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Research Article
Learning Concepts and
Categories
Is Spacing the ‘‘Enemy of Induction’’?
Nate Kornell and Robert A. Bjork
University of California, Los Angeles
ABSTRACT—Inductive learning—that is, learning a new
concept or category by observing exemplars—happens
constantly, for example, when a baby learns a new word or
a doctor classifies x-rays. What influence does the spacing
of exemplars have on induction? Compared with massing,
spacing enhances long-term recall, but we expected spac-
ing to hamper induction by making the commonalities that
define a concept or category less apparent. We asked
participants to study multiple paintings by different ar-
tists, with a given artist’s paintings presented consecutive-
ly (massed) or interleaved with other artists’ paintings
(spaced). We then tested induction by asking participants
to indicate which studied artist (Experiments 1a and 1b) or
whether any studied artist (Experiment 2) painted each of
a series of new paintings. Surprisingly, induction profited
from spacing, even though massing apparently created a
sense of fluent learning: Participants rated massing as
more effective than spacing, even after their own test
performance had demonstrated the opposite.
The spacing effect refers to the nearly ubiquitous finding that
items studied once and revisited after a delay are recalled better
in the long term than are items studied repeatedly with no in-
tervening delay (e.g., Cepeda, Pashler, Vul, Wixted, & Rohrer,
2006; Dempster, 1996; Glenberg, 1979; Hintzman, 1974;
Melton, 1970). The positive effects of spacing on long-term re-
call are large and robust, and have been demonstrated in a va-
riety of domains, such as conditioning (even in animals as simple
as Aplysia; see Carew, Pinsker, & Kandel, 1972), verbal learning
(e.g., Bahrick, Bahrick, Bahrick, & Bahrick, 1993; Ebbinghaus,
1885/1964), motor learning (e.g., Shea & Morgan, 1979), and
learning of educational materials (e.g., Bjork, 1979; Dempster,
1988).
In many everyday and educational contexts, however, what is
important to learn and remember transcends specific episodes,
instances, and examples. Instead, it is most important to learn
the principles, patterns, and concepts that can be abstracted
from related episodes or examples. In short, educators often
want to optimize the induction of concepts and patterns, and
there are reasons to think that such induction may be enhanced
by massing, rather than by spacing. As stated by E.Z. Rothkopf
(personal communication, September 1977), ‘‘spacing is the
friend of recall, but the enemy of induction.’’
SPACING AS THE ENEMY OF INDUCTION
There is a compelling logic behind Rothkopf’s assertion.
Massing allows one to notice the similarities between successive
episodes or exemplars, whereas spacing makes doing so more
difficult. Thus, for example, spacing presentations of individual
paintings by a given artist will make it more difficult to notice
any characteristics that define the artist’s style because spacing
increases the chances that those characteristics will be forgotten
between successive presentations.
MASSING AS THE FRIEND OF INDUCTION
The logic behind Rothkopf’s assertion is so compelling that, to
our knowledge, it has never been tested. Perhaps the most direct
evidence that massing facilitates induction comes from a study
by Kurtz and Hovland (1956), who asked participants to study
simple drawings that varied on four dimensions: size, shape,
position, and coloring. There were four categories of drawings
that participants learned to identify; for example, a ‘‘Kem’’ was
defined as a drawing containing a circle positioned near the top
of the display. Each category was presented eight times, with the
individual items either interleaved with items from the other
Address correspondence to Nate Kornell, Department of Psychology,
1285 Franz Hall, UCLA, Los Angeles, CA 90095, e-mail: nkornell@
ucla.edu.
PSYCHOLOGICAL SCIENCE
Volume 19—Number 6 585Copyright r2008 Association for Psychological Science
categories (i.e., spaced) or massed together. No item was ever
repeated exactly. On a memory test following the study phase,
participants’ performance was better for drawings in the massed
condition than for drawings in the spaced condition. Gagne
´
(1950) obtained a similar result using four categories of non-
sense-figure/nonsense-syllable pairs: Error rates were reduced
when the highly similar category members were grouped to-
gether, instead of being interleaved.
Less direct evidence comes from experiments that compared
exact and nonexact repetitions (i.e., verbatim repetitions vs.
paraphrased or gist repetitions). In such experiments, the spac-
ing effect appears to diminish or disappear altogether for non-
verbatim repetitions (e.g., Appleton-Knapp, Bjork, & Wickens,
2005; Dellarosa & Bourne, 1985; Glover & Corkill, 1987).
