The ability to learn multiple languages simultaneously is a fundamental human
linguistic capacity. Yet there has been little attempt to explain this in evolutionary
terms. Perhaps one reason for this lack of attention is the idea that
monolingualism is the default, most basic state and so needs to be explained before
considering bilingualism. When thinking about bilingualism in this light, a
paradox appears: Intuitively, learning two languages is harder than learning one,
yet bilingualism is prevalent in the world. Previous explanations for linguistic diversity
involve appeals to adaptation for group resistance to freeriders. However,
the first statement of the paradox is a property of individuals, while the second
part is a property of populations. This thesis shows that the properties of cultural
transmission mean that the link between individual learning and population-level
phenomena can be complex. A simple Bayesian model shows that just because
learning one language is easier than two, it doesn't mean that monolingualism
will be the most prevalent property of populations.
Although this appears to resolve the paradox, by building models of bilingual language
evolution the complexity of the problem is revealed. A bilingual is typically
defined as an individual with "native-like control of two languages" (Bloomfield,
1933, p. 56), but how do we define a native speaker? How do we measure proficiency? How do we define a language? How can we draw boundaries between
languages that are changing over large timescales and spoken by populations with
dynamic structures? This thesis argues that there is no psychological reality to
the concept of discrete, monolithic, static `languages' - they are epiphenomena
that emerge from the way individuals use low-level linguistic features. Furthermore,
dynamic social structures are what drives levels of bilingualism. This leads
to a concrete definition of bilingualism: The amount of linguistic optionality that
is conditioned on social variables.
However, integrating continuous variation and dynamic social structures into existing
top-down models is difficult because many make monolingual assumptions.
Subsequently, introducing bilingualism into these models makes them qualitatively
more complicated. The assumptions that are valid for studying the general
processes of cultural transmission may not be suitable for asking questions about
bilingualism. I present a bottom-up model that is specifically designed to address
the bilingual paradox. In this model, individuals have a general learning mechanism
that conditions linguistic variation on semantic variables and social variables
such as the identity of the speaker. If speaker identity is an important conditioning
factor, then `bilingualism' emerges. The mechanism required to learn one
language in this model can also learn multiple languages. This suggests that the
bilingual paradox derives from focussing on the wrong kind of question. Rather
than having to explain the ability to learn multiple languages simultaneously as
an adaptation, we should be asking how and why humans developed a flexible
language learning mechanism.
This argument coincides with a move in the field of bilingualism away from asking
`how are monolinguals and bilinguals different?' to `how does the distribution of
variation affect the way children learn?'. In this case, while studies of language
evolution look at how learning biases affect linguistic variation, studies of bilingualism
look at how linguistic variation affects learning biases. I suggest that the
two fields have a lot to offer each other