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Presence of Life-Like Robot Expressions Influences
Children’s Enjoyment of Human-Robot Interactions in
the Field
David Cameron1, Samuel Fernando2, Emily Collins1, Abigail Millings1, Roger Moore2, Amanda Sharkey2,
Vanessa Evers3, and Tony Prescott1
1 Dept. of Psychology, University of Sheffield, S10 2TN, UK.
Email: {d.s.cameron, e.c,.collins, a.millings, t.j.prescott}@sheffield.ac.uk
2 Dept. of Computer Science, University of Sheffield S10 2TN, UK.
Email: {s.fernando, r.k.moore, a.sharkey}@sheffield.ac.uk
3 Dept. of Electrical Engineering, Mathematics and Computer Science,
University of Twente, NL. Email: v.evers@utwente.nl
Abstract. Emotions, and emotional expression, have a broad
influence on the interactions we have with others and are thus a
key factor to consider in developing social robots. As part of a
collaborative EU project, this study examined the impact of life-
like affective facial expressions, in the humanoid robot Zeno, on
children’s behavior and attitudes towards the robot. Results
indicate that robot expressions have mixed effects depending on
the gender of the participant. Male participants showed a positive
affective response, and indicated greater liking towards the robot,
when it made positive and negative affective facial expressions
during an interactive game, when compared to the same robot
with a neutral expression. Female participants showed no marked
difference across two conditions. This is the first study to
demonstrate an effect of life-like emotional expression on
children’s behavior in the field. We discuss the broader
implications of these findings in terms of gender differences in
HRI, noting the importance of the gender appearance of the robot
(in this case, male) and in relation to the overall strategy of the
project to advance the understanding of how interactions with
expressive robots could lead to task-appropriate symbiotic
relationships.
1 INTRODUCTION
A key challenge in human robot interaction (HRI) is the
development of robots that can successfully engage with people.
Effective social engagement requires robots to present engaging
personalities [1] and to dynamically respond to and shape their
interactions to meet human user needs [2].
The current project seeks to develop a biologically grounded
[3] robotic system capable of meeting these requirements in the
form of a socially-engaging Synthetic Tutoring Assistant (STA).
In developing the STA, we aim to further the understanding of
human-robot symbiotic interaction where symbiosis is defined as
the capacity of the robot, and the person, to mutually influence
each other in a positive way. Symbiosis, in a social context,
requires that the robot can interpret, and be responsive to, the
behavior and state of the person, and adapt its own actions
appropriately. By applying methods from social psychology we
aim to uncover key factors in robot personality, behavior, and
appearance that can promote symbiosis. We hope that this work
will also contribute to a broader theory of human-robot bonding
that we are developing drawing on comparisons with our
psychological understanding of human-human, human-animal and
human-object bonds [4].
A key factor in social interaction is the experience of emotions
[5]. Emotions provide important information and context to social
events and dynamically influence how interactions unfold over
time [6]. Emotions can promote cooperative and collaborative
behavior and can exist as shared experiences, bringing individuals
closer together [7]. Communication of emotion can be thought of
as a request for others to acknowledge and respond to our
concerns and to shape their behaviors to align with our motives
[8]. Thus emotional expression can be important to dyadic
interactions, such as that between a teacher and student, where
there is a need to align goals.
Research with a range of robot platforms has demonstrated the
willingness of humans to interpret robot expressive behavior –
gesture [9], posture [10], and facial expression [1] – as affective
communication. The extent to which robot expression will
promote symbiosis will depend, however, on how well the use of
expression is tuned to the ongoing interaction. Inappropriate use
of affective expression could disrupt communication and be
detrimental to symbiosis. Good timing and sending clear signals is
obviously important.
Facial expression is a fundamental component of human
emotional communication [11]. Emotion expressed through the
face is also considered to be especially important as a means for
communicating evaluations and appraisals [12]. Given the
importance of facial expressions to the communication of human
affect, they should also have significant potential as a
communication means for robots [13]. This intuition has lead to
the development of many robot platforms with the capacity to
produce human-like facial expression, ranging from the more
iconic/cartoon-like [e.g., 14, 15] to the more natural/realistic [e.g.,
16, 17, 18].
Given the need to communicate clearly it has been argued that,
for facial expression, iconic/cartoon-like expressive robots may be
more appropriate for some HRI applications, for instance, where
the goal is to communicate/engage with children [16, 15].