Similarly, Melton (1970) demonstrated that spacing effects do
not occur when participants fail to recognize that a repeated item
is a repetition. Given such findings, and given that inductive
learning involves exposure to a variety of different exemplars
and does not involve exact repetition, it seems possible that
spacing effects will disappear or turn into massing effects in
tasks requiring induction.
Finally, research in the domain of motor learning also pro-
vides indirect evidence that inductive learning may profit from
massing, rather than spacing. Learning a motor skill, such as a
tennis serve, involves induction in the sense that exposure to
one’s own proprioceptive feedback is an important component of
learning—and such repetitions are, by necessity, not exact,
especially for novices learning complex skills. Although spaced
practice is effective in many areas of motor learning (e.g., Shea &
Morgan, 1979), massed practice can be more effective for learn-
ing complex motor skills (Wulf & Shea, 2002).
GOALS OF THE PRESENT RESEARCH
Thus, there are both logical and empirical reasons to expect
massing, not spacing, to facilitate induction. One goal of the ex-
periments reported here was to investigate the size of the mass-
ing effect—that is, the advantage of massed study over spaced
study—in an inductive-learning context relevant to educational
practice.
Another goal was to investigate participants’ subjective as-
sessments of massed versus spaced study in the context of in-
duction. Prior research has demonstrated that people often rate
massing as more effective than spacing, even in contexts in
which spacing is actually superior (Baddeley & Longman, 1978;
Simon & Bjork, 2001; Zechmeister & Shaughnessy, 1980). Such
massing illusions may derive from the fact that metacognitive
judgments are often grounded in feelings of fluency (e.g., see
Benjamin, Bjork, & Schwartz, 1998). Presenting the same item
twice consecutively makes processing the second presentation
seem highly fluent, providing a (misleading) impression of learn-
ing, whereas spacing decreases the fluency of processing the
second presentation. In other words, massing provides a sense of
ease, which learners assume will translate to good memory on a
later test, whereas spacing is often a ‘‘desirable difficulty’’
(Bjork, 1994) in the sense that it enhances long-term retention.
If massing produces a sense of fluency of induction, participants
may prefer massing to spacing in the induction task we used in
these experiments.
EXPERIMENTS 1A AND 1B
In this experiment, participants were asked to learn the styles of
12 different artists by viewing six different paintings by each
artist. In Experiment 1a, spacing was manipulated within par-
ticipants: Paintings by each of 6 of the artists were presented
massed, and paintings by each of the other 6 artists were
presented spaced. In Experiment 1b, spacing was manipulated
between participants: For a given participant, the paintings
were presented either all massed by painter or all interleaved
(spaced). After the learning phase, participants were shown new
paintings by the same 12 artists and asked to select, from a list of
all the artists’ names, the artist who had painted each new
painting. After the test, participants in Experiment 1a were
asked what presentation condition, massing or spacing, they felt
had been more effective for learning a given artist’s style.
Method
Participants
The participants were University of California, Los Angeles,
undergraduates, who participated for course credit. There were
120 participants in Experiment 1a and 72 participants, 36 in
each condition, in Experiment 1b.
Materials
The materials were 10 paintings by each of 12 artists (Georges
Braque, Henri-Edmond Cross, Judy Hawkins, Philip Juras, Ryan
Lewis, Marilyn Mylrea, Bruno Pessani, Ron Schlorff, Georges
Seurat, Ciprian Stratulat, George Wexler, and YieMei). Six paint-
ings by each artist were presented during the study phase, and 4
more were presented during the test phase. All the paintings were
landscapes or skyscapes. We selected artists who would be rela-
tively unknown to the participants, although some of the paintings
by Braque and Seurat may have been familiar to some of the
participants (however, on the final test, average performance on
paintings by those two artists was not better than average perfor-
mance on paintings by all 12 artists). The paintings were cropped
to remove identifying characteristics such as names and signa-
tures, if necessary, and then resized to fit into a 15- 11-cm
rectangle on a computer screen.
Procedure and Design
Participants were instructed about the nature of the study and
test phases and were then shown 72 paintings, 6 paintings by
each of the 12 artists. Each painting was shown for 3 s on a
586 Volume 19—Number 6
Spacing and Induction
computer screen, with the last name of the artist displayed
below.
