's benevolent actions was found to be related with higher activation in the head of the caudate nucleus (King-Casas et al. 2005
). Studies have also shown that, compared with normal subjects, depressed subjects had significantly lower mean volumes for the bilateral heads of the caudate nucleus; moreover, such volume reduction was correlated with depression severity (Butters et al. 2009
). Depressed patients may thus have difficulty being benevolent because of dysfunctions in the caudate, and therefore fail to respond altruistically. This in turn may prevent them from building advanced relationships with others and lead to their failure in normal social interactions.
Depressed patients also appear to be quite sensitive to negative stimuli (Hamilton and Gotlib 2008
; Baert et al. 2010
). It is logical to speculate that they harbor strong negative feelings, including pain and anger, with respect to malevolent treatment. Indeed, previous studies have shown that people rejected (malevolent response) an unfair offer (malevolent requirement) with anger (Pillutla and Murnighan 1996
), suggesting that the negative emotion (i.e., anger) plays an important role in reacting to malevolence. Therefore, the fact that the depressed patients in this study made fewer malevolent (i.e., deceptive) responses might be attributed to their difficulty in converting the emotion of anger into an actual action of revenge. This opinion is consistent with the findings of a recent study by Harle et al. (2010)
that depressed individuals reported a more negative emotional reaction (anger, disgust, and surprise) to unfair offers, but still accepted significantly more of these offers than did the controls. Malevolence has been previously reported to be related to higher activation in the anterior insula. Furthermore, this increased activation predicted participants’ decisions to make a malevolent response (e.g., rejecting offers) (Sanfey et al. 2003
). The anterior insula may thus be important in converting the feeling of anger into a malevolent response to others’ malevolent actions. A recent study showed that, compared with healthy controls, major depressed patients showed significantly reduced neural activity, particularly in the bilateral anterior insula (Wiebking et al. 2010
). In line with this thought, depressed patients facing a malevolent requirement may find transforming the feeling of anger into a response of revenge (deceptive repayment) rather challenging. Revenge against a malevolent requirement has been proposed to serve as a fundamental adaptive mechanism by which people assert and maintain a social reputation (Nowak et al. 2000
). Therefore, depressed patients in normal social life may fail to adjust to others’ malevolence by revenge and fall deeply into the mire of negative feelings, which may in turn further enhance the severity of their symptoms.
Destoop et al. (2012)
investigated decision making in people with depression using a modified ultimatum game paradigm. Participants were asked to play as responder and then proposer against the same partner. The results showed that depressed patients in the role of responder accepted both fair and unfair offers. Following our speculation above, depressed patients in Destoop et al.'s study might have found it difficult to fight back the unfair offers. Future studies may contribute to clarify the mechanisms of this particular behavioral presentation of people with depression.
Additionally, only when the risk of being detected was low did the patients in the present study make fewer deceptive responses than the controls. That is, the controls tended to lie more frequently when the risk of being detected was low because they would be exposed and punished less frequently in this condition. Compared with the healthy participants, depressed patients might have tried to avoid risky decision making (deception) even when the risk was low. This idea is supported by a study by Smoski et al. (2008)
, who observed that depressed patients performed better than controls in the Iowa gambling task, a finding that could be understood only from the perspective that depressed patients were risk avoidant.
In sum, the behavior of people with depression of being relatively less altruistic as well as less deceptive than their healthy counterparts reflects their tendency to be very self-focused. Depressed patients may have difficulty in integrating information of both risk and others’ intentions into social decision making. Their impaired interpersonal interaction could have a biological basis, which would be worth further exploring in future studies. Indeed, both animal (Grippo et al. 2007
) and human (Hinojosa et al. 2011
) studies have shown that social isolation is a predictor of depression. Our results provide further evidence that depressed patients behave in a particular way that isolates them from other people in social interactions. This specific behavioral pattern might contribute to their further social isolation and may thus intensify their depression.
This study investigated the decision making of depressed patients in interpersonal interactions in an economic exchange game. This lab task did not reflect a real social interaction. Rather, it was only a very simplified model of social behavior that failed to capture other important domains of social interaction, for example, communication through verbal language (Duff et al. 2009
), nonverbal language (Brune et al. 2009
), facial expressions (Mojzisch et al. 2006
), and eye contact (Voncken et al. 2006
). Future studies may advance our understanding of the social behaviors of depressed patients by involving more factors of social interaction. Pairing behavioral with neuroimaging studies in the future could also help unravel the neural mechanisms underlying the behaviors. Moreover, Fujiwara (2009)
have recently shown that people who make altruistic financial contributions to individuals other than family members may be at risk of developing major depression. Therefore, it is difficult to conclude that the depressed patients’ special behavioral pattern in social decision making is the consequence of their mental disorder. Future longitudinal studies may contribute to addressing the causal relationship between major depression and abnormal choices in social decision making.