Nevertheless, as the technology for constructing robot faces has
become more sophisticated, robots are emerging with richly-
expressive life-like faces [16, 17, 18], with potential for use in a
range of real-world applications including use with children. The
current study arose out of a desire to evaluate one side of this
symbiotic interaction – exploring the value of life-like facial
expression in synthetic tutoring assistants for children. Whilst it is
clear that people can distinguish robot expressions almost as well
as human ones [16, 18], there is little direct evidence to show a
positive benefit of life-like expression on social interaction or
bonding. Although children playing with an expressive robot are
more expressive than those playing alone [19], this finding could
be a result of the robot’s social presence [20] and not simply due
to its use of expression. A useful step toward improving our
understanding would be the controlled use of emotional
expression in a setting in which other factors, such as the presence
of the robot and its physical and behavioral design, are strictly
controlled.
In the current study the primary manipulation was to turn on or
off the presence of appropriate positive and negative facial
expressions during a game-playing interaction, with other features
such as the nature and duration of the game, and the robot’s
bodily and verbal expression held constant. As our platform we
employed a Hanson Robokind Zeno R50 [21] which has a
realistic silicon rubber (“flubber”) face, that can be reconfigured,
by multiple concealed motors, to display a range of reasonably
life-like facial expressions in real-time (Figure 1).
Figure 1. The Hanson Robokind Zeno R50 Robot with example
facial expressions
By recording participants (with parental consent), and through
questionnaires, we obtained measures of proximity, human
emotional facial expression, and reported affect. We hypothesized
that children would respond to the presence of facial expression
by (a) reducing their distance from the robot, b) showing greater
positive facial expression themselves during the interaction, and
c) reporting greater enjoyment of the interaction compared to
peers who interacted with the same robot but in the absence of
facial expression. Previous studies have shown some influence of
demographics such as age and gender on HRI [22, 23, 24]. In our
study, a gender difference could also arise due to the visual
appearance of the Zeno robot as similar to a male child, which
could prompt different responses in male and female children. We
therefore considered these other factors as potential moderators of
children’s responses to the presence or absence of robot emotional
expression.
2 METHOD
2.1 Design
Due to the potential of repeated robot exposure prejudicing
participants’ affective responses, we employed a between-subjects
design, such that participants were allocated to either the
experimental condition – interaction with a facially expressive
robot, or to the control condition of a non-facially-expressive
robot. Allocation to condition was not random, but determined by
logistics due to the real-world setting of the research. The study
took place as part of a two-day special exhibit demonstrating
modern robotics at a museum in the UK. Robot expressiveness
was manipulated between the two consecutive days, such that
visitors who participated in the study on the first day were
allocated to the expressive condition, and visitors who
participated in the study on the second day were allocated to the
non-expressive condition.
2.2 Participants
Children visiting the exhibit were invited to participate in the
study by playing a game with Zeno. Sixty children took part in the
study in total (37 male and 23 female; M age = 7.57, SD = 2.80).
Data were trimmed by age to ensure sufficient cognitive capacity
(those aged < 5 were excluded4) and interest in the game (those
aged >11 were excluded) leaving 46 children (28 male and 18
Female; M age = 8.04, SD = 1.93).
2.3 Measures
Our primary dependent variables were interpersonal responses to
Zeno measured through two objective measures: affective
expressions and interpersonal distance. Additional measures
comprised of a self-report questionnaire, completed by
participating children, with help from their parent/carer if
required, and an observer’s questionnaire, completed by
parents/carers.
2.3.1 Objective Measures
Interpersonal distance between the child and the robot over the
duration of the game was recorded, using a Microsoft Kinect
sensor, and mean interpersonal distance during the game
calculated. Participant expressions were recorded throughout the
game and automatically coded for discrete facial expressions:
Neutral, Happy, Sad, Angry, Surprised, Scared, and Disgusted,
using Noldus FaceReader version 5. Mean intensity of the seven
facial expressions across the duration of the game were calculated.
Participants’ game performances (final scores) were also recorded.
FaceReader offers automated coding of expressions at an accuracy
comparable to trained raters of expression [25].
2.3.2 Questionnaires
Participants completed a brief questionnaire on their enjoyment of
the game and their beliefs about the extent to which they thought
that the robot liked them. Enjoyment of playing Simon Says with
Zeno was recorded using a single-item, four-point measure,
ranging from ‘I definitely did not enjoy it’ to ‘I really enjoyed it’.
