For the majority of the 20th century, one of the central dogmas of linguistics was
that, at the level of the lexicon, the relationship between words and meanings is
arbitrary: there is nothing about the word ‘dog’ for example that makes it a
particularly good label for a dog. However, in recent years it has become
increasingly recognized that non-arbitrary associations between words and meanings
make up a small, but potentially important portion of the lexicon. This thesis
focuses on exploring the effect that non-arbitrary associations between words and
meanings have on language learning and the structure of the lexicon. Based on a
critical analysis of the existing literature, and the results of a number of experiments
presented here, I suggest that the overall prevalence and developmental timing of
two forms of non-arbitrariness in the lexicon– systematicity and motivatedness – is
shaped by the pressure for languages to be learnable while remaining expressive. The
effect of pressures for learnability and expressivity have been recognized to have
important implications for the structure of language generally, but have so far not
been applied to explain structure at the level of the lexicon.
The central claim presented in this dissertation is that features of the perceptual and
cognitive organization of humans results in specific types of associations between
words and meanings being easier for naïve learners to acquire than others, and that
the pressure for languages to be learnable results in lexica that leverage these
human biases. Taking advantage of these biases, however, induces constraints on the
structure of the lexicon that, left unchecked, might limit its expressivity or penalize
subsequent learning. Thus, lexica are structured such that early-acquired words are
able to leverage these biases while avoiding the limitations imposed by those biases
when they are extended past a certain point
