38
Psychologically Plausible Methods
for Character Behavior Design
Phil Carlisle

38.1 Introduction
38.2 Perception and
Abstractions
38.3 Developmental
Psychology and the
Teleological Stance
38.4 Attribution Theory

38.5 Problem of Characters as
Tokens38.6 Practical Application of
Psychology in Character
Behavior Design
38.7 Conclusion
References

38.1 Introduction
If you have ever worked with 3D game character artists, you’ll have heard them talk obsessively about silhouette and how a character reads. They do this because they understandthat for video games especially, there is the issue of viewpoint. As players move arounda world in 3D, they can change their viewpoint with respect to the character considerably, changing distance and angle. Artists want their work to be understood by players, sothey try and maximize that understanding by taking care to make a “readable” character.
A readable character is one that can be viewed from different directions and at different
distances and still remain recognizable.
As behavior designers, we have very similar issues to deal with; we have to make our
behaviors “readable” by players in different situations and have our players recognize andunderstand the behavior. The aim of this chapter is to encourage you to think about this
aspect of your behavior design and to ground you in a sample of the psychological aspects
that come into play from the player’s perspective. Using a number of studies performedby groups of undergraduate game design students as examples, the intention is to raiseawareness rather than to be academically rigorous. As psychology is a very complex field,
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it is recommended that these studies not be taken at face value, but instead should be
used to consider how psychology might inform the development of character behavior, byadding methods of evaluation and ways of thinking about player understanding.

38.2 Perception and Abstractions
When working on character behaviors, one of the principal tools at our disposal is theuse of animation and movement. By the use of movement, we can convey to the player
different behavioral meanings based on the nature of the movement, but how do players
actually perceive movement?
In the 1970s, psychologist Gunnar Johansson ran a series of studies involving pointlight animations [Johansson 73]. He took a number of his students and dressed themcompletely in black clothing. He then attached small reflective markers to various joints
and got the students to act out particular motions while filming them such that only themarkers were visible. The resulting films bear a striking resemblance to modern motioncapture data; an example of his work can be viewed online [Maas 11].
Johansson was interested in our perception of motion. What he found was that fromthese relatively sparse data, his test subjects were able to describe what types of objects
were involved and in some circumstances what gender the subject was. He varied theangles of the motions and otherwise manipulated the viewpoints in order to study
where perception broke down. Interestingly, when he tested occluding some of themarkers, the participants were able to perceive the motion until a relatively high percentage of the markers were obscured. Similarly, he altered the orientation and foundthat perception broke down only in fairly extreme circumstances (e.g., when the subject
was turned upside down).
What this perception study showed is that humans (and presumably other animals)
have a capability to take relatively sparse or abstract motions and parse them into familiar
mental models of motion. We can think of this process as shape fitting, taking motion, andmapping it to known patterns of movement and then attributing that movement to somepreviously learned behavior.
It seems likely that this capability is actually a very useful one in terms of neuroevolution; for instance, it might be used to spot a prey animal from a distance by its movement
pattern or for selecting a weaker prey animal from a herd.
So what does this all have to do with character AI? The basic premise proposed here isthat when we create behavior, we should take care that the motion the behavior exhibits
matches motions that can then be shape fitted by players into mental models that allow
them to associate the motion of the character with learned patterns.
Just to highlight this point, consider a character behavior in the abstract for a moment.
If we want to portray an aggressive character, what styles of movement would we most
likely select to show that aggression? One suggestion would be that short, fast movements
would be better for showing aggression than long, slow movements. Why would this be
the case? One possibility is that we have evolved a capability to detect sharp fast movements as aggressive in response to the types of movements we might have seen from predator animals. For example, think of a cat trying to attack a mouse; how does the movementchange as the cat approaches? Think of the explosive force used for when the cat “pounces”
on the mouse.
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This sharp fast movement happens in other animals too. Fish, insects, and snakes all
exhibit striking motions. Most 3D animators know this instinctively, but this notion of
movement speed was actually outlined much earlier in traditional stop-motion animation
as the principle of force described well in The Illusion of Life [Thomas 81] and essentiallydeals with the biomechanical effort involved in specific motions. The act of stalking in
the cat movement might be seen as aggressive by a viewer who had previously seen the cathunting. From the perspective of the prey, however, the slower movement is less threatening, which suggests that viewpoint has a large impact on the way movement is perceived
and is informed by the previous experience of the viewer.
Another interesting aspect to come out of Johansson’s work is the fact that somemotions suggest particular feelings or relationships. In one study, the motions of two
point lights were arranged such that a synchronized circular motion was used, as shownin Figure 38.1. Where the point lights gained close proximity and mirrored the motion ofeach other, this was perceived by viewers as having affinity between the points. Similarly,
points that moved in contrary motions were perceived to have animosity.
In order to cement this notion of perception of motion in the abstract, let us present
another study that will also ease us into another aspect of psychology.
In 1944, Fritz Heider and Marianne Simmel presented a paper called An Experimental
Study of Apparent Behavior [Heider 44]. In it, they describe a study where they presented
viewers with a film showing a number of abstract shapes (lines, circles, triangles) with corresponding motions filmed using stop-motion animation. They asked the participants to
explain what they saw in the film. In almost all of the cases, the participants explained thefilm in terms of what the “characters” were doing. They described the scene with explanations such as the following: “A man has planned to meet a girl and the girl comes along

