In historical linguistics, we pay a lot of attention to the mechanisms of language change in terms of languages as systems. We try to explain how a change may first have arisen by looking at other facts about that language. For example, we might explain the change in the language of many English speakers whereby ‘th’ is pronounced like ‘f’ (saying ‘fink’ for ‘think’, etc.) by pointing out that these two sounds are acoustically similar and that this may have led to them being confused by children learning the language. I wrote about this sort of explanation in a previous blog post.

But there’s a second layer of explanation to be done. The very idea of ‘languages as systems’ is an abstraction. There’s no such thing as ‘English’, a single entity: instead, each speaker with some level of English proficiency (perhaps 840,000,000 people according to Wikipedia) produces language which has a lot in common but some differences. So for English as a whole to change—or just the English of the UK, or the English of New York, or even the English of a single village—the newly minted pronunciation (or word, or phrase, or piece of grammar) has to spread from the first person who produced it to other people.

In historical linguistics and in sociolinguistics we distinguish two different flavours of this process of spread: ‘transmission’, where the new form is learnt by young children as they acquire the language for the first time, and ‘diffusion’, where the new form is passed among adults.

Early stages of a change spreading by contagious diffusion/wave model
Early stages of a change spreading by contagious diffusion/wave model
Late stages of a change spreading by contagious diffiusion/wave model
Late stages of a change spreading by contagious diffiusion/wave model

Our ideas about how diffusion works have traditionally been based on historical dialect studies. Huge survey-style dialectology projects in the nineteenth and twentieth centuries created maps of differences in traditional dialects, especially in German- and English-speaking Europe. These showed that new forms appeared to have spread continuously outwards in patterns that resembled the ripples produced by a stone dropped into water. A recent new form would be found in a single, connected area. An older form might have spread everywhere except for a few small regions which stood out as conservative islands. This idea of change spreading outwards continuously is sometimes described as the ‘wave’ model.

However, later in the twentieth century, studies of ongoing changes which were still diffusing actively through the population found a different pattern. Here it was found that, instead of spreading continuously across the map, changes tended to start in the largest city in a region and then proceed to ‘jump’ from city to city without ever being found in the intervening countryside. Having spread to progressively smaller cities, the change would then start to spread out from these to the surrounding rural areas. This observation led to a revised suggestion that there were two possible patterns of diffusion: the ‘contagious diffusion’ observed in historical studies, where a change spread continuously across space, and ‘hierarchical diffusion’, where the change spread down a ‘hierarchy’ of increasingly smaller settlements.

Early stages of a change spreading by hierarchical diffusion
Early stages of a change spreading by hierarchical diffusion
Later stages of a change spreading by hierarchical diffusion
Later stages of a change spreading by hierarchical diffusion

Finally, in much more recent research, a third, rarer pattern has been identified. Changes can apparently sometimes first spread throughout a rural region, then into smaller towns, and only then finally into cities. This pattern, the mirror image of hierarchical diffusion, has been labelled ‘contra-hierarchical diffusion’.

So why do different changes diffuse in different ways? Perhaps we need another reminder not to think exclusively in terms of big abstractions. I’ve written here about changes being found in particular locations and about changes spreading across space. But language doesn’t actually exist in physical space. Really what we’re talking about is not changes spreading to particular places, but changes spreading to the language of people who live in particular places.

Changes can clearly only spread between people when those people talk to one another. So when changes spread continuously across space, that must reflect that people are more likely to know and talk to people who live near to them. Really, what we’re seeing is that changes spread continuously through social networks—and those social networks, for very obvious practical reasons, mostly reflect the physical reality of where people live and work.

Once we remember this, hierarchical diffusion also becomes easy to explain. If we compare the modern era to any historical period we find very different patterns of population movement and communication. With public transport and cars people habitually travel much further to work and study. They also relocate more often and move much greater distances when they do. With these factors and electronic communications, they keep in more regular touch with people living far further away than ever before. And cities are crucially important to all these processes: people commute in and out of cities much more than between rural areas and they are more likely to migrate to cities for work. All this means that people’s social networks have much less to do with the geography of continuous space than ever before. Most people are communicating regularly with people who live much, much further away from us than our ancestors ever did—and such long-distance contacts particularly connect cities.

Given all that, we really shouldn’t be surprised to find that changes tend to spread first between cities and only later to the surrounding countryside. In fact, this isn’t a different process to that of contagious diffusion at all! Both are really just the process of changes spreading through people’s social networks.

So what about contra-hierarchical diffusion? This is a little harder to explain. The best explanation here is probably to do with the social meaning that speakers ascribe to changes. In regions where there is significant inward migration especially into urban areas, local speakers may actively participate in championing distinctively local ways of speaking in order to differentiate themselves from newcomers. As cities have historically been involved in hierarchical diffusion process, it is rural regions that are most likely to still preserve distinctively local forms. As a result, a drive to speak in a more local way (and thus express that one is a ‘real’ local person and not an outsider) will tend to cause rural forms to spread to urban areas.

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