If we are to study language as communication, then we will need to take into account the properties both of human communicators and of the situations in which linguistic communication occurs. Indeed, a further important claim of functionalism is that language systems are not self-contained with respect to such factors, and therefore autonomous from them, but rather are shaped by them and so cannot be properly explained except by reference to them. Linguists who make this claim. . .undoubtedly form the largest and most influential group of functional theorists. The main language-external motivating factors are of two kinds: the biological endowment of human beings, including cognition and the functioning of language processing mechanisms, and the sociocultural contexts in which communication is deeply embedded. We might also expect that a functionalist approach would pay serious attention to the interaction between these factors and the ways in which languages change over time, although in practice this varies considerably from one model to another.
The question of motivation for linguistic systems is, of course, not a simple one. Much of the formalist criticism of functionalist positions has assumed a rather naïve view of functional motivation, in which some linguistic phenomenon is explicable in terms of a single factor. Functionalists, however, have never seen things this way, but rather accept that there may be competing motivations, pulling in different directions and often leading to compromise solutions.
Butler, C S. Concise Encyclopedia of Philosophy of Language and Linguistics. (Elsevier Science, 2009), 268.
In other words, functionalists are free to (indeed, they must) incorporate a variety of linguistic tools in their final analysis of a language system. For example, in order to analyze a language system’s socio-cognitive elements, we must also understand the internal pressures under which it seeks to produce maximally-effective forms of communication—a task too messy to be neatly categorized by a single theoretical framework. Different tools are needed for different tasks.