A

A
B
B

(a)

(b)

Figure 38.1
An example of complementary (a) and contrary (b) motions used in the study.

Psychologically Plausible Methods for Character Behavior Design473

with another man. The first man tells the second to go; the second tells the first, and heshakes his head. Then the two men have a fight, and the girl starts to go into the room to
get out of the way and hesitates and finally goes in,” clearly showing that they had generated an internal narrative to explain what they saw. Indeed it is hard not to see the samefootage [Smith-Welch 08] and offer a similar explanation.
The fact that viewers of a film are able to take abstract shapes with motion and construct an internal narrative should really drive home the point that we need to consider
the qualities of motion inherent in behavior when we are trying to design characters. Onceagain, this points to the ability to map relatively abstract movements to known behavioral patterns. As character designers, this should give us some ideas on how to shape theunderstanding of a character’s behavior from a player’s perspective. Altering the qualities
of movement and the viewpoint they are seen from could allow us to alter how a characteris perceived to our advantage, much like an author setting up a reader’s mental view of acharacter before getting them to perform an action.

38.3 Developmental Psychology and the Teleological Stance
The process by which we are able to translate movements into mental models and the specifics of that process is an ongoing area of research. The term “shape fitting” relates it to theprocesses used in computer vision, but this is merely a shorthand description for a complex
set of processes. It is quite useful to review this aspect within the literature on developmental psychology. A useful starting point is the Blackwell Handbook of Childhood Cognitive
Development [Goshwani 10]. In particular, the chapter by Gergely [Gergely 10] describes
an early infant process that allows inference about actions through a naive theory of rational action, basically a process of mapping movements to intended actions that is referred
to as the teleological stance. As infants grow, they develop more capabilities in terms of
the ability to read a given motion and infer its intention, so developers of games for infants
should probably consider the stages of development their target audience has reached.
This ability to read the intentions of others uses gaze direction as a key component.
Children follow the gaze direction of their parents in order to learn what is important
for them to be aware of [Goshwani 10]. Over time, they develop the capacity to model
the knowledge of others, using gaze as a significant aspect of that model. For example, asa child grows, they begin to understand that another person might have knowledge of aparticular object or they might not. They learn that if the other person has actually seen
the object (i.e., directed their gaze toward it), then they know about the object. Thus, theylearn that the stronger the gaze, the more important the object.
This aspect of gaze direction is important for us as behavior designers because it is oneof the primary tools we can use to direct the player toward the correct reading of behavioral
intent. The player can read the behavior quite differently if they see that the character is notaware of an interacting object; in other words, if we direct the gaze of the character awayfrom an object, we can alter how the player reads any corresponding interaction with it.
Consider Figure 38.2 as an example. In the leftmost pair, we have no way of determining the gaze direction. If you focus on that pair exclusively, you are unlikely to ascribe any
intention to the objects. In the middle pair, the gaze is pointing inward; this is likely toindicate antagonism or aggression, unless we have previously seen context in which thepair had displayed positive affect, in which case we might view them as being in a friendly
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Figure 38.2
A simple example of how perceived gaze direction can influence our reading of a situation.

chat or a conspiratorial congress. The final pair we might well view as being unaware ofeach other, unless we had seen previous context that suggested they knew each other,might then change the reading of the pair to one of a social rebuff. However, if you interpret the “characters” in the figure, you should be aware that the only condition that ischanging is the perceived gaze direction of each pair. This effect is happening even withina very abstract scene but can be particularly strong when seen in motion.

