Morphology and MLU


My post from two weeks ago, Form, Content, Use- Repeat., spoke about phonology, so now it’s time to talk about the next component of form in language: morphology. A morpheme is the smallest unit of a word that carries meaning. Take the word “kick” for example. This word has one morpheme since it cannot be broken down into smaller components each with their own meanings. The word “kicks” has 2 morphemes (1st= kick, 2nd= s).

There are two categories morphemes can be placed into. Free morphemes can stand on their own, like “kick”. It doesn’t need to be attached to anything to make sense. Bound morphemes, on the other hand, need to attach onto another morpheme. Prefixes and suffixes fit into this category (the fancy categorical name for these is “derivational” morphemes because they can change the class of a word) as well as the plural “s” added to the ends of words (a kind of tense marker in the category of “inflectional” morphemes). 

Now onto MLU…which is pretty impossible to understand without knowing what a morpheme is, but we have that covered.  MLU stands for mean length of utterance. To calculate the MLU, first you need a language sample from a child consisting of about 50-100 utterances. To get this sample the SLP can record a session in which they ask the child questions and create a dialogue through play interactions. After the language sample is attained, the SLP then counts the number of morphemes the child said and divides this by the number of utterances.

Counting morphemes in child samples is a bit different than counting morphemes in fully developed adult speech. Young children often say words together, as if they were one word. If a child says “choo choo train” when you ask what he is playing with, it would count as one morpheme if there is no other evidence that he says “train” by itself.  Same thing goes for catenative forms of words such as “gonna.” It would count as one morpheme instead of the normal two for an adult who knows it is a shortened way to say “going to.” Fillers such as “um,” “oh,” and “well” do not get assigned morphemes at all.

After the MLU is calculated (total number of morphemes divided by number of utterances), the SLP will be able to see which developmental stage the child is in with regards to language development. There is a high correlation between MLU and chronological age of the child. This is partly due to the child’s increase in working memory (short term memory) as they get older allowing for sentences to be longer. A longer sentence has a better chance of having more morphemes.

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