Feeding 35generalized source of motivation. The magnitude of this
generalized drive was determined by summing all unsatisfied
physiological needs; any perturbation of homeostasis re-
sulted in an increase in a common source of behavioral
energy. Empirically, Hull’s generalized drive theory failed
because separate sources of motivation most often do not
generalize. Thirsty animals tend not to eat, and frightened an-
imals forsake eating and drinking. It also became clear that
learning was at least as important a source of motivation as
was homeostatic need. Often we eat because the situation
tells us to. Past experience informs us that this is the proper
time or place to eat.
To account for empirical challenges to Hull’s (1943) gen-
eralized drive principle, incentive motivational theories sug-
gested that two types of motivation could be activated by
Pavlovian means. Conditional stimuli (CSs; see also chapter
by Miller and Grace in this volume) that predicted desirable
outcomes, such as the occurrence of food or the absence of
pain, activated an appetitive motivational system; CSs that
predicted undesirable outcomes activated an aversive moti-
vational system. Anything that activated the appetitive
system stimulated appetitively related behaviors and sup-
pressed aversively motivated behaviors. The opposite was
true for stimuli that excited the aversive system. This expla-
nation was an improvement because learning, in the form of
Pavlovian conditioning, could provide a source of motiva-
tion. Additionally, the notion of two systems provides more
selectivity than Hull’s (1943) generalized drive principle.
The problem with this view is that it simply does not go far
enough. As we shall see, cues associated with food do not
simply cause an enhancement of food-associated behaviors.
Rather, the cue signals that a particular class of food-related
behavior is appropriate and that others are inappropriate. On
the aversive side, fear and pain are organized in an antago-
nistic manner. Because fear inhibits pain-related behavior,
how can fear, pain, and hunger simultaneously hold mutu-
ally reciprocal relationships? As we shall see, organizing
these systems around their function makes sense of the
relationships between classes of behavior. By combining
function, antecedent cause, and behavioral effect into our
definition of a motivational system, we are also successful
in limiting the number of motivational systems that can be
generated.
What Is Motivation?
The idea that we eat because we are hungry seems intuitively
obvious. Both lay and several formal descriptions of behavior
suggest that hunger is a response to food deprivation and
that hunger engenders behaviors that correct the depletion. In
this way, factors such as body weight or caloric intake are
regulated about some set point. This homeostatic view has
directed much research, and in many situations body weight
appears to be held relatively constant. However, caloric in-
take and body weight are influenced by many variables, such
as the type and quantity of food available, activity levels, sea-
son, and palatability.
Bolles (1980) has noted that if the experimenter holds sev-
eral of these variables constant, the others will come to rest at
some set of values. Thus, an observed set point or consistency
may be an artifact of relatively static conditions. Additionally,
because all these factors are variables in an equation, the ex-
perimenter is free to solve for any of them as a function of the
others. In effect, body weight may appear to be regulated
simply because you have kept the other variables constant.
Alternatively, if you held body weight and the other variables
constant, you could solve the equation for palatability and
thereby conclude that palatability is regulated. From a func-
tional perspective what is critical is that an animal ingests
the necessary substances in sufficient quantities; how that is
accomplished does not matter. Natural selection favors any
scheme that satisfies the goal. In this regard, regulating
palatability may make a lot of sense—and is a topic to which
we will return later.
This idea is a general point about motivational terminol-
ogy and motivational systems. We have to recognize that mo-
tivation is organized about the evolutionary requirement that
the system needs to solve (see also chapter by Capaldi in this
volume). Hunger, sexual arousal, and fear really refer to a be-
havioral organization that is imposed on an organism when
the environment demands that a particular problem be
solved. Motivation is no longer conceived of as a blind force
that impels an animal forward. It is something that gives
form, structure, and meaning to behavior, and it is from this
vantage that we will begin to analyze some exemplars of spe-
cific motivational systems.FEEDINGThe vast majority of animal species gain the nutrients they re-
quire to survive and grow by harvesting them from other liv-
ing creatures. This strategy requires that animals have means
to detect and capture these nutrients and that the behavioral
systems governing these actions be sensitive to the availabil-
ity of required nutrients and the physiological demands of the
animal. Psychological examination of these requirements
typically focuses on either the factors that initiate the behav-
ior or the response topography of food-gathering behavior.
We examine each of these aspects in turn.