How do people talk to each other? Psycholinguist Mathias Barthel studied this, taking account of pauses, eye movements and even the position of the tongue.Conversing is a complex skill with no explicit rules. What determines who speaks when and how quickly we prepare our sentences in our minds, while still listening to the other person? During research at the Max Plank Institute in the Netherlands, German psycholinguist Mathias Barthel (currently working at Humboldt University of Berlin) discovered that while listening to someone, people will do anything to prepare the ultimate response as quickly as possible while still listening. This month, he received his PhD on this at Radboud University.You write that most conversations run smoothly despite all kinds of unwritten rules. What are these rules?“At the beginning of a conversation, it's unclear who will speak when and for how long, or how long the conversation will last – we 'manage' the course together during the conversation. The basic rules are that we take turns to speak, one after the other, without overlaps or overly long silences between turns.Numerous clues tell us where a sentence is headed and when it's our turn to speak – grammar, word choice, intonation or the lengthening of the final syllable.Everyone involved in the conversation takes collective care to make sure that the stream of con-versation continues to flow. The fact is that we don't like overly long silences. They cause dis-comfort or distrust and not only that, we might miss our chance to say something. A silence lasting just one second is long. The average silence lasts 250 milliseconds. This is true of all languages and cultures, with tiny differences. For example, the Danish leave slightly longer pauses and the Japanese, even shorter ones.”But people regularly talk at the same time, or interrupt each other?“If interruptions are cooperative, we don't experience them as disruptive. We continually talk briefly at the same time – if we didn't, conversations would be really stilted.It sometimes seems as though two people start talking at the same time, but in fact that’s rarely the case. Generally, one person is ahead by a half or whole syllable. It's an unwritten rule that the person who starts speaking first, carries on.Sometimes, a person will behave as though they started speaking earlier, even though they did-n't. They can then claim their turn by not stopping speaking, lengthening the syllables and talk-ing louder. No-one likes that, so more often the opposite occurs – you keep quiet to let the other person speak until the next opportunity. Except sometimes during political debates or disagreements.”Your research shows that while listening, people start to prepare their reaction at a very early stage. Are they actually still listening?“People try to complete their cognitive tasks with the least possible effort. That would mean first listening and only then preparing our response, two successive tasks. But we don't do this, since it would result in much longer silences. Apparently, we find the smooth running of a conversation, a social objective, more important than the burden of extra cognitive effort.My research shows that people do listen carefully to what the other person is saying and to clues that signal it's their turn, and at the same time start planning their own response very early on. We know this partly by looking at the size of the pupils, which shows what's happening subconsciously: the more cognitive power you are using, the more enlarged your pupils be-come.”So if someone's pupils are small, they're not actually listening to me?“It’s something that is only visible to some professionals, and the reason poker players often wear sunglasses. But the brightness of the light, for example, or a person's physical condition have much more effect on the size of your pupils.”How did you discover that we plan early on?“By a number of tests. In one of these tests, I put two people in different rooms. They spoke to each other through an intercom and had to name a list of objects together. Number one, a col-league, began by naming two or three objects. Number two, the test subject, saw images of these and some other objects. The task was to name the remaining objects.I registered the test subject's eye movements. I could see when this person was mentally cross-ing off objects named by the other person and when he or she began preparing what to say next. That's how I discovered that people start planning their own sentence as soon as possible. Sometimes I had the first person make a longer sentence: I was given a telephone, a pen and a monitor, or bought them.An extra verb at the end only shortened the silence on the part of the test subject: the person had been preparing his or her own sentence for a long time. You can see that the breathing and po-sition of the tongue of the person listening changes just before he or she starts to speak.Obviously, real conversations are less task-driven, but we do have to converse in a controlled fashion. Another notable point: I hadn’t asked for the task to be carried out as quickly as possi-ble, and yet that's exactly what all the test subjects did. I suspect this has to do with the social standards associated with holding a conversation: as few overlaps and silences as possible. I venture to suggest that this is no different in other conversations.”This article previously appeared (in Dutch) in de Volkskrant. Image by Trung Thanh through Unsplash.