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 mixing. 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 are commonly grouped together as ‘processes‘.

Amongst the processes most characteristic of early development there is fronting with tongue-tip T and D replacing back of the tongue K and G. But the opposite happens  by assimilation in doggy commonly said as GOGI. But doggy as DODI and tickle as TITU are seemingly never heard. The oddness of this 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 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.

Typically, these errors involve the loss of a contrast by what is known as ‘assimilation. or (less often) increasing the contrast by what is known as ‘dissimilation’. (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 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 consonants, not in English, and not in any other language.

The opposite of stopping also happens, in other words, saying tea as SEA. But this, like ‘backing’ is rare.

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

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‘.