In Experiment 1a, the paintings by each of six of the artists
were presented consecutively (massed), whereas the paintings by
each of the other six artists were intermingled with paintings by
other artists (spaced). The artists assigned to the massed and
spaced conditions were determined randomly for each partici-
pant. Each successive block of six paintings consisted of six
paintings by a given artist (massed, or M) or one painting by each
of the six artists (spaced, or S). The order of the blocks was
MSSMMSSMMSSM (see Fig. 1). In Experiment 1b, depending on
the condition to which a participant was assigned, either all of the
paintings were presented in the massed condition or all of the
paintings were presented in the spaced condition.
At the end of the study phase, there was a 15-s distractor task,
during which participants counted backward by 3s from 547; the
test phase began when participants completed the distractor
task. On each test trial, an unfamiliar painting by one of the 12
artists was presented. Participants indicated who they thought
had created each painting by clicking their computer’s mouse on
1 of 13 buttons, 12 labeled with the names of the artists and 1
labeled ‘‘I don’t know.’’ After this response, feedback was pro-
vided : The word ‘‘correct’’ followed a correct selection, and the
correct artist’s name was presented following an error.
There were 48 test trials divided into four blocks of 12
paintings. Each block consisted of one new painting by each of
the 12 artists, presented in random order. After the test phase in
Experiment 1a, participants were told the meanings of the terms
massed and spaced and asked, ‘‘Which do you think helped you
learn more, massed or spaced?’’ They were given three response
options: ‘‘massed,’’ ‘‘about the same,’’ and ‘‘spaced.’’ The same
question could not be asked in Experiment 1b because partic-
ipants did not experience both conditions.
Results
In marked contrast to our expectations, spaced study resulted in
significantly better test performance than did massed study, as
measured by the proportion of artists identified correctly on the
test (Fig. 2). (Experiment 1b was conducted after we came up
with a convoluted conjecture that mixing massed and spaced
paintings in a single learning phase created a spacing effect in
Experiment 1a.) The advantage of spacing was significant in
both Experiment 1a, F(1, 119) 577.35, p<.0001, Zp25.39,
and Experiment 1b, F(1, 70) 515.63, p<.001, Zp25.18.
Not surprisingly, given that feedback was provided, test per-
formance increased across test blocks—Experiment 1a: F(3,
357) 526.99, p<.0001, Zp25.18; Experiment 1b: F(3, 210)
511.56, p<.0001, Zp25.14. The interaction of presentation
condition and test block was significant—Experiment 1a: F(3,
357) 513.25, p<.0001, Zp25.10; Experiment 1b: F(3, 210)
53.33, p<.05, Zp25.046. This interaction appears to reflect
the large increase from the first to the second test block in the
massed condition, which may have been a consequence of the
first test block acting as an additional, spaced study opportunity
that benefited previously massed items in particular. A planned
comparison of performance during the first test block, which
Fig. 1. The first 12 paintings presented to 1 of the participants in Ex-
periment 1a (the artists in each condition were determined randomly for
each participant). The first 6 paintings (left column) were all by the same
artist (massed, or M), and the next 6 paintings (right column) were all by
different artists (spaced, or S). In total, there were 12 blocks of 6
paintings in the order MSSMMSSMMSSM. Therefore, in the spaced
condition, a given artist was represented by 1 painting in each S block.
Volume 19—Number 6 587
Nate Kornell and Robert A. Bjork
was, presumably, largely unaffected by the presence of feed-
back, showed that participants performed significantly better in
the spaced condition than in the massed condition, in both
Experiment 1a (M5.61, SD 5.24 vs. M5.35, SD 5.24),
t(119) 510.82, p<.0001, p
rep
1.00, d50.99, and Ex-
periment 1b (M5.59, SD 5.22 vs. M5.36, SD 5.18), t(70) 5
4.94, p<.0001, p
rep
1.00, d51.28.
The advantage of spacing over massing is all the more sur-
prising given participants’ responses on the questionnaire ad-
ministered after the test. As Figure 3 shows, participants in
Experiment 1a judged massing to be more effective than spacing,
regardless of their performance in the two conditions. Overall,
78% of the participants did better with spaced presentations than
they did with massed presentations, but 78% of the participants
said that massing was as good as or better than spacing.
Discussion
The results of Experiments 1a and 1b pose two puzzles. First,
why did spacing, not massing, foster induction when there were
compelling reasons to expect otherwise? Second, why did par-
ticipants remain unaware that spacing was more effective than
massing, even after taking the test? With respect to the second
puzzle, we hypothesized that participants, while taking the test,
might not have remembered which artists had been presented in
which condition. To investigate this hypothesis, we presented 28
participants in Experiment 1a with a list of the artists’ names, and
asked them to indicate—after they had completed the test—how
each artist’s paintings had been presented (spaced or massed).