38.4 Attribution Theory
The work by Heider, Simmel, and others eventually led to the field of psychology known as
attribution theory [Kelley 67]. For behavior design, this concept of attribution is an important one. The basic idea is to consider how we attribute the intention of an action. We
commonly think of actions as internal (i.e., the character intended to do it) or external (itwas done to the character or caused by other factors). As behavior designers, we should be
able to influence a player’s view of the behavior such that they ascribe the action as being
intentional by the character or unintentionally happening to the character. For instance,
we might hint at the incompetence of a character, which leads the player to likely ascribe
positive actions as being external; in other words, if something good happens, it happens
by chance and not skill.
An interesting aspect of the Heider/Simmel study was that the narrative given to
explain the scene changed when the scene was presented in reverse; each viewer waspresented with both a “forward” normal viewing of the film and a “reverse” viewing, inwhich the film was played backward. In many areas, the narrative broke down because
the context of the motions was changed. For instance, the larger triangle in the forward
presentation is seen as dominant or bullying through the remaining narrative due to an
early segment where it performs a fast motion toward the smaller triangle and pushes it ashort distance; however, in the reverse presentation, the first aggressive “push” movement
is not seen, and thus the narrative of dominance is changed. This implies that in order for aviewer to correctly attribute a behavior, that behavior must be seen within a context that is
temporally plausible; in other words, the order in which we present behavioral movements
is important. It can be useful to recreate this aspect of the study with your own section ofa film, playing the film in the reverse and thinking about how the behaviors are affected.
Are some behaviors still understandable? Do you read the motivations of characters differently? What is causing that effect?
We can consider a simple illustration of this effect. Think of a cat trying to catch amouse; imagine the cat slowly creeping up behind the mouse, keeping close to the ground,slowly moving up, and then suddenly moving with rapid force during a “pounce” behavior.
Psychologically Plausible Methods for Character Behavior Design475

Now consider the behavior with the order of actions reversed. Instead of a creep-thenpounce, think how a pounce-then-creep would look.
The concept of attribution of behavior is an interesting one, in that it poses questions
about awareness and intent. If we are constantly evaluating the motions of others in a bid
to understand their intention in terms of our own beliefs about them, then perhaps we
can start to manipulate that evaluation in order to achieve a desired effect. One of the key
aspects we need to think about is the attribution of intent. Does a behavior we want toportray show intention for the character or not? For example, imagine two characters anda ball. Character A throws the ball at character B; only character B is facing away from A
and is not aware of the ball, so consequently B gets hit on the head. In this simple scene,
we can actually influence different readings of the inference of the viewer.
Consider the case where we have previously shown character A throwing balls at othercharacters. In this case, it is likely a viewer would ascribe the ball-throwing behavior as
an intentional act. The viewer will have learned that character A throws balls and thus
would see the ball-throwing action as fitting in with what they know about A’s likelihood
to intentionally throw a ball at another character.
Now imagine the same scene, but instead we have previously seen character B being hit
by several other characters throwing balls. In this case, we are more likely to attribute thebehavior to be a lack of awareness by B than any particular malice by character A.
The fact that a simple ball-throwing behavior could be understood differently depending on our understanding of the attributed qualities of the participants is an important
point to note in any character behavior design. We can begin to manipulate a player’s
reading of a given character by reinforcing their mental attribution processes if we can set
up scenes in a manner that takes into account the teleological stance of the behavior andreinforces a given view of the character.

38.5 Problem of Characters as Tokens
Before we get on to discussing the practical application of these psychological principles
in terms of behavior design, it is worth noting an issue that most character designers face.
A number of years ago at the University of Bolton, we ran an (unpublished) undergraduate study into the methods that players use to select companion characters in games. In
this particular study, the test involved choosing companion characters for further gameplay from a roster of characters, which were presented in different ways. The charactersthemselves were portrayed using different clothing and gender options, with the aim of
the study being to understand more about the effect of the communication mode (text,
cinematic) on the choice. One of the more surprising results from the study came fromthe qualitative interviews conducted with participants after they had made the characterchoice. Although this was not the aim of the study, we found that most players chose characters not based on their narrative delivery method, but instead on a measure of perceived
usefulness in the game. In essence, they had ignored the “character” of the choice and had
instead chosen based on the utility value. This was not a choice that was based on evidence(no statistics were provided for comparing the characters) but rather on a simple perceived
value, that is, “they looked like they would be good in a fight.”
The fact that players understand that characters in games are also part of the gamemech­anics suggests an underlying problem in terms of making more believable characters,
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in that for all our best efforts, we may never be able to break that relationship in the player’s
mind. It might well be that games with more focus on the story and setting of the game
could convince players to choose characters for their narrative value; however, it is worth
considering what aspects of a character players are likely to base their choices on at thedesign stage. Is the player presented with information on the character that will explicitly
state their utility? How can we identify players who make choices based on other aspects ofcharacter and perhaps alter our offering of choices based on that identification?

