Making decisions is an integral part of everyday life. Social psychologists have demonstrated in many studies that humans' decisions are frequently and strongly influenced by the opinions of others-even in simple perceptual decisions, where, for example, participants have to judge what an image looks like. However, because the effect of other people's opinions on decision making has remained largely unaddressed by the neuroimaging and neurophysiology literature, we are only beginning to understand how social influence is integrated into the decision-making process. We put forward the thesis that by probing the neurophysiology of social influence with perceptual decisionmaking tasks similar to those used in the seminal work of Asch (1952, 1956), this gap could be remedied. Perceptual paradigms are already widely used to probe neuronal mechanisms of decision making in nonhuman primates. There is also increasing evidence about how nonhuman primates' behavior is influenced by observing conspecifics. The high spatial and temporal resolution of neurophysiological recordings in awake monkeys could provide insight into where and how social influence modulates decision making, and thus should enable us to develop detailed functional models of the neural mechanisms that support the integration of social influence into the decision-making process.
An experienced dermatologist has to decide whether a mole on the sole of a patient's foot looks suspicious for melanoma. After she examines it carefully, she thinks that the mole is benign. Then she is told that two other dermatologists have diagnosed melanoma and said that the mole should be removed. What should the dermatologist do? Now, consider a monkey foraging for food in the trees. The fruit on its current tree has turned out not to be very juicy, so the monkey moves on. In three directions, it sees a few spots of color glinting between the leaves that might indicate fruit; it remembers vaguely that some of these might belong to a tree that yielded nice fruit only yesterday, and it sees another monkey moving about in one of the trees. Which tree should the first monkey move to next?
Obviously, both dermatologists and monkeys are faced with decisions every day of their lives. Often, as illustrated by the examples above, decisions are made in a social context. Thus, the decision of the dermatologist is likely to be influenced by hearing what her colleagues think (Bonaccio & Dalal, 2006; Yaniv, 2004). Similarly, observing a conspecific foraging in a tree nearby may be a cue for a monkey, biasing its decision about where to move next (Bonnie & de Waal, 2007; Meunier, Monfardini, & Boussaoud, 2007; Myers, 1970; Subiaul, Cantlon, Holloway, & Terrace, 2004).
In general, three aspects of social decision making can be distinguished: The first aspect refers to situations in which the success of one's own decision directly depends on the concomitant decisions of others. To illustrate, imagine deciding about the level of a first request in a negotiation about a pay raise. The level at which the first request is pitched will, on the one hand, be dependent on its presumed reception by the other person; on the other hand, it also has direct consequences for the offer given in response. In most of the scenarios used for research on such social interactions, the decision maker is in direct competition with others for a specific reward. We will refer to this aspect of social decision making as strategic decision making. Utilizing tasks derived from game theory (von Neumann & Morgenstern, 1944), neuroeconomics has recently begun to successfully delineate the neuronal circuits that contribute to strategic social decision making (for reviews, see Fehr & Camerer, 2007; Glimcher, 2003; Loewenstein, Rick, & Cohen, 2008; Sanfey, 2007). Neuroimaging experiments and brain lesions in humans and animals have identified an important role in strategic decision making for brain areas that are involved in reward evaluation, reinforcement learning, and the representation of mental states of others, such as the ventral striatum, caudate nucleus, orbitofrontal cortex, insula, and anterior cingulate cortex. …