Accuracy on the identification task was significantly above chance
for artists whose paintings had been presented spaced (M5.74,
SD 5.17), t(27) 57.48, p<.0001, p
rep
1.00, d51.41, but
not for artists whose paintings had been presented massed (M5
.55, SD 5.21), t(27) 51.19, p5.25, p
rep
5.69,d50.23.
Participants’ inability to remember which artists’ paintings had
been presented massed suggests that participants often, if not
always, made their metacognitive judgments on the basis of their
subjective experience during the study phase.
With respect to the puzzle of why spacing enhanced induction,
one possible explanation is that the test required recalling a given
artist’s name, not just knowing his or her style, and spacing facil-
itates recall. It seems possible that participants did indeed induce
an artist’s style more effectively in the massed condition than in the
spaced condition, but recalled the name associated with that style
better in the spaced condition. Experiment 2 was designed to test
that possibility by assessing participants’ recognition, not recall, so
that they did not need to remember name-style associations.
EXPERIMENT 2
The learning phase in Experiment 2 was identical to the learning
phase in Experiment 1a. However, during the test phase, par-
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Spaced
Massed
0.2
0.3
0.4
Proportion Correct Proportion Correct
0.5
0.6
0.7
0.8
1234
Test Block
Spaced
Massed
Fig. 2. Proportion of artists selected correctly on the multiple-choice
tests in Experiments 1a (top panel) and 1b (bottom panel) as a function of
presentation condition (spaced or massed) and test block. Error bars
represent standard errors.
0
10
20
30
40
50
60
70
80
Massed >
Spaced
Massed =
Spaced
Spaced >
Massed
Judged Effectiveness
Number of Participants
Spaced > Massed
Massed = Spaced
Massed > Spaced
Actual Effectiveness
Fig. 3. Number of participants (out of 120) who judged massing as more
effective than, equally effective as, or less effective than spacing in Ex-
periment 1a. For each judgment, the number of participants is divided
according to their actual performance in the spaced condition relative to
the massed condition.
588 Volume 19—Number 6
Spacing and Induction
ticipants were given a style-recognition test. All of the tested
paintings were new paintings, as in Experiment 1, but partici-
pants were asked only to categorize a given test painting as by a
‘‘familiar artist’’ (i.e., by an artist whose paintings had been
presented during the study phase) or as by an ‘‘unfamiliar artist.’’
Thus, the test required remembering only studied artists’ styles,
not their names.
Method
Participants
The participants were 80 undergraduate students at the Uni-
versity of California, Los Angeles, who participated for course
credit.
Materials
The materials consisted of the same set of paintings used in
Experiment 1, plus, for each studied artist, an additional set of
four distractor paintings. Each distractor painting was chosen to
be stylistically similar to a studied artist’s paintings, and each
distractor was by a different artist (see Fig. 4).
Procedure
The study phase was exactly the same as in Experiment 1a, as
was the questionnaire at the end of the experiment. The only dif-
ference from Experiment 1a was in the test phase (and associ-
ated instructions).
During each trial of the test phase, a painting was presented
with two buttons on the computer screen; one button was labeled
‘‘familiar artist,’’ and one was labeled ‘‘unfamiliar artist.’’ Par-
ticipants were instructed to select the ‘‘familiar artist’’ button if
they thought the painting was by an artist whose paintings had
been presented during the study phase, and to select the ‘‘un-
familiar artist’’ button if they thought the painting was by an
artist whose paintings had not been presented during the study
phase. There were four test blocks, each of which included one
target painting and one distractor painting by a corresponding
nonstudied artist, making a total of 24 paintings per block. No
feedback was given during the test.
Results
Recognition test trials are, inevitably, also learning events. A
side effect of falsely endorsing a painting as by a familiar artist
was that a participant might alter his or her concept of the fa-
miliar artist’s style by incorporating aspects of a painting by an
unfamiliar artist into that concept. On each successive test
block, the potential for contamination created by false alarms
grew, resulting in a significant decrease in recognition accuracy
across test blocks, F(3, 237) 53.60, p<.05, Zp25.04.
Therefore, to gain maximum leverage on the question of interest,
we restricted our analyses to the first test block, which provided
the purest measure of the learning that occurred during the study
phase of the experiment.
