Within the field of clinical linguistics there are four crucial, but difficult, inter-linked questions.

1. How is it that children both learn to talk and yet often have problems with this? Most children learn to talk without any special guidance, training or instruction. There are plainly random variations in what chidlren hear said. Typically, this is in the language of the family and the general environment. As shown by David Adger (2019), very early in life children are saying things which, in all probability, have never been said before in the whole of human history. But by around ten, children have a common understanding of what is interpretable, what isn’t, and what is interpretable in more than one way. This common understanding is reached without children ever being given what is known as ‘privileged information’ about how things work in their target language. Of course, children wouldn’t understand if anyone ever tried to explain any of the critical points. But no two children hear the same things. In the case of any given language, what children happen to hear said, and what they are taught (if they are taught at all) vary widely and randomly. What they say varies infinitely. They can say an infinite number of things. So the learning of speech and language cannot be just by copying what has been heard. This is a very cursory statement of what is commonly known as ‘the logical problem of language acquisition. But difficulties with this are the commonest sort of developmental disorder. The more we know about this the better we can help those children for whom learning to talk is not so easy.
2. How did speech and language evolve in the human species? It is clear from numerous experiments that even with the most intense training, chimpanzees, bonobos, and gorillas do not progress to anything like the language skills of a typical human two-year old. There is clearly a profound biological difference on this point. Humans have evolved a capacity which seems to be on a different level from any comparable system. Putting this differently, it is incomparable.
3. Why should it be that different sorts of speech and language problem commonly go together – by what are known as ‘co-morbidities’?
4. Why is it that children’s speech errors pattern in the way they clearly do? Children make many sorts of mistakes in learning to talk. But not quite all sorts of mistakes. Order in disorder is a strange anomaly. There shouldn’t be any order at all.

On the one hand, it is obvious that no two children are the same, not even supposedly identical twins. On the other, there has to be a commonality in the acquisition of speech and language. Despite the huge range of childhood experiences, people brought up in a given language community have a common understanding of the community language. Without this common understanding, there would be a monstrous unfairness in the idea of law and the principle that ignorance of the law is no defence. Business contracts would be meaningless. And there would be no wit or plays on words or verbal comedy.

Children seem to know what to do with words like who, what, when, where and why without being told or taught. Consider two pairs of questions

“Who do you want to play with?” and “Who do you want to talk to?”

“Who do you want to play with you?’ and “Who do you want to talk to you?”

The questions differ only in the last or the last but one word – about who is to do the playing or the talking. But in both cases, who is understood in a position to the right of the position in which it is pronounced. The difference is commonly understood without this ever being explained.

Not really questions

Something quite contradictory happens in expressions of surprise, as in “He said what?” or “She went where?” In English, these are not really questions. But such expressions, with words like what or where said in the position where they are understood, are not generally requests for information. Children don’t seem to confuse these expressions of surprise with proper questions where the question word is pronounced at the beginning.

Multiple Wh questions

Or take what are sometimes known as ‘multiple’ Wh questions like “Who ate what?” or “What did who say when?” Such questions are rare in everyday conversation. But in most languages, including English, only one of the question words is said at the beginning of the sentence. When children learning English do ask such questions (even though they only do so very rarely), they do not make mistakes like ‘Who what ate?” or “What who ate?” with the question words together before any other words. Why don’t children ever make this sort of mistake? And how do they correctly and reliably understand multiple Wh questions (which they do) even when they have never heard such structures?

Solutions, answers, and a hypothesis

In my research I outline an approach to answering these questions. I propose that  speech and language evolved by a series of approximately seven significant steps. These steps provided an apparatus for both speech and language. Provisions by these steps can be used incorrectly by child learners, sometimes very incorrectly. Although this is uncommon, it does happen. For the most part children’s speech and language develop in characteristic and well-ordered ways, although at different tempos.

Application

Clinical linguistics is sometimes, mistakenly I think, seen as a very minor specialism in linguistics. It investigates one area where theoretical advances can be put to immediate, practical use. It can improve the speed and efficacy of interventions, and suggest interventions for conditions which may seem to be untreatable. There is relevant data in almost every other area from theoretical syntax to field studies in typology. Clinical discoveries may provide useful and relevant data in almost any area.