The main finding is that neurostimulation influencing cortical excitability in the VPC elicits changes in utilitarian judgments. When we applied tDCS to the VPC but not when we applied it to the OC, we also identified distinct gender-related differences in utilitarian responses to moral as well as non-moral dilemmas. These differences might help to explain the known gender-related differences in human utilitarian reasoning. We also found a significant reduction in RTs for utilitarian responses after cathodal VPC-tDCS, regardless of type of dilemmas and of sex. These findings acquire strength because they come from a study investigating utilitarian judgments by tDCS in a large study sample, 78 subjects, balanced for sex and age and controlled for religious beliefs and type of education.

Our tDCS study using the moral judgment task to assess various material and non-material factors influencing human behavior and decisions therefore advances current knowledge on decision-making processes and utilitarian judgment.

The gender-related differences in utilitarian responses we found before applying tDCS agree well with current knowledge. Gender-related differences in cognitive and behavioral processes are associated with functional and structural gender differences in the brain [44] , [45] , [46] , especially in the frontal lobe [47] , [48] , [49] , an area also involved in moral behavior [2] , [3] , [5] , [6] , [7] , [9] , [10] . In their study assessing altruistic cooperativeness, Yamasue et al. (2008) found that the greater cooperativeness in females correlated with larger gray matter volumes in the social brain regions such as the bilateral inferior frontal cortex. A remarkable gender-related difference has been found also in the frontal lobe neurotransmitters related to behavior [50] , [51] . Finally, hormones greatly influence behavior and their receptor distribution differs between sexes in the brain structures involved with cognition [42] . The gender-related difference we found in the performance of the moral judgment task therefore fits in well with anatomical, functional, neurochemical and neuroendocrinological evidence of gender-related differences in brain areas involved in moral behavior.

The analyses of baseline RTs confirmed Greene et al. (2001) study, showing that utilitarian responses are slower than non-utilitarian responses specifically in PM dilemmas, but not in NM and IM dilemmas. Whereas these baseline RTs differences are independent of gender, we found significant gender-related differences in utilitarian responses studied before tDCS. These results are in line with the observation that males differ from females in cognition, decisional processes [41] , [42] , [43] , moral judgments [28] , [30] and in brain activation patterns during moral tasks [29] . These differences are independent of cultural factors, such as education levels and religious beliefs.

Effect of VPC-tDCS on Utilitarian Judgments

Whereas tDCS left response patterns to the moral judgment task in males unchanged, in females anodal VPC-tDCS increased the utilitarian responses for all types of dilemmas tested. Also, cathodal VPC-tDCS reduced RTs for utilitarian responses in both males and females. We therefore conclude that anodal and cathodal tDCS both interfere with rational decisions, or rational evaluation of the advantages and disadvantages of each option in both sexes, but do so more strongly in females. Our experiments indicate that tDCS-induced changes in utilitarian reasoning are site-specific and that both anodal and cathodal VPC-tDCS differentially modified subjects' performance in the moral judgment task, suggesting that the effects are specific and depend on factors other than skin perception. Equally important, the tDCS-induced changes we observed are not related to mood changes or cultural factors.

In our experiments the charge flows ventrally from the prefrontal surface to the right arm, thereby most probably stimulating the most ventral portion of prefrontal cortex. Because skull resistivity is higher than scalp resistivity, most of the current delivered by tDCS gets shunted through the scalp. The current density in the scalp also tends to decrease with the distance between electrodes. As Nathan et al. (1993) confirmed, the current density generated in the cortex by the stimulation decreases rapidly with depth – i.e. it decreases by one order of magnitude in 8 mm [52]. Also, intra-operative results show that eliciting a motor evoked potential by directly stimulating the human brainstem requires a current density of about 2–9 mA/cm2 [53]. For these reasons, it is unlikely that charge flows in the brainstem and structures other than the cerebral cortex below the stimulating electrode. tDCS might, however, also influence neighboring cortical areas. Even if the main effect on cortical excitability is localized beneath the stimulating electrode [54], we cannot totally exclude the possibility that tDCS also modulates other areas of the prefrontal cortex (directly, but also indirectly). Hence, notwithstanding possible tDCS-induced changes in other brain areas, we believe that tDCS induces its most important effect by modulating the VPC below the stimulating electrode.

Explaining why tDCS affects utilitarian responses in a gender-specific manner and RTs in both sexes is challenging. The female susceptibility to the effects of anodal VPC-tDCS we found in utilitarian responses could arise in several ways. For example, the gender-related effects of tDCS on utilitarian judgments might agree with the known gender specificity in the effects of cathodal and anodal tDCS on brain excitability [55], [56], [57]. Yet if they do, we find it hard to explain why none of the previous studies on the influences of brain stimulation on decisional processes reported gender-related effects [17], [18], [19].

