One of the most important insights in my research so far has been to understand that consensus is not the same as unanimity. In a nutshell, unanimity is when everyone agrees and consensus is when no one disagrees. Although this seems like a hair-splittingly small distinction, the difference between the two can be huge in practice.
The French sociologist Philippe Urfalino has done some pioneering work to clarify how unanimity and consensus are different (Urfalino 2007, 2010, 2014). Amongst other things he points out that in order to establish unanimity, it is necessary that everyone forms their individual preference on the matter and expresses it in a vote. Unanimity is reached if everyone votes for the same option. Consensus, in contrast, allows some (or even a majority) to not make up their mind and to simply remain silent. Margaret Gilbert (1987: 194) speaks of allowing something “to stand as the view of [the group]”. Both Urfalino and Gilbert emphasize independently of each other the non-summative nature of consensus (Gilbert’s term is “collective belief”), meaning that collective decisions are not necessarily the sum of individual decisions.
Indeed, I would argue that most of the time we make collective decisions by consensus (i.e. in a non-summative way). This is neither inherently good nor bad, but it is important that we understand what we are doing. In particular, we should recognize that when we decided something by consensus, we cannot assume that each member of the group actually preferred the option that became the decision of the group. It is even possible that some participants (and again: even the majority) are in fact against the decision, but - for whatever reason - remain silent and let it pass.
Having studied decision-making among democratic and often strongly egalitarian activists (Haug 2010), I am aware that some proponents of consensus-decision making will say that if a large numbers (or even just a few) are against the decision but they let it pass anyway instead of vetoing it, then this is not a real consensus decision. However, my impression is that such insisting on a narrow definition of consensus as the only “real” consensus is not very helpful in actually reaching consensus.
Of course, it works fine if everyone shares this narrow definition and understands how it works, but if diverse groups come together and each of them has a different understanding of what consensus is, then it makes more sense to acknowledge that consensus can be practiced in different ways and to see that the commonality between all of them is that they are all constituted by successfully observing the absence of disagreement (Urfalino  speaks of “the rule of non-opposition”): “Is anyone against it?” - “No? Then it’s decided.”
The crux is in how this happens in practice. I suggest that we can distinguish at least four types of consensus in meetings: imposed consensus, acclaimed consensus, basic consensus, and deliberative consensus (Haug 2015). In a nutshell, the difference between these is in how much opportunity there is to voice dissent:
In an imposed consensus, someone (usually the chair) simply claims that consensus has been reached and directly moves on to the next topic or closes the meeting. Participants who feel that this claim to consensus is not accurate will have to be quite articulate in order to make their objections heard while the meeting is already proceeding to a new topic. In fact, they have to disrupt the meeting and demand that the observation of consensus be annulled.
In an acclaimed consensus, there is an explicit “slot” for participants to express their views, but its official purpose is not to express dissent but consent. Participants are asked to confirm that they agree with “the consensus” (e.g. “Does everyone agree?”). The assumption is that, yes, everyone agrees and participants are expected to merely connfirm this assumption (as indicated by the “few muttered ‘agreed’”). A “No” is clearly the less preferred option (implied in the wording of the question and how it is articulated).
In basic consensus, participants are explicitly asked if there is anyone who disagrees. A frequent wording in the meetings I observed was, “Is there anyone who cannot live with this decision?” or simply “anyone against?”, thus providing an explicit slot for dissenters to, “speak now or forever remain silent”, as it were. In this practice, consensus appears as a stretch of silence and by observing this silence as the absence of dissent, participants determine that consensus exists. The silence in this practice affords the voicing of disagreement, making it easy (as compared to imposed and acclaimed consensus) dissenters to raise objections.
Deliberative consensus (the term is inspired by Beatty and Moore’s (notion of “deliberative acceptance”) not only gives participants the opportunity to express dissent, but actively encourages that dissent is articulated in order to make sure that no one is silenced. For example, the facilitator may take up previously contested issues and ask the dissenters whether their concerns have meanwhile been sufficiently addressed, or if there are tentative signs of disagreement such as “rumours” from the coffee break or someone frowning, these might be taken up by asking something like “Are you sure we’ve reached consensus? Peter, you look unhappy.” In contrast to the other forms of consensus, deliberative consensus encourages dissent – not in the sense that dissent is welcomed, but in that there is a particular concern for dissenters being silenced. The encouragement of objections serves to counteract institutional pressures by creating an atmosphere of appreciation for each individual’s point of view.
Although deliberative consensus seems to come close to unanimity, it is still fundamentally different from unanimity because even when dissent is encouraged, participants are still free to not form a preference on the matter at all and there may still be some who disagree but who don’t want to stop the group from going ahead, be it because the decision is not important to them or because they see no point in arguing any further, or for whatever other reasons they may have.
I think that this is one of the main reasons why consensus decision-making is so popular, even in meetings where by-laws suggest that decisions are made by voting. Having to make up your mind about every single damn issue that’s on the agenda can be very demanding. Consensus allows people to let others whom they trust and/or deem competent basically make the decision for them.
Hence, despite its reputation, consensus is not necessarily an egalitarian form of collective decision-making because “incorporates a general acceptance of inequality in individual contributions to collective decision-making. The general equality of participation to the process coexists with a recognition of the legitimacy of unequal influence of individuals, depending on social status or expertise” (Urfalino 2014: 339). Rather than being inherently democratic in a liberal sense, it seems that the benefits of consensus are mainly in it’s capacity for “deciding without dividing” (Urfalino 2014: 333).
Gilbert, Margaret (1987): Modelling collective belief. In: Synthese, Vol. 73, No. 1. pp. 185–204. DOI: 10.1007/BF00485446
Haug, Christoph (2010): Discursive Decision-making in Meetings of the Global Justice Movements: Cultures and Practices. Doctoral thesis. Freie Universität Berlin, Fachbereich Politik- und Sozialwissenschaften. Berlin.
Haug, Christoph (2015): What is consensus and how is it achieved in meetings? Four practices of consensus decision-making. In: Joseph A. Allen, Nale Lehmann-Willenbrock & Steven G. Rogelberg (Eds.): The Cambridge Handbook of Meeting Science. New York: Cambridge Univ. Press. pp. 556–584. DOI: 10.1017/CBO9781107589735.024
Moore, Alfred & O’Doherty, Kieran (2014): Deliberative Voting: Clarifying Consent in a Consensus Process. In: Journal of Political Philosophy, Vol. 22, No. 3. pp. 302–319. DOI: 10.1111/jopp.12028
Urfalino, Philippe (2007): La décision par consensus apparent: Nature et propriétés. In: Revue européenne des sciences sociales, Vol. 45, No. 1. pp. 47–70. DOI: 10.4000/ress.86
Urfalino, Philippe (2010): Deciding as bringing deliberation to a close. In: Social Science Information, Vol. 49, No. 1. pp. 111–140. DOI: 10.1177/0539018409354812
Urfalino, Philippe (2014): The Rule of Non-Opposition: Opening Up Decision-Making by Consensus. In: Journal of Political Philosophy, Vol. 22, No. 3. pp. 320–341. DOI: 10.1111/jopp.12037