Richard Marshall interviews Jennifer Lackey in 3:AM Magazine:
3:AM: You’re a leading figure in social or collective epistemology. There are various positions one can take under that term, so there’s what’s called a summative and a non-summative approach isn’t there? What’s the difference between these two positions?
JL: The distinction between summative and non-summative approaches in the collective epistemology literature applies to a broad range of phenomena, including group belief, group justification, group knowledge, and group testimony. According to summativism, a group’s state can be understood in the sense that all or some members of the group are, or would be, in that state. So, for instance, on a summative account of group belief, a group’s believing a proposition amounts to all or some members of the group believing that proposition. On such a view of group testimony, a group’s testifying to a proposition means that all or some members of the group would testify to that proposition were the relevant opportunity to arise. For summativists, then, collective states can be understood entirely in terms of the states of individuals.
In contrast, non-summativists claim that a group’s state cannot be understood in this way. Instead, the group itself is the bearer of the state, where this is something over and above, or otherwise distinct from, the states of the individual members. Such views in general are supported by divergence arguments, which purport to establish that phenomena at the group level can diverge from what is happening at the individual level among the group’s members. For instance, the divergence arguments with respect to group belief hold that a group can be properly said to believe a proposition even if not a single one of its members believes it. This might happen, for example, when a department agrees to put forward a candidate as the best applicant for admission to its Ph.D. program, despite the fact that not a single one of its members actually believes this is correct; instead, they all think that this is the candidate who is most likely to be approved by the administration. In such a case, non-summative accounts of group belief take this to be a good reason to accept that the department itself, rather than any particular individual(s), believes this proposition. For non-summativists, then, a new, collective epistemology is needed to understand the states of collective entities.
3:AM: So where are you on this?
JL: There are important differences between my view of group belief and, say, my view of group testimony, but, broadly speaking, much of my work in collective epistemology aims to develop reductionist views of collective phenomena. The central idea is that collective states can be understood entirely in terms of the states of individuals and the relations between them. This does not mean, however, that the states of groups with a particular content are merely the summation of individual states with the same content.