Rejecting unfairness: emotion-driven reaction or cognitive heuristic?

In the following paragraphs, I am arguing that rejecting inequality, even when it means sacrificing available resources, could be interpreted as a default response that occurs when there is no other reason to choose otherwise. Moreover, I am reviewing some of our latest findings suggesting that emotions might not be the sole mechanism that ultimately explains this response, as claimed instead by the most accredited account (e.g., Sanfey et al., 2003; van't Wout et al., 2006; Crockett et al., 2008; Tabibnia et al., 2008). The idea that a 50-50 share is preferred over other distributions, when there is no reason to support one of the contending parties, is not new to the psychological debate: it has been suggested that people use equality heuristically, because it has psychological advantages, such as being a cognitive simple strategy, easy to use and to be understood by everyone, quickly implemented, defensible, and, moreover, a useful starting point from which, in case, adjustments can be made (Messick and Schell, 1992; Messick, 1995). Furthermore, less-equal distributions are consistently rejected more often among different human populations (Henrich et al., 2006). The central claim of Bicchieri's book The Grammar of Society (2006) is that an equal-division norm plays a critical and under-appreciated role in driving behavior in bargaining games (Bicchieri, 2006; Nichols, 2010). Research in the field of behavioral economics has demonstrated that the model of homo economicus often fails to predict human behavior: the Ultimatum Game (UG) (Guth et al., 1982), a widely employed tool to investigate socio-economic decision-making, perfectly shows how people do not always make decisions driven by the principle of maximizing monetary payoff. In this game, a proposer has to share some money with a responder, who can either accept or reject the offer: if he accepts, the money is divided as the proposer has established, otherwise both of them get nothing. To maximize their payoffs, the proposer should offer the smallest amount of money, and responder should always accept, as even one is better than zero. However, numerous findings show that the proposer tends to make fair offers, around 50% of the share, while the responder prefers to reject a sure amount of money rather than accepting an unfair division. Models of social preferences provide a formal explanation for the apparently irrational behavior. Two families of theories, i.e., theories of negative reciprocity (e.g., Rabin, 1993; Falk and Fischbacher, 2006) and theories of inequality aversion (e.g., Fehr and Schmidt, 1999) tried to explain rejections: the former focuses on intentions and describes rejections as a tool to punish the unfair proposer, whereas the latter focuses on the outcome and claims that people are naturally averse to unequal distributions, especially when disadvantageous. Recently, Tricomi et al. (2010) found support for this claim, showing that basic reward brain structures, such as ventral striatum and ventromedial prefrontal cortex (vMPFC), are involved in both advantageous and disadvantageous inequity. From a psychological perspective, negative emotions, such as anger and frustration, elicited by the unfair treatment, are accounted to cause rejections (Pillutla and Murnighan, 1996), and a number of neuroscientific findings support this hypothesis: for example, van't Wout et al. (2006), using the skin conductance response (SCR) as a measure of emotional activation, reported that people were more emotionally aroused, showing a higher SCR when rejecting, as opposed to accepting, unfair offers. Moreover, areas known to be involved in emotional control, such as vMPFC, and in processing negative emotions, such as anterior insula (AI), are found also to be activated by rejections, and not acceptances, of UG unfair offers (e.g., Sanfey et al., 2003; Koenigs and Tranel, 2007).