A further more conjectural possibility is that the female tendency towards altruism is also more easily modulated by external factors. Hence whereas altruism in males is preprogrammed, in females it might be more sensitive to changes in brain plasticity. According to this hypothesis, tDCS influences utilitarian judgments in females but not in males, who responded to brain stimulation only with a reduction in RTs. This hypothesis fits in with the behavioral differences existing between genders during life: whereas in males altruistic behavior has no need to change during life, in females it has to change in relation to behavioral changes linked to reproduction and parental care [58]. This gender-related difference in altruism's sensibility to external factors is also supported by the known gender-related anatomical and functional differences in the brain structures controlling behavior and decision making. In females, a greater propensity to altruistic behavior correlates with a larger-sized inferior frontal cortex [46], so that reducing ventral prefrontal activity with tDCS could mean enhancing the cognitive and rational control of behavior.

Functional neuroimaging data with moral tasks also support a gender-related pattern of brain activation. For example, Harenski et al. (2008) found a greater activation of posterior cingulate cortex and anterior insula in females, and a greater activation of the inferior parietal cortex in males. Because the VPC is tightly linked to the cingulate cortex, tDCS over the VPC could indirectly modulate activity in the cingulate cortex. In conclusion, the differential sensitivity to tDCS in males and females could reflect gender-related anatomical, functional and neurochemical differences in the brain areas involved in utilitarian behavior. Moreover, even if gender, which also includes educational aspects, is a crucial factor for tDCS efficacy, socio-economic status and personal education might be relevant too. The relative importance of biological and social factors remains an interesting question for future researches.

A central point to clarify is how anodal and cathodal tDCS differentially modify utilitarian choices. Anodal stimulation could do so by inducing excitatory effects on the underlying cerebral cortex [11], [13], [59]. This possibility notwithstanding, even if in studies investigating tDCS-induced changes in primary cortices anodal stimulation delivered close to neurons depolarizes the neuronal membrane and cathodal stimulation hyperpolarizes it, cognitive studies leave the relation between polarity and the effects on the neuronal membrane unclear [60]. In accordance with this point of view, in our study anodal tDCS had the same effect as a lesion in the ventral portion of the frontal cortex. In their study, Koenigs et al. (2007) described lesioned patients as characterized by deficits in decision-making tasks, namely they produced an abnormally utilitarian pattern of judgments on moral dilemmas endorsing highly emotionally aversive behaviors despite an undamaged social knowledge of normative conduct [39]. Our results nevertheless showed that cathodal tDCS decreased RTs for utilitarian responses but left the proportion of utilitarian responses unchanged. Indeed, it decreased, albeit not significantly, utilitarian responses in females. These data are congruent with the study by Knoch et al. (2008) who showed that cathodal tDCS on the prefrontal cortex reduces the propensity to punish unfair behavior in the Ultimatum Game [61]. In this task, punishing unfair behavior means rejecting an unfair offer in order to obtain a fairer proposal. This is a utilitarian behavior because the subject aims to achieve a personal gain, even at the expense of damaging others. Hence cathodal and anodal tDCS both induce a functional brain system imbalance through the same mechanism. However, whereas cathodal tDCS alters the time of utilitarian reasoning in both sexes, anodal stimulation interferes more incisively, modifying utilitarian reasoning and its possible consequent actions, also in a gender-specific way. Insofar as the ventral part of the frontal lobe is phylogenetically older than the dorsolateral part [62], it might be more related to defence of the individual and to survival than to social interaction and cooperativeness.

Among other possible explanations, besides influencing cortical excitability, tDCS could induce neurochemical changes in the brain that could involve several neurotransmitters [63]. Dopamine is important for behavior, motivation and decision-making [64]. Parkinson's disease, characterized by reduced dopamine production in the brain, is associated with impaired decisional processes [65]. Anodal VPC-tDCS might therefore alter the outcomes of utilitarian decision-making processes by enhancing prefrontal dopamine. Yet, because dopamine is an anionic catecholamine that during electrophoresis migrates toward the anode, anodal VPC-tDCS could increase dopamine levels in the frontal lobe, influencing the reward circuit and ultimately altering decisional processes, increasing the rate of utilitarian responses. Interestingly, the gender-specific effects of VPC-tDCS in our experiments agree with the female-specific features of the dopaminergic system in the frontal cortex [50], [51]. Although this is a theoretical hypothesis, emerging data in humans support the interaction between tDCS and the dopaminergic system [66], [67].

The gender-related differences in utilitarian responses we found in this study using the moral judgment task in healthy subjects are a good starting-point for explaining criminal behavior. Mental illness – the inability to distinguish good from bad – reflects immoral reasoning. Our study provides evidence showing that males are by nature more utilitarian than females, and that non-invasive brain stimulation more easily and incisively alters feminine than male utilitarian thinking, thus confirming the prevalence of criminal behavior in males. Future studies designed to investigate the possibility of modulating morality and utilitarianism could better account for criminal behavior and male propensity to violate the law.