Year

2004

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Psychology - Faculty of Health & Behavioural Sciences

Abstract

This thesis is concerned with investigating causal explanations for the observed patterns of sex differences in spatial cognition. Researchers in this area have traditionally reported a male advantage in many spatial tasks, particularly mental rotation. However, a number of more recent results suggest that the pattern of sex differences in spatial ability is more complicated: females show a spatial memory advantage on some tasks involving large arrays of objects, and navigational tasks seem to reveal sex differences in attentional bias toward different types of spatial information. After reviewing three main types of causal explanations, it is concluded that the evolutionary approach has at least two compelling advantages. Firstly, it is inherently multi-disciplinary, and readily incorporates both biological-proximal and social-developmental explanations, as these mechanisms are integral to the expression of evolved traits. Secondly, it not only fits the available data, but provides a framework for generating novel theories and predictions; indeed, Silverman and Eals (1992) used an evolutionary model of sex differences in spatial ability � the �hunter-gather model, based on the sexual division of labour hypothesized to underlie hominid foraging and navigational requirements � to successfully predict the previously unobserved female advantage on certain object array tasks. Thus the primary aim of this thesis is to expand the hunter-gatherer model by further examining the computational demands placed on spatial ability in ancestral humans, and by making increasingly focused predictions about the cognitive skills underlying sex differences in spatial performance. Following a detailed discussion of these computational demands, it is argued that male-typical hunting as foraging strategy is associated with long range survey-based wayfinding, which requires the allocentric encoding of spatial relationships and enhanced sensitivity to coordinate information such as distance, based on the use of external geometric reference points. By contrast, it is argued that female-typical gathering as a foraging strategy is associated with sort range route-based wayfinding, which requires the ability to encode relational egocentric-categorical information about a large number of objects, but does not require enhanced sensitivity to categorical information. In order to examine this proposed model, several different spatial tasks are used and devised, with the aim of measuring sex differences in bias and sensitivity toward different types of spatial information. In all cases the computational demands of the task are related to the computational demands hypothesised to underlie hunting and gathering. Examples of manipulations of established tasks include variations on mental rotation (including measurements of the effects of angular distance), object array tasks, object recognition, categorical and coordinate sensitivity tasks, the judgement of line angle and position, and the tilt categorical and coordinate sensitivity tasks, the judgement of line angle and position, and the tilt illusion. Examples of novel lower-level tasks designed to measure the skills underlying mental rotation and object location memory include a rapid object array task, a judgement of vertical, and a judgement of parallelism. In almost all cases, results showed strong support for the hypotheses, and in some cases the novel tasks produced extremely large sex differences in the predicted direction. Basically, males show a clear advantage in sensitivity to coordinate information when external geometric reference points are available; furthermore, they are able to encode that information allocentrically, which allows the formation of spatial representations with the property of space constancy. Females show a clear advantage in object-to-position assignment based on egocentric-categorical encoding of a large number of relational positions. However, as anticipated, the female advantage seems to be limited to encoding capacity, and does not extend to sensitivity to categorical spatial information. Conclusions offer support for the use of evolutionary psychology as a tool in generating theorectical and experimental development. While further research is required to investigate the biological implementation and developmental mechanisms underlying the observed sex differences, it is hoped that this thesis contributes to the hunter-gatherer model of spatial ability, and more generally to the field of sex differences in cognition.

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