Participants’ perceptions of the extent to which Zeno liked them
single-item on a thermometer scale, ranging from ‘I do not think
he liked me very much’ to ‘I think he liked me a lot’. They were
also asked if they would like to play the game again. Parents and
4 Additional reasons for excluding children below the age of 5 were
questionable levels of understanding when completing the self-report
questionnaires, and low reliability in FaceReader’s d etection of
expressions in young children.
carers completed a brief questionnaire on their perceptions of
their child’s enjoyment and engagement with the game on single-
item thermometer scales, ranging from ‘Did not enjoy the game at
all’ to ‘Enjoyed the game very much and ‘Not at all engaged’ to
‘Completely engaged’.
2.4 Procedure
The experiment took place in a publicly accessible lab and
prospective participants could view games already underway.
Brief information concerning the experiment was provided to
parents or carers and informed consent was obtained from parents
or carers prior to participation.
During the game, children were free to position themselves
relative to Zeno within a ‘play zone’ boundary marked on the
floor by a mat (to delineate the area in which the system would
correctly detect movements) and could leave the game at their
choosing. The designated play zone was marked by three foam
.62msq mats. The closest edge of the play zone was 1.80m from
the robot and the play zone extended to 3.66m away. These limits
approximate the ‘social distance’ classification [26]. This range
was chosen for 2 reasons i) Participants would likely expect the
game used to occur within social rather than public- or personal-
distance ii) This enabled reliable recordings of movement by the
Kinect sensor. The mean overall distance for the participants from
the robot fell well within social-distance boundaries (2.48m).
At the end of the game, participants completed the self-report
questionnaire, while parents completed the observer’s
questionnaire. Participant-experimenter interaction consistency
was maintained over the two days by using the same experimenter
on all occasions for all tasks.
Interaction with the robot took the form of the widely known
Simon Says game (Figure 2). This game was chosen for several
reasons: children’s familiarity with the game, its uncluttered
structure allows autonomous instruction and feedback delivery by
Zeno, and its record of successful use in a prior field study [27].
The experiment began with autonomous instructions delivered
by Zeno as soon as children stepped into the designated play zone
in front of the Kinect sensor. Zeno introduced the game by saying,
“Hello. Are you ready to play with me? Let's play Simon Says. If I
say Simon Says you must do the action. Otherwise you must keep
still.” The robot would then play ten rounds of the game or play
until the child chose to leave the designated play zone. In each
round, Zeno gave one of three simple action instructions: ‘Wave
your hands’, ‘Put your hands up’ or ‘Jump up and down’. Each
instruction was given either with the prefix of 'Simon says’ or no
prefix.
Figure 2. A child playing Simon Says with Zeno
The OpenNI/Kinect skeleton tracking system was used to
determine if the child had performed the correct action in three
seconds following instruction. For the ‘Wave your hands’ action,
our system monitored the speed of the hands moving. If sufficient
movement for the arms were detected following instruction then
the movement was marked as a wave. For the ‘Jump up and
down’ action the vertical velocity of the head was monitored,
again with a threshold to determine if a jump had taken place.
Finally for the ‘Put your hands up’ action, our system monitored
the positions of the hands relative to the waist. If the hands were
found to be above the waist for more than half of the three
seconds following the instruction then the action was judged to
have been executed. The thresholds for the action detection were
determined by previous trial and error during pilot testing in a
university laboratory. The resulting methods of action detection
were found to be over 98% accurate in our study. In the rare cases
where the child did the correct action and the system judged
incorrectly then the experimenters would step in and say “Sorry,
the robot made a mistake there, you got it right”.
If children followed the action instruction after hearing ‘Simon
says’ the robot would say, “Well done, you got that right”. If the
child remained still when the prefix was not given, Zeno would
congratulate them on their correct action with “Well done, I did
not say Simon Says and you kept still”. Conversely, if the child
did not complete the requested movement when the prefix was
given Zeno would say, “Oh dear, I said Simon Says, you should
have waved your hands”. If they completed the requested
movement in the absence of the prefix, Zeno would inform them
of their mistake with, “Oh dear, I did not say Simon Says, you
should have kept still”. Zeno gave children feedback of a running
total of their score at the end of each round (the number of correct
turns completed).
If the child left the play zone before ten rounds were played,
the robot would say, “Are you going? You can play up to ten
rounds. Stay on the mat to keep playing”. The system would then
wait three seconds before announcing, “Goodbye. Your final
score was (score)”. This short buffer was to prevent the game
ending abruptly if the child accidentally left the play zone for a
few seconds.
At the end of the ten rounds, the robot would say, “All right,
we had ten goes. I had fun playing with you, but it is time for me
to play with someone else now. Goodbye.”
