34 Motivation
excess. The behavior is variable; learning and genetics alone
cannot account for all of the behavior. Consequently, we in-
voke hunger, a motivational construct, to capture the remain-
der of the variance. For example, a theory of feeding might
suggest that genes determine what we eat, and memory of
past experiences tells us where to forage. Hunger activates
foraging behavior and determines when we eat.
Any complete theory of behavior can be viewed as an
analysis of variance with learning, genetics, and motivation
configured to explain behavior as best as possible. Accord-
ingly, any concept of motivation will be defined partly by the
particular matrix of learning and genetics within which it
is embedded. As a consequence, as our ideas about learn-
ing or behavior genetics change, so must our ideas about mo-
tivation. Indeed, our concept of motivation is dramatically
different from the generalized need-based drive and the recip-
rocally inhibitory incentive motivation theories that charac-
terized the earlier and later parts of the twentieth century.
Although those theories have been very influential to the ideas
developed here, we do not review them in this chapter. Instead,
the reader is urged to consult Bolles (1975) for, arguably, the
most authoritative review of those earlier approaches.
The analogy to analysis of variance highlights another
important aspect of motivation, learning, and genetics. It is
incorrect to think of these factors as independent “main” ef-
fects. Most of the variance in behavior is accounted for by
the interactions between these factors. For example, research
into constraints on learning demonstrated that even basic
learning processes, such as Pavlovian and operant condi-
tioning, have powerful and specific genetic influences that
determine what information is readily acquired and what in-
formation is virtually impossible to assimilate (Seligman &
Hager, 1972). Conversely, recent research on the neurobiol-
ogy of learning suggests that the mechanism by which
we encode information involves gene expression and that
learning influences which genes are expressed (Bolhuis,
Hetebrij, Den Boer-Visser, De Groot, & Zijlstra, 2001;
Rosen, Fanselow, Young, Sitcoske, & Maren, 1998). Thus,
learning and genetic factors affect behavior, and each other.
We raise these examples to foreshadow that our explanation
of motivation will also primarily reside within a description
of these interactions.
A Definitional Framework for Motivation
The framework we advocate for understanding motivation is
calledfunctional behavior systems(Timberlake & Fanselow,
1994). Two aspects to defining a functional behavior system
are common to the definition of any motivational construct:
environmental cause and behavioral effect. These are the
necessary components to any empirically tractable definition
of an intervening variable. A functional behavior system must
be anchored to objectively defined environmental causes.
These are the antecedent conditions for activation of the be-
havior system and the things an experimenter can manipulate
to turn on the system. The functional behavioral system must
also have objectively observable behavioral consequences of
activating the system.
Functional behavior systems have a third component to
the definition that is unique to this approach. The naturally
occurring problem that the system has evolved to solve is a
component of the definition. This component is critical be-
cause modern views of motivation see behavior as being
tightly organized around these functional concerns. Environ-
mental causes and behavioral effects are grouped together
about the naturally occurring problems that the successful
organism is built to solve. This problem-oriented view fo-
cuses the analysis on how multiple behaviors relate to each
other in a manner that is coordinated to solve a problem.
Hunger and feeding are understood as a means to ensure that
the necessary nutrients and calories are harvested from the
environment. Hunger and feeding cannot be understood sim-
ply in terms of the amount eaten or the number of lever
presses a rat makes for a food pellet. Nor can it be under-
stood simply in terms of the postingestional consequences of
food that satisfy some homeostatic requirement. Rather, for
each species, food-related motivation is tailored to the niche
that the animal occupies. An animal must search for appro-
priate items, procure them, prepare them, consume them,
and digest them. The sequence is all-important, and a failure
anywhere along the chain means that the organism fails to
meet critical environmental demands. Different behaviors
are necessary for each step; different rules apply to each
component; and the analysis of behavior is a description of
the path. A theory of motivation must capture the structure
of this organization.Impetus for the Development of Functional
Behavior SystemsA metatheoretical concern in approaching motivation is how
many separate motivations does a complex organism have?
Freud (1915) voiced one extreme when he suggested that all
motivation stemmed from a single unconscious source of
energy. The instinct theorists of the early part of the twentieth
century voiced another when they linked instincts directly to
behaviors (e.g., Lorenz, 1937). Eventually, instinct theory
crushed itself because there were no constraints on the num-
ber of instincts that could be generated. To avoid such prob-
lems, Hull (1943), like Freud (1915), argued for a single