By the second law of thermodynamics, applying to systems of all sorts from galaxies to molecules, order is naturally lost rather than gained. The law explains why saltwater and fresh water, once mixed cannot be unmixed. Mahommed, one of the first people to think in a modern scientific way, was rightly curious about this. The answer came more than a thousand years later, at the end of the 18th century. By this law, speech errors of all sorts should vary randomly. But they don’t. There are patterns or signs of ordering in speech errors. Why should this be?

Particular mispronunciations at particular ages

The various patterns in children’s speech disorders are commonly grouped together as ‘processes‘.

Amongst the processes most characteristic of minor, early developmental issues there is ‘fronting’ with tongue-tip T and D replacing back of the tongue K and G. But the opposite happens much less often. It can’t be that tongue tip articulations are easier than back of the tongue articulations. In doggy commonly said as GOGI a tongue tip articulation at the beginning of the word in a stressed syllable assimilates to a back of the tongue articulation in the middle of the word in an unstressed syllable. But doggy as DODI and tickle as TITU are seemingly never heard. And ‘backing’ or replacing all tongue tip articulations by back of the tongue articulations, saying tar as CAR, and do as GOO, is rare. The oddness of this distribution and the apparent polarity reversal between fronting and the apparent back-of-the-tongue assimilation was noted, but not explained, by Alan Cruttenden in 1978. Later in development, as shown by Nunes (2002), as children reach seven or eight this pattern changes in a very specific, narrowly defined set of contexts including cardigan said as KARDIDAN, calculator as KALTALATOR, hippopotamus as HITOPOTAMUS, archeopteryx as ARTIOPTERIX, with a tongue tip articulation replacing both the back of the tongue and lip articulations. The polarity of the loss gets reversed, this time over the course of development.

The assimilation, as this is known, involves a loss of a contrast. There is the opposite, known as ‘dissimilation’, increasing the contrast, in little as LIKU and middle as MIGU, where the tip of the tongue T and D dissimilate to K and G next to what the child correctly perceives as an underlyingly tongue tip L. The notion of ‘underlying’ elements is discussed here as an instance of ‘underspecification‘. (In descriptions of children’s speech, dissimilation is, for some reason, commonly overlooked).

What is happening to the articulators in these child-speech forms has little or no equivalent in competent speech where there are many instances of assimilation between adjacent stops and many instances of long range phenomena between vowels. But in competent speech there are no known cases of either assimilation or dissimilation between the articulators of non-adjacent consonants, not in English, and not in any other language.

Some typically developing children and many with impaired speech say sea as TEA, zoo as DOO, four as POUR, van as BAN.  S, Z, F and V are all what are known as ‘fricatives’. The airstream is allowed to pass through a narrow gap in the mouth, the tongue tip and the hard palate in S and Z, and between upper teeth and the lower lip in F and V. The sound we perceive is by the friction as the air is forced through the gap. The replacement sounds, T, D, P and B, all involve a complete blockage or occlusion of the air stream. This is commonly known as ‘stopping’ for obvious reasons. The opposite of stopping also happens, in other words, saying tea as SEA. But this, like backing is rare.

By contrast, in competent speech, the opposite of stopping is common. For instance, when electric is turned into a noun as electricity, the stop in the final K sound in electric turns into the fricative S sound in electricity. But stopping in competent speech is almost unknown.

The overall pattern of incompetence cannot be characterised in terms of the internal properties of the speech sounds known as their distinctive features.

Running in the family

In a  way that has been intensely studied for almost a century, in roughly a third of all developmental issues with speech and language, there is a closely related family member with a similar issue. On some counts the percentage of family involvement is even higher. But the effect seems to hold at least five times more often than it would if there was no significant connection. In such cases, of course, it is hard to disentangle the social and the genetic aspects. Family members have contact with one another as well as sharing some of the same DNA. But in the case of one child I was treating, half the family lived on the far side of Europe. They only got together for weddings and funerals. So there was no plausible for any social factor to play any role in a speech disorder. But exactly the same speech issue was observed in several members of the family on both sides of Europe.  On the basis of more robust statistical data, most researchers conclude that there has to be a significant genetic effect.

A linguistic approach

By my proposal here, what is characteristically happening in child speech is by misuses or over-simplifications (usually slight) of a highly evolved architecture which operates in a necessary ‘learnability space‘.