Why should it be that developmental problems with speech and / or language are typically complex, with seemingly separate issues commonly happening together, far more often than would be expected by chance’?  

If a set of issues commonly occur together, they are known as co-morbidities. A co-morbidity is not a disease, but just the co-occurrence of more than one condition in a significant proportion of individuals.

Common co-morbidities

Considered separately, common issues involve :

• Disorders involving ‘speech sounds’ or ‘phonemes’ or delays in their development;
• Getting the words or phonemes in the wrong order;
• Difficulty putting words together into sentences with subjects like I in “I fell over”;
• Problems with both auditory memory and auditory discrimination – often known as ‘Central Auditory Processing Disorder’.
• Problems with reading and writing;
• Problems with physical co-ordination;
• Poor ability to discriminate and identify sounds against a background of noise of some different sort;
• Poor memory of sequence of random items. such as numbers;
• Poor awareness of speech and language – known as ‘metalinguistics’.

Disorders commonly involve more than one of these. These co-morbidities are well-agreed and have been extensively studied over many years, as by Nunes (2002), Hugh Catts (2005, 2006), Dorothy Bishop (2009), Sean Redmond (2015, 2016), Maggie Snowling (2019), and many others.

But how is it that a physical  problem sequencing articulations is commonly co-morbid with the detection of a similarity between the sounds of words, a common defect of metalinguistics? Is there an over-arching commonality?

These co-morbidities challenge to the notion of ‘Specific Language Impairment’, SLI. If co-morbidities are characteristic, what is the sense in calling SLI ‘specific’? So the term SLI has now generally been abandoned in favour of ‘Developmental language disorder’, DLD.

Disorders or delays with phonemes

Difficulties with the ordering and articulation of speech sounds or phonemes are often attributed to the the positioning of the tongue in the mouth and sequencing of the necessary movements and gestures. But why should a seemingly physical issue be significantly co-morbid with a metalinguistic issue?

Missing subjects

Of children having any sort of difficulty with English, many have difficulty learning that in the overwhelming majority of sentences there is a subject, or a form taking the place of the subject, i.e. it and there, as in “It seems that there is a problem”. If such children are shown a picture of someone eating, and are asked: What is happening? they may think that “Eat” or “Eating” is an OK answer. Of course, as an answer that shows that the question has been more or less understood. But it also shows a misunderstanding about what makes English English, and different in a very particular way from languages like Italian, Greek, Spanish, in which the equivalent of “Eating” or “Eating dinner” would be perfectly OK. Such children misunderstand how English works. This may limit their understanding of sentences where there is some reference to something like the ‘subject’. In other words, most sentences. A serious issue for the children affected.

But again, why should this c0-occur with one or more metalinguistic issues?

The engineering question – and approaching an explanation

Why should these ‘co-morbidities’ exist? Engineers would ask: What is the commonality? The question is sharpest in relation to the co-morbidity between difficulties with respect to speech, where there is clearly and obviously a physical aspect, and a defect with respect to metalinguistics, where there is clearly no physical aspect of any sort.

One possible approach to the issue is by the idea that speech and language have a single architecture and that an underdeveloped sense of metalinguistics is an intrinsic part of the problem for many children who can’t make themselves understood, rather than a separate issue.

The puzzle here was the central issue approached bv Nunes (2002). But in the light of subsequent research, the analysis by Nunes (2002) has to be rejected. But by the proposal here at least some of the co-morbidity issues can be attributed to an under-development of an over-atching Universal Grammar in some individuals. If Universal Grammar is common to all aspects of the speech and language faculty, it is not surprising that the effect is commonly many-sided.