The sole experimental manipulation coincided with Zeno’s
spoken feedback to the children after each turn. In the expressive
robot condition, Zeno responded with appropriate ‘happiness’ or
‘sadness’ expressions, following children’s correct or incorrect
responses. These expressions were prebuilt animations, provided
with the Zeno robot, named ‘victory’ and ‘disappointment’
respectively. These animations were edited to remove gestures so
only facial expression were present. In contrast, in the non-
expressive robot condition, Zeno’s expressions remained in a
neutral state regardless of child performance. Previous work
indicates that children can recognize these facial expression
representations by the Zeno robot with a good degree of accuracy
[28].
3 RESULTS
A preliminary check was run to ensure even distribution of
participants to expressive and non-expressive conditions. There
were 9 female and 16 male participants in the expressive
condition and 9 female and 12 male participants in the non-
expressive condition. A chi square test was run before analysis to
check for even gender distribution across conditions indicates no
significant difference (X2 (1,48) = 2.25, p = .635).
3.1 Objective Measures
Overall, we did not observe any significant main effects of Zeno’s
expressiveness on objective measures of interpersonal distance or
facial expressions between conditions. However, there were
significant interaction effects, when gender was included as a
variable.
There was a significant interaction of experimental condition
and child’s gender on average child’s expressions of happiness
F(1,39) = 4.75, p = .038. While male participants showed greater
average happiness in the expressive robot condition in comparison
to those in the non-expressive condition (19.1%, SE 3.3% versus
5.3%, SE 4.1%), female participants did not differ between
conditions (7.4%, SE 4.3% versus 12.6%, SE 4.6%). Simple
effects tests (with Bonferroni correction) indicated that the
observed differences between conditions for male participants was
significant (p = .012).
A contrasting interaction was found for average expressions of
surprise F(1,39) = 5.16, p = .029. Male participants in the
expressive robot condition showed less surprise than those in the
non-expressive condition (6.1%, SE 3.2% versus 19.6%, SE
4.0%), whereas female participant expressions for surprise did not
differ between conditions (11.9%, SE 4.2% versus 7.1%, SE
4.5%). There were no further significant interactions for any of
the remaining expressions.
There was a near significant interaction for experimental
condition and child’s gender for interpersonal distance F(1,41) =
2.81, p = .10 (Figure 3). Male participants interacting with the
expressive robot tended to stand closer (M = 2.28m, SE .10m)
than did those interacting with the non-expressive robot (M =
2.57m, SE .13m), whereas female participants interacting with the
expressive robot tended to stand further away (M = 2.59m, SE
.14m) than those interacting with the non-expressive robot (M =
2.45m, SE .14m). A follow-up simple effect test indicates that the
difference between conditions for male participants was also near
significant (p = .086).
Figure 3. Mean interpersonal distance during game
Controlling for participant age or success/failure in the game
made no material difference to any of the objective measures
findings.
3.2 Questionnaires
No significant main effects of condition were seen for self-
reported measures or observer reported measures. However, there
were significant gender effects, and significant gender X
condition effects. Gender had a main effect on children’s beliefs
about the extent to which the robot liked them F(1,38) = 5.53, p =
0.03. Female participants reported significantly lower ratings (M
= 3.08, SE .34) than did male participants (M = 4.17, SE .31).
We observed a significant interaction of gender and
experimental condition for participants’ enjoyment in interacting
with Zeno F(1,38) = 4.64, p = .04. Male participants interacting
with the expressive Zeno reported greater enjoyment of the
interaction than those who interacted with the non-expressive
Zeno (M = 3.40, SE .18 versus M = 3.00, SE .23), whereas female
participants interacting with the expressive Zeno reported less
enjoyment than those interacting with the non-expressive Zeno
(M = 3.22, SE .23 versus M = 3.78, SE .23). Simple effects tests
did not indicate that the difference found between conditions were
significant for either male participants (p > .10) or female
participants (p > .10).
Results from the observer reports generated by the participants’
parents or carers showed the same trends as those from the self-
report results but did not show significant main or interaction
effects. Controlling for participant age or success/failure in the
game made no material difference to any of the questionnaire data
findings.
4 DISCUSSION
The results provide new evidence that life-like facial expressions
in humanoid robots can impact on children’s experience and
enjoyment of HRI. Moreover, our results are consistent across
multiple modalities of measurement. The presence of expressions
could be seen to cause differences in approach behaviors, positive
expression, and self-reports of enjoyment. However, the findings
are not universal as boys showed more favorable behaviors and
views towards the expressive robot compared to the non-
expressive robot, whereas girls tended to show the opposite.
