A constraint is a kind of limitation on what is possible. Consider constraints that come from the nature of the human body itself. For spoken language, the physical and physiological properties of the vocal tract constrain what can be produced, and the auditory system constrains what can be perceived. For sign language, the physical and physiological properties of the hands, arms, upper body, and face constrain what can be produced, and the visual system constrains what can be perceived.
Just as important are cognitive constraints, constraints arising from the nature of the human mind. First, there are limitations on human memory. Cognitive scientists divide human memory into short-term and long-term memory. Short-term memory is used to temporarily store the information that is needed as we interpret the world around us. When we are faced with a complicated visual scene, we can’t look at everything at once, and we have to scan the scene in order to come up with some kind of interpretation. But we need to temporarily store some sort of record of what we’ve already seen in the scene as we do this. That information is kept in short-term memory. Likewise, when we are listening to a sentence, we need to remember the parts that we’ve already heard as we are trying figure out the meaning of the whole. Short-term memory is used for this as well. The same applies to reading and to the visual processing of sign language. Cognitive science research has shown that our short-term memory has a very restricted capacity, and this places strong constraints on how sentences and discourses are organized.
We use our long-term memory to store information that we learn through experience. Information may remain in long-term memory for a very long time, even indefinitely, and human long-term memory is very large, larger than the memory of any current computer. But long-term memory is still limited. We obviously do not have an infinite memory capacity, and just because we have stored an item in memory doesn’t mean we can retrieve it when we need it. We will see later how the finiteness of long-term memory matters for how language works.
Since languages have to be learned, limitations on human learning are obviously also relevant. This is perhaps the most complex and controversial topic of all. Everyone at least agrees that language must be learnable because children obviously learn it. That is, it must be possible to figure out the forms, the functions, the meanings, and the associations between them on the basis of the examples of language that are available to young children. It turns out that coming up with an explanation for how this might happen has proven to be very challenging though.
“Speaker” Orientation and “Hearer” Orientation
Finally there are constraints that are specific to the two ways in which language is processed, language production, that is, speaking, writing, and signing, and language comprehension, that is, listening, reading, and interpreting linguistic signs. To simplify matters, I will be referring to a language producer as the “Speaker”, even though writers and signers as well as speakers proper are intended. And I will be referring to a language comprehender as the “Hearer”, even though readers and sign interpreters as well as hearers proper are included. (To remind ourselves of these distinctions, I will capitalize Speaker and Hearer when they are used in this way.)
For the Speaker, the main constraint is that the production of linguistic forms be easy. It is easier for the Speaker to make relatively few distinctions because the Speaker has to remember what the distinctions are and to make the extra effort to keep things distinct. For example, maintaining the agreement between subject and verb in the present tense in English (the girl sings, the girls sing) requires an effort on the part of the Speaker. What is easy for the Speaker depends in turn on constraints from the body. For example, large movements of the tongue tip are more difficult to execute than short movements, so short movements should be preferred from the Speaker’s perspective.
For the Hearer, the main constraint is that linguistic forms that need to be distinguished can be easily distinguished in comprehension. This constraint also depends on the body, specifically the parts of the nervous system that are responsible for hearing (for spoken language) and vision (for written and signed language). For example, if a language contains a large number of homophones, that is, different words which sound the same (such as two, too, and to in English), this may put a burden on the Hearer.
Speaker-oriented constraints and Hearer-oriented constraints often oppose each other; what simplifies things for the Speaker (for example, not making many distinctions) complicates things for the Hearer. The opposition of Speaker orientation and Hearer orientation is particularly clear as languages change. Languages change for a variety of reasons — contact with other languages, imperfect learning by children, random fluctuation — but it appears that all languages are always changing. Most of the changes can be seen as either Speaker-oriented or Hearer-oriented. For example, the grammar of a language may become simplified as some suffixes are dropped, a change that seems to result in less work for the Speaker. But the two kinds of trends always balance each other out in the end, and the simplification of the grammar in one way will probably be compensated for by an increase in complexity (from the perspective of the Speaker) somewhere else in the grammar of the language. Otherwise language would fail as a communicative device. These built-in pressures in favor of the Speaker and the Hearer apparently prevent the world’s languages from moving in some general overall direction. That is, at least in recent history, it does not appear that languages have generally been getting simpler or more complicated (in any sense of these words) as they evolve.