
et al.,2015), as parts of the state of the world. For
communication to be successful, the audience must
be able to reconstruct the original communicative
goal: their decoded transformation of the world,
Da:W→W
, must be such that
Da(w)≈Gs(w)
.
1
Communicative goals shape and constrain a
speaker’s production choices: different utterance
types typically correspond to different goals. The
communicative goal of a referring utterance (“The
black and white cat”), for example, is a state of
the world where the audience is able to identify an
entity in context. The transformation
Da
required
to achieve
w∗
is a change of attention by the
audience. Statements (“The Sun is a star”) are
typically used when the purpose of an interaction is
pure information transmission—e.g., when giving
a scientific talk. In this case, the communicative
goal is a state of the world in which the audience
holds new beliefs, the ones intended by the speaker.
Da
is a transformation of the belief state of the
audience, and the communicative goal is achieved
when
Da(w)≈Gs(w)= w∗
. All utterance types—
e.g., questions, directives, and performatives—can
be seen as strategies to achieve communicative
goals. The same utterance type, and even the same
utterance, can fulfil different goals: a blatantly
false statement (“It never rains in Amsterdam”)
can be used for comedic effect rather than for
conveying facts. For simplicity, in the rest of this
paper, we describe utterances as having a single
communicative goal. Often, however, different
goals are associated with the same utterance at
the same time: a teacher can use a question (“Are
you sure this is the right answer?”) to inform
their student that their answer is incorrect, while
showing a positive attitude towards them—thereby
striving for both epistemic and social utility. Our
framework naturally generalises over such cases;
when multiple communicative goals are involved,
states of the world can be designed accordingly.2
2.2 Production Costs
Given the current and the intended future state
of the world,
w
and
w∗=Gs(w)
, a speaker en-
codes the communicative goal
Gs(w)
into a mental
representation of the intended state of the world:
1
We sometimes refer to
Da(w)
and
Gs(w)
as
Da
and
Gs
,
as in our formulation these functions are always applied to the
current state of the world w.
2
To account for epistemic and social utility, for example,
states of the world can be defined to include the audience
belief state as well as their emotional state.
Es(Gs(w))= e
. To use a slightly different vocabu-
lary, this is the speaker’s conception of the intended
environment state. The speaker then realises
e
as
an utterance
r
which is presented to the audience:
Rs(e) = r
. Two types of cost are associated with
the encoding and realisation processes. Because
the encoding process is inevitably lossy—mental
representations are compressed representations of
the real state of the world—the speaker makes an
effort to reduce information loss; we refer to this
as the
encoding cost CE
. The cost associated with
executing a bit of behaviour
r
meant to be per-
ceived by the audience (e.g., speaking, writing, or
typing) is the
realisation cost CR
. Both costs af-
fect the decision making process of speakers. In
addition, the speaker is influenced by the expected
comprehension costs of the audience.
2.3 Comprehension Costs
The speaker’s communicative goal
Gs(w)
is not
observable by comprehenders. Given a state of the
world
w
and the speaker’s behaviour
r
, compre-
henders process
r
into a reconstruction of the orig-
inal mental representation,
Pa(r) = e0≈e
, from
which they decode the speaker’s communicative
goal:
Da(e0)= w0≈Gs(w)
. Two types of cost are
associated with the comprehension of an utterance.
Speaker and comprehender are different individu-
als and therefore have different ways of encoding
communicative goals into messages (Connell and
Lynott,2014). In the absence of a perfect model of
the speaker’s encoding mechanism, reconstructing
e
is a lossy and effortful process; we denote the
corresponding cost as
processing cost
,
CP
. The
second cost results from interpreting
e0
in context—
i.e., decoding from
e
the state of the world intended
by the speaker. In other words, this is the effort
required to ground the message in the environment.
We refer to it as the
decoding cost CD
. It is impor-
tant to note that although processing and decoding
costs are on the side of comprehenders, speakers
estimate them and take them into account when
making production decisions.
2.4 Utility
In what ways is the decision making process
of speakers affected by these costs? Speakers
are thought to be driven by efficiency concerns
(Zipf,1949;Jaeger and Tily,2011): they strive
to minimise the collaborative effort required
to achieve their communicative goals (Clark
and Wilkes-Gibbs,1986;Clark and Schaefer,