Why is conversation so easy? Simon Garrod 1 and Martin J. Pickering 2 1 University of Glasgow, Department of Psychology, Glasgow, UK, G12 8QT 2 University of Edinburgh, Department of Psychology, Edinburgh, UK, EH8 9JZ Traditional accounts of language processing suggest that monologue – presenting and listening to speeches – should be more straightforward than dialogue – hold- ing a conversation. This is clearly not the case. We argue that conversation is easy because of an inter- active processing mechanism that leads to the align- ment of linguistic representations between partners. Interactive alignment occurs via automatic alignment channels that are functionally similar to the automatic links between perception and behaviour (the so-called perception – behaviour expressway) proposed in recent accounts of social interaction. We conclude that humans are ‘designed’ for dialogue rather than monologue. Whereas many people find it difficult to present a speech or even listen to one, we are all very good at talking to each other. This might seem a rather obvious and banal observation, but from a cognitive point of view the appa- rent ease of conversation is paradoxical. The range and complexity of the information that is required in mono- logue (preparing and listening to speeches) is much less than is required in dialogue (holding a conversation). In this article we suggest that dialogue processing is easy because it takes advantage of a processing mechanism that we call ‘interactive alignment’. We argue that interactive alignment is automatic and reflects the fact that humans are designed for dialogue rather than monologue. We show how research in social cognition points to other similar automatic alignment mechanisms. Problems posed by dialogue There are several reasons why language processing should be difficult in dialogue. Take speaking. First, there is the problem that conversational utterances tend to be elliptical and fragmentary. Assuming, as most accounts of language processing do, that complete utterances are ‘basic’ (because all information is included in them), then ellipsis should present difficulty. Second, there is the problem of opportunistic planning. Because you cannot predict how the conversation will unfold (your addressee might suddenly ask you an unexpected question that you have to answer), you cannot plan what you are going to say far in advance. Instead, you have to do it on the spot. Third, there is the problem of making what you say appropriate to the addressee. The appropriateness of referring to some- one as ‘my next-door neighbour Bill’, Bill, or just him depends on how much information you share with your addressee at that point in the conversation. Does she know who Bill might be? Does she know more than one Bill? Is it obvious to both of you that there is only one male person who is relevant? Similarly, in listening, you have to guess the missing information in elliptical and fragmentary utterances, and also have to make sure that you interpret what the speaker says in the way he intends. If this were not enough, conversation presents a whole range of interface problems. These include deciding when it is socially appropriate to speak, being ready to come in at just the right moment (on average you start speaking about 0.5 s before your partner finishes [1]), planning what you are going to say while still listening to your partner, and, in multi-party conversations, deciding who to address. To do this, you have to keep task-switching (one moment speaking, the next moment listening). Yet, we know that in general multi-tasking and task switching are really challenging [2]. Try writing a letter while listening to someone talking to you! So why is conversation easy? Part of the explanation is that conversation is a joint activity [3]. Interlocutors (conversational partners) work together to establish a joint understanding of what they are talking about. Clearly, having a common goal goes some way towards solving the problem of opportunistic planning, because it makes your partner’s contributions more predictable (see Box 1). However, having a common goal does not in itself solve many of the problems of speaking and listening alluded to above. For instance, it does not ensure that your contributions will be appropriate for your addressee, alleviate the problems of dealing with fragmentary and elliptical utterances, or prevent interface problems. One aspect of joint action that is important concerns what we call ‘alignment’. To come to a common under- standing, interlocutors need to align their situation models, which are multi-dimensional representations containing information about space, time, causality, intentionality and currently relevant individuals [4–6]. The success of conversations depends considerably on the extent to which the interlocutors represent the same elements within their situation models (e.g. they should refer to the same indi- vidual when using the same name). Notice that even if interlocutors are arguing with each other or are lying, they have to understand each other, so presumably alignment is not limited to cases where interlocutors are in agreement. But how do interlocutors achieve alignment of situation models? We argue that they do not do this by explicit negotiation. Nor do they model and dynamically update every aspect of their interlocutors’ mental states. Instead, Corresponding author: Simon Garrod (simon@psy.gla.ac.uk). Opinion TRENDS in Cognitive Sciences Vol.8 No.1 January 2004 8 http://tics.trends.com 1364-6613/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.tics.2003.10.016