Sex differences towards facially expressive robots during HRI
could have profound impact on the design and development of
future robots; it is important to replicate these experimental
conditions and explore these results in more depth in order to
identify why these results arise. At this stage, the mechanisms
underpinning these differences still remain to be determined. We
outline two potential processes that could explain our results.
The current results could be due to children’s same-sex
preferences for friends and playmates typically exhibited at the
ages range tested (ages five to ten) [29]. Zeno is nominally a ‘boy’
robot and expressions may be emphasizing cues seen on the face
to encourage user perceptions of it as a boy. As a result, children
may be acting in accordance with existing preferences for play
partners [30]. If this is the case, it would be anticipated that
replication of the current study with a ‘girl’ robot counterpart
would produce results contrasting with the current findings.
Alternatively, results could be due to the robot’s expressions
emphasizing the existing social situation experienced by the
children. The current study took place in a publically accessible
space, with participants in the company of museum visitors, other
volunteers, and the children’s parents or carer. Results from the
current study could represent children’s behavior towards the
robot based on existing gender driven behavioral attitudes. Girls
may have felt more uncomfortable than boys when in front of
their parents whilst engaging in explorative play [20] with a
strange person (in the form of their perceived proximity to the
experimenter) and an unfamiliar object (the robot). Social cues
from an expressive robot, absent in a neutral robot, may reinforce
these differences through heightening the social nature of the
experiment.
Behavioral gender differences in children engaging in public or
explorative play are well established, and the link between these
gender differences and the influence of direct parents/carers
differential socialization of their children dependent upon the sex
[31,32], is a further established link of developmental study. To
better explore the gender difference observed in our study we
must take into consideration existing observed behavioral patterns
in children engaging in explorative play around their parents.
Replication in a familiar environment away from an audience
including children’s parents may then impact on apparent sex
differences observed in the current HRI study.
The current study is a small-sample field experiment. As with
the nature of field studies, maintaining an exacting control over
experimental conditions is prohibitively difficult. Along with
possible confounds from the public testing space, the primary
experimenter knew the condition each child was assigned to;
despite best efforts in maintaining impartiality, the current study
design cannot rule out potential unconscious experimenter
influence on children’s behaviors. In studies concerning emotion
and expression, potential contagion effects of expression and
emotion [33] could impact on participant’s expressions and
reported emotions. The current results therefore offer a strong
indication of the areas to be further explored under stricter
experimental conditions.
We aim to repeat the current study in a more controlled
experimental environment. Children will complete the same
Simon-says game in the familiar environment of their school, this
time without an audience. Rather than allocation by day to
condition, the study protocol will be modified to randomly
allocate children to conditions, and the study will be conducted by
an experimenter naïve to conditions. Testing at local schools
offers better controls over participant sample demographics as
children can be recruited based on age and having similar
educational and social backgrounds. The environment of this
study also removes any direct influence by the presence of
parents/carers. Thus, a repeat of the current study under stricter
conditions also offers opportunity to further test the proposed
hypotheses for the observed sex differences in enjoyment in
interacting with a facially expressive robot.
We have previously proposed that human-robot bonds could be
analyzed in terms of their similarities to different types of existing
bond with other human, animals, and objects [4]. Our
relationships with robots that are lacking in human-like faces may
have interesting similarities to human-animal bonds which can be
simpler than those with other people—expectations are clearer,
demands are lower, and loyalty is less prone to change. Robots
with more human-like faces and behavior, on the other hand, may
prompt responses from users that include more of the social
complexities of human-human interaction. Thus, aspects of
appearance that indicate gender can become more important,
subtleties of facial and vocal expression may be subjected to
greater scrutiny and interpretation. Overall, as we progress
towards more realistic human-like robots we should bear in mind
that whilst the potential is there for a richer expressive
vocabulary, the bar may also be higher for getting the
communication right.
5 CONCLUSION
This paper offers further steps towards developing a theoretical
understanding of symbiotic interactions between humans and
robots. The production of emulated emotional communication
through facial expression by robots is identified as a central factor
in shaping human attitudes and behaviors during HRI. Results
from both self-repot and objective measures of behavior point
towards possible sex differences in responses to facially
expressive robots; follow-up work to examine these is identified.
These findings highlight important considerations to be made in
the future development of a socially engaging robot.
6 ACKNOWLEDGMENTS
This work is supported by the European Union Seventh
Framework Programme (FP7-ICT-2013-10) under grant
agreement no. 611971. We wish to acknowledge the contribution
of all project partners to the ideas investigated in this study.
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