Languages evolve, adapting to pressures arising from their learning and use. As these pressures may be different in different sociocultural environments, non-linguistic factors relating to the group structure of the people who speak a language may influence the features of the language itself. Identifying such factors, and the mechanisms by which they operate, would account for some of the diversity seen in the complexity of different languages. This thesis considers two key hypotheses which connect group structure to complex language features and evaluates them experimentally. Firstly, languages spoken by greater numbers of people are thought to be less morphologically complex than those employed by smaller groups. I assess two mechanisms by which group size could have such an effect: different degrees of variability in the linguistic input learners receive, and the effects of adult learning. Four experiments conclude that there is no evidence for different degrees of speaker input variability having any effect on the cross-generational transmission of complex morphology, and so no evidence for it being an explanation for the effect of population size on linguistic complexity. Three more experiments conclude that adult learning is a more likely mechanism, but that linking morphological simplification at the level of the individual to group-level characteristics of a language cannot be simply explained. Idiosyncratic simplifications of adult learners, when mixed with input from native speakers, may result in the linguistic input for subsequent learners being itself complex and variable, preventing simplified features from becoming more widespread. Native speaker accommodation, however, may be a key linking mechanism. Speakers of a more complex variant of a language simplify their language to facilitate communication with speakers of a simpler language. In doing so, they may increase the frequency of particular simplifications in the input of following learners. Secondly, esoteric communication | that carried out by smaller groups in which large amounts of information is shared and in which adult learning is absent | may provide the circumstances necessary for the generation and maintenance of more complex features. I assess this in four experiments. Without a learnability pressure, esoteric communication illustrates how complexity can be maintained, but there is generally no evidence of how smaller groups or those with greater amounts of shared information would develop comparatively more complex features. Any observable differences in the complexity of the languages of different types of groups is eliminated through repeated interaction between group members. There is, however, some indication that the languages used by larger groups may be more transparent, and so easier for adult learners to understand.

The effects of dual tasking in Parkinson's disease (PD) have been studied for a number of years. Previous research has generally focused on changes in gait patterns while another task has been performed concurrently. Very few studies have focused on the impact of a concurrent task on speech or language. Language is key for communication: to express wants and needs, to maintain familial relationships, and for social interaction. Thirty-seven individuals participated in the study: 10 with PD, 14 neurologically healthy older (HO) adults, and 13 healthy younger (HY) adults. The participants were given a list of topics to consider and were invited to select several to talk about during the experiment. Their monologues were recorded as they spoke under three conditions: standing still, walking on a treadmill, and walking over randomly presented obstacles on a treadmill. The monologue recordings were transcribed, marked for processing by Systematic Analysis of Language Transcripts (SALT), and analyzed for subordinate clauses by a language expert. The language variables measured were the mean length of utterance in morphemes (MLUm), relative clauses per utterance, adverbial clauses per utterance, noun clauses per utterance, total clauses per utterance, words per minute, different words per minute, relative clauses per minute, adverbial clauses per minute, noun clauses per minute, total clauses per minutes, and utterances per minute. There were significant changes across the conditions of standing, walking, and obstacle in the language variables of words per minute, different words per minute, noun clauses per minute, total clauses per minute, and utterances per minute. A downward trend was noted for adverbial clauses per minute as the gait task became more demanding. The PD and HO groups had less complex language than the HY group, as reflected by the following language variables: adverbial clauses per minute, noun clauses per minute, and total clauses per minute. These findings suggest that as attentional resources used for the production of language are directed to increasing levels of motoric activity, language complexity will significantly decrease across conditions.

Several studies using standardized tests have provided evidence for the presence of language disorders in persons with Multiple Sclerosis (MS) and have suggested that persons with MS may have language that is less complex. One aspect not thoroughly studied is that of syntactic complexity in naturalistic, conversational settings. The present study collected language samples from 10 adults with MS and 10 age-matched controls and compared scores on five quantitative measures derived from those samples. No significant differences were found between groups on any measure. This finding suggests that in mild cases of MS or during periods of remission, individuals may retain their ability to use complex language structures in naturalistic discourse.

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