  Physical embodiment is a required component for robots that are structurally
coupled with their real-world environments. However, most socially interactive
robots do not need to physically interact with their environments in order to
perform their tasks. When and why should embodied robots be used instead of
simpler and cheaper virtual agents? This paper reviews the existing work that
explores the role of physical embodiment in socially interactive robots. This
class consists of robots that are not only capable of engaging in social
interaction with humans, but are using primarily their social capabilities to
perform their desired functions. Socially interactive robots provide
entertainment, information, and/or assistance; this last category is typically
encompassed by socially assistive robotics. In all cases, such robots can
achieve their primary functions without performing functional physical work. To
comprehensively evaluate the existing body of work on embodiment, we first
review work from established related fields including psychology, philosophy,
and sociology. We then systematically review 65 studies evaluating aspects of
embodiment published from 2003 to 2017 in major peer-reviewed robotics
publication venues. We examine relevant aspects of the selected studies,
focusing on the embodiments compared, tasks evaluated, social roles of robots,
and measurements. We introduce three taxonomies for the types of robot
embodiment, robot social roles, and human-robot tasks. These taxonomies are
used to deconstruct the design and interaction spaces of socially interactive
robots and facilitate analysis and discussion of the reviewed studies. We use
this newly-defined methodology to critically discuss existing works, revealing
topics within embodiment research for social interaction, assistive robotics,
and service robotics.