38.6 Practical Application of Psychology in
Character Behavior Design
In another study, an undergraduate group was interested in how the perception of a character shaped a player’s view of their own performance. They conducted an experiment inwhich test subjects played an online game against a series of three “bots” where each bot
was represented with a different narrative designed to relate to easy, medium, and hard
opponent difficulty levels. What the test subjects were not aware of was that each of thethree “bots” was actually being controlled by a human player, who was deliberately playing to a script that essentially reduced the combat to the same level of difficulty for eachplay session. What the study found was that the players were more likely to score theirown performance highly if they managed to win against the “hard” enemy. Similarly,
they sought to attribute a loss to the difficulty level of the opponent. This study essentiallyshowed that players can be influenced to feel differently about their own performance
depending on the narrative context of their actions. If the context is one where the player
feels they are likely to lose, then they ascribe a loss to the game, whereas if they win they
are more likely to ascribe the win to their own mastery or skill, no matter of the actual
difficulty involved in the action.
In practical terms, this means we can start to encourage players to feel more skillful by
presenting them with challenges described as hard, but which are biased toward them winning. This can be as subtle as describing enemies in a manner that inflates their ability withrespect to other agents, for example, calling an agent a major instead of a soldier. We can also
influence a player’s perception by making sure they see a character fail at a task; from thatpoint, it is more likely they will perceive that character as more likely to fail other tasks.
Similarly, we can influence a player’s perception of character interactions within agroup by carefully manipulating their motions. Considering the case where a player iscontrolling a group of role-playing characters, we can make two characters within thatgroup appear more friendly toward each other by simply biasing their movements to be
more complementary (i.e., often aligned in the same direction, being in close proximity).
We can increase the perception of animosity by making sure that the characters avoid
close proximity and bias their movements such that they never mirror each other.

38.7 Conclusion
When designing the behaviors of characters in a game, it can be too easy to focus on theindividual behavior and lose sight of the overall readability of the character. If we want tocreate characters that players understand and believe in, we must embrace the aspects ofpsychology that inform how a player perceives them.
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In this chapter, we have tried to offer some simple psychological theories that you caninvestigate and use to your advantage when considering your initial character designs.
Hopefully, this chapter has convinced you that many of the psychological tools you can
use to affect players’ perceptions function at a high level of abstraction and can often be
implemented with very minimal development cost.
Psychology itself is an ongoing area of research and we are only just beginning to investigate the psychology involved in games. From a practical point of view, many of the psychological effects described in this chapter are quite subtle or do not apply universally toall player types. Even with these caveats, it is well worth the time for a designer to familiarize themselves with psychology and its measurement as part of the design toolset. We haveseen many of our students gain design roles within the game industry due in large partto their experience working on an aspect of game psychology that is under researched,offering a unique experience and perspective that many employers seem to find valuable.

References[Gergely 10] Gergely, G. 2010. Kinds of agents: The origins of understanding instrumental and communicative agency. In Blackwell Handbook of Childhood Cognitive
Development (2nd edn.), ed. U. Goshwani, pp. 76–105. Oxford, U.K.: Blackwell.
[Goshwani 10] Goshwani, U. ed. 2010. Blackwell Handbook of Childhood Cognitive
Development (2nd edn.). Oxford, U.K.: Blackwell Publishers.
[Heider 44] Heider, F. and Simmel, M. 1944. An experimental study of apparent behavior.
The American Journal of Psychology (University of Illinois Press) 57(2): 243–259.
[Johansson 73] Johansson, G. 1973. Visual perception of biological motion and a model for
its analysis. Perception and Psychophysics (Springer-Verlag) 14(2): 201–211.
[Kelley 67] Kelley, H. H. 1967. Attribution theory in social psychology. Nebraska Symposium
on Motivation (University of Nebraska Press) 15: 192–238.
[Maas 11] Maas, J. 2011. Johansson: Motion perception. https://www.youtube.com/
watch?v=1F5ICP9SYLU (accessed June 1, 2014).
[Smith-Welch 08] Smith-Welch, M. Youtube.com. https://www.youtube.com/
watch?v=sZBKer6PMtM (accessed June 2, 2014).
[Thomas 81] Thomas, F. and Johnson, O. 1981. The Illusion of Life: Disney Animation.
New York: Hyperion.

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