Instrumental Responding 387preference in concurrent chains (Nevin, 1979). Nevin and
Grace (2000) proposed an extension of behavioral momen-
tum theory in which behavioral mass (measured as resistance
to change) and value (measured as preference in concurrent
chains) are different expressions of a single construct repre-
senting the reinforcement history signaled by a particular
stimulus. Their model describes how stimulus-reinforcer
(i.e., Pavlovian) contingencies determine the strength of an
instrumental response, measured as resistance to change.
Thus, it complements Herrnstein’s (1970) quantitative law of
effect, which describes how response strength measured as
response rate depends on response-reinforcer (i.e., instru-
mental) contingencies.
Ecological-Economic Analyses of
Instrumental Conditioning
A third approach towards the study of instrumental behavior
was inspired by criticisms of the apparent circularity of the
law of effect: If a reinforcer is identified solely through its
effects on behavior, then there is no way to predict in advance
what outcomes will serve as reinforcers (Postman, 1947).
Meehl (1950) suggested that this difficulty could be overcome
if reinforcers were transsituational; an outcome identified as a
reinforcer in one situation should also act as a reinforcer in
other situations. However, Premack (1965) demonstrated ex-
perimentally that transsituationality could be violated. Cen-
tral to Premack’s analysis is the identification of the reinforcer
with the consummatory response, and the importance of ob-
taining a free-operant baseline measure of allocation among
different responses. His results led to several important recon-
ceptualizations of instrumental behavior, which emphasize
the wider ecological or economic context of reinforcement in
which responding—both instrumental (e.g., lever press) and
contingent (e.g., eating)—occurs. According to this view, re-
inforcement is considered to be a molar adaptation to the con-
straints imposed by the instrumental contingency, rather than
a molecular strengthening process as implied by the law of ef-
fect. Two examples of such reconceptualizations are behavior
regulation theory and behavioral economics.
Behavior Regulation
Timberlake and Allison (1974) noted that the increase in
responding associated with reinforcement occurred only if
the instrumental contingency required that the animal per-
form more of the instrumental response in order to restore the
level of the contingent (consummatory) response to baseline
levels. For example, consider a situation in which a rat is
allowed free access to a running wheel and drinking tube
during baseline. After recording the time allocated to these
activities when both were freely available, a contingency is
imposed such that running and drinking must occur in a fixed
proportion (e.g., 30 s of running gives access to a brief period
of drinking). If the rat continued to perform both responses at
baseline levels, it would spend far less time drinking—a con-
dition Timberlake and Allison (1974) termed response depri-
vation. Because of the obvious physiological importance of
water intake, the solution is for the rat to increase its rate of
wheel running so as to maintain, as far as possible, its base-
line level of drinking. Thus, reinforcement is viewed as an
adaptive response to environmental constraint.
According to behavior regulation theory (Timberlake,
1984), there is an ideal combination of activities in any given
situation, which can be assessed by an organism’s baseline al-
location of time across all possible responses. The allocation
defines aset pointin a behavior space. The determiners of set
points may be complex and depend on the feeding ecology of
the particular species (e.g., Collier, Hirsch, & Hamlin, 1972).
The effect of the instrumental contingency is to constrain the
possible allocations in the behavior space. For example, the
reciprocal ratio contingency between running and drinking
previously described implies that the locus of possible alloca-
tions is a straight line in the two-dimensional behavior space
(i.e., running and drinking). If the set point is no longer possi-
ble under the contingency, the organism adjusts its behavior
so as to minimize the distance between obtained allocation
and the set point. Similar regulatory theories have been pro-
posed by Allison, Miller, and Wozny (1979), Staddon (1979),
and Rachlin and Burkhard (1978). Although regulatory theo-
ries have been very successful at describing instrumental
performance at the molar level, they have proven somewhat
controversial. For example, the critical role of deviations from
the set point seems to imply that organisms are able to keep
track of potentially thousands of different responses made
during the session, and able to adjust their allocation accord-
ingly. Opponents of regulatory theories (e.g., see commen-
taries following Timberlake, 1984) claim this is unlikely and
that the effects of reinforcement are better understood at a
more molecular level. Perhaps the most likely outcome of this
debate is that molar and molecular accounts of instrumental
behavior will prove complementary, not contradictory.Behavioral EconomicsAn alternative interpretation of set points is that they repre-
sent the combination of activities with the highest subjective
value or utility to the organism (e.g., so-called bliss points).
One of the fundamental assumptions of economic choice
theory is that humans maximize utility when allocating their