Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Alaa A. Ahmed

Second Advisor

Virginia L. Ferguson

Third Advisor

Mark E. Rentschler

Fourth Advisor

Shalom D. Ruben

Fifth Advisor

Leaf Van Boven

Abstract

Nearly every aspect of our behavior is framed by risk, and people can respond to risk very differently. Some individuals readily confront highly variable situations in order to obtain large rewards (risk-seeking behavior), while others prefer more certain situations even if it means obtaining smaller rewards (risk-averse behavior).

At present, the role that risk plays in movement decisions is poorly understood. This dissertation investigates human movement decisions under risk, exploring the rationality of movement behavior in risky constructs and whether sensitivity to risk generalizes across movements. In the first study, I compared risk-sensitivity between two continuous motor tasks (arm-reaching and whole-body leaning). Individuals were irrationally risk-seeking in both movements, and more so in whole-body leaning. In the second and third studies, I examined the effect of postural threat on discrete, risky decisions for the same motor tasks as well as for an economic task (performed while sitting and while standing). Postural threat was manipulated using an elevated platform. At ground level, individuals were again more risk-seeking in whole-body leaning than in arm-reaching. Increasing elevation only affected the task that was most salient to the threat, resulting in overweighting the probability of errors for the whole-body movement. There was no consistent effect of threat on arm-reaching, and economic choices were robust to body posture and postural threat. The first three studies establish that seemingly irrational movement behavior stems from poor estimations of motor variability and distorted utility and probability weighting, and relevant threat exacerbates irrationality. In the fourth study, I investigated whether irrationalities were also present in the valuation of effort, a predominant movement cost. I compared relative valuations of gains and losses between an effort-based movement domain and an economic domain. Individuals exhibited loss aversion in effort, suggesting that movement decisions may be geared toward avoiding higher effort over acquiring lower effort. Loss aversion was not as pronounced in this effort task as it was in equivalent financial decisions.

Ultimately, this work demonstrates that humans exhibit irrational movement decisions under risk. There is evidence that risk-sensitivity in movement generalizes, to an extent, between motor tasks, and is influenced by emotional states.

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