, 2005), and neurobiological variables have only rarely been used as predictors of individual differences in altruism (de Quervain et al., 2004, Harbaugh et al., 2007, Hare et al., 2010, Moll et al., 2006 and Tricomi et al., 2010). Recent applications of brain morphometry indicate that individual differences in brain structure can be useful in understanding individual differences in traits and skills (Kanai and Rees,

2011). We therefore conjectured that variables reflecting relatively stable neuroanatomical Navitoclax individual differences—such as gray matter (GM) volume—may help predict individual differences in altruism. In humans, altruism is likely to be related to perspective taking, i.e., the ability to take other individuals’ perspectives

into account. In fact, developmental data suggest that preschoolers who have already acquired theory of mind skills behave more prosocially (Takagishi et al., 2010), and experiments with adults indicate that subjects with better skills in reading others’ mental states show more altruistic behavior (Underwood and Moore, 1982). One brain region that has been repeatedly and reliably found to be implicated in tasks requiring the ability find more to represent and understand others’ perspectives is the temporoparietal junction (TPJ) (Decety and Lamm, 2007, Frith and Frith, 2007, Ruby and Decety, 2001, Saxe and Kanwisher, 2003 and Young et al., 2010). We therefore hypothesized that GM volume in the TPJ may provide a neuroanatomical basis for individual differences in human altruism. Research on human social preferences provides behavioral (Bolton

and Ockenfels, 2000, Charness and Rabin, 2002 and Fehr and Schmidt, 1999) and neural (Tricomi et al., 2010) evidence that other-regarding behaviors and motives depend on the initial payoff allocation between the subject and the subject’s partner. In particular, if subjects have a lower initial payoff than their partner (“disadvantageous initial inequality”), they are much less willing to behave altruistically toward the partner compared to a situation with advantageous initial inequality (i.e., when the heptaminol subject has a higher initial payoff than the partner). In fact, some individuals even reduce the partner’s payoff if possible if the latter has a higher initial payoff. In view of the radically different propensities for behaving altruistically in the domain of advantageous and disadvantageous inequality, it may be possible that the neuroanatomical basis for human altruism is not identical across these domains. In the present study, subjects had to allocate money between themselves and anonymous partners (Figure 1; task description in Experimental Procedures) in a series of binary choice problems. In each trial, subjects faced a binary choice in which they could increase or decrease the partner’s monetary payoff.