Many tests of language development distinguish between ‘expressive’ or ‘productive’ language’ and ‘receptive language’ or ‘comprehension’. But if these tests use different criteria according to what is being tested, as many tests do, it is not clear that the same cognition is being tested in the two cases. Small children can often respond appropriately to instructions before they give equivalent instructions themselves. The apparent discontinuity with reception before expression in almost all areas of language in almost all children may be due to any number of intervening factors – including memory, familiarity, confidence in the use of language, access to the necessary words, and the demands of the context. Where there seems to be a developmental issue, the difference can seem even more stark between comprehension and expressive language. So there is a notion of Specific Expressive Language Delay or Disorder or SELD. This is sometimes used as a diagnostic category, often based on a difference between children’s scores on tests of their comprehension and their expressive language. But this is not sufficient to force the conclusion that expressive and receptive language represent separate areas of cognition. Part of the problem here is that it is difficult to test these things symmetrically. For instance, many of the most naturally plausible ways of testing comprehension are with respect to answers to questions beginning with who, what, where, when, why and how, or requiring Yes or No as an answer. But it is difficult to devise test protocols which require children to ask corresponding sorts of questions in similarly natural and plausible ways. So tests of comprehension and expressive language may be testing different things. And the differences on the scores may be an artifact of the differences between the protocols. A similar difference may underly the common observation about more normally developing children.

It is possible, as the framework here would suggest, that there are just different degrees of competence, yielding different results according to how these are tested.

One area where there may be a substantial difference between what is said and what is understood may be with respect to the lexicon. Children may confidently and accurately point to pictures of lions, tigers, rhinoceroses and other exotic animals from their pictures, long before they are using these words in their own speech. The passive vocabulary is likely to be several times larger than the active one. But as the lexicon expands, both of these things are very difficult to measure with any accuracy.

More generally, it may be that the notion of an ‘expressive language disorder’ is less well-defined than sometimes assumed.

It might seem possible in principle to devise a fully symmetrical test of expressive and receptive language. But in the absence of such a thing, the difference here may be artifactual. If a test is designed on the basis that speech and language are behaviours, rather than capacities, the test may be doing little more than testing this assumption. Tests of ‘expressive’ and ‘receptive’ language may may be just tapping different cognitions.