Again, we were surprised to find that performance in the
spaced condition was superior to performance in the massed
condition. As Figure 5 shows, the spaced and massed conditions
produced similar rates of false alarms (i.e., saying that a painting
by a nonstudied artist was by a studied artist), but the hit rate
(i.e., correctly categorizing a new painting by a studied artist as
by a familiar artist) was higher in the spaced condition (M5.77,
SD 5.22) than in the massed condition (M5.67, SD 5.24),
t(79) 53.28, p<.01, p
rep
5.98, d50.41. Consequently, there
was a significant interaction between spacing condition and
response type, F(1, 79) 57.84, p<.01, Zp25.09. There was
also a main effect of response type, F(1, 79) 5177.82, p<
.0001, Zp25.69, with more hits than false alarms; this pattern
of results shows that participants could distinguish between the
target and distractor paintings. Thus, even in a situation that did
not require participants to recall name-style associations,
spacing led to more effective induction than did massing.
Fig. 4. Examples of four target (a) and four associated distractor (b) paintings from the test phase of Experiment 2. The target
paintings were the same as those used in Experiment 1. The distractors were all by different artists, and each was selected to be
similar to the paintings of a given studied artist.
Volume 19—Number 6 589
Nate Kornell and Robert A. Bjork
As in Experiment 1a, the participants’ metacognitive judg-
ments were strikingly at odds with their actual prior perfor-
mance. Of the 72 participants who did not say that learning in
the massed condition and learning in the spaced condition were
‘‘about the same,’’ 64 thought massing had been more effective
than spacing.
One possible explanation of the current findings is that
schema induction happened early in the study phase. For ex-
ample, the induction for each artist may have been ‘‘done’’ by the
third study trial, so that the next three study trials amounted to
either massed or spaced memory practice. To test this possi-
bility, we conducted an additional experiment. The test phase
was the same as in Experiment 2, but in the study phase, only two
paintings by each artist (instead of six) were presented. We
obtained the same pattern of results: There was a significant
benefit of spaced study, but participants thought massed study
had been more effective.
GENERAL DISCUSSION
A common way to teach students about an artist is to show, in
succession, a number of paintings by that artist. Counterintu-
itive as it may be to art-history teachers—and our participants—
we found that interleaving paintings by different artists (spac-
ing) was more effective than massing all of an artist’s paintings
together. A possible key to understanding the present findings
involves the relationship between induction and discrimination.
Induction and Discrimination
Experiment 1 required that participants discriminate among
different artists’ styles; that is, on the test, they had to decide
which artist, among the 12 studied artists, had painted a given
new painting. The interleaving of artists that was intrinsic to the
spaced condition might have fostered such discrimination. For
example, the key to deciding whether a tree is a maple or an
oak (or some other tree) is learning to appreciate the differences
among trees, not learning about a given type of tree in isolation.
Interleaving had the effect of juxtaposing different paintings and
therefore might have enhanced discrimination learning. (In fact,
we have presented no evidence regarding the effects of temporal
spacing in the absence of interleaving, and it may be inter-
leaving, not spacing itself, that is the key to enhancing inductive
learning.)
With respect to this possibility, the following observation by
Kurtz and Hovland (1956) seems relevant: ‘‘When the degree of
discriminability is low, it might be expected that placing of in-
stances from different concepts in juxtaposition would facilitate
discrimination learning, whereas with greater discriminability,
like that obtaining in the present study, the reverse might ob-
tain’’ (p. 242). Thus, if discrimination is not difficult, as was the
case in Kurtz and Hovland’s experiment, massing may be ad-
vantageous, but if discrimination is difficult, as it was in our ex-
periments, spacing might be more effective.
This argument is appealing, but it is not entirely consistent
with the results from Experiment 2. The recognition test in Ex-
periment 2 required discriminating between paintings by pre-
viously studied artists and similar paintings by artists who had
not been studied; it did not require distinguishing among artists
whose work had been presented, and yet there was a benefit from
spacing.
It could be argued that a by-product of being better able to
distinguish among the presented artists is being able to distin-
guish those artists, as a group, from other artists. It may be rare,
in fact, that a concept or category (such as what psychology is or
how to fly a kite) is ever learned without the need to discriminate
it from other categories (such as sociology or ways to make a kite
fall).
Our results notwithstanding, there surely are situations in
which massing is more effective for induction than is spacing.
We attempted to create one such situation by asking participants
to figure out, and remember on a later test, the single word that
could be used to fill in the blanks in each of 12 sets of six words;
for example, in the case of _____ cracker, _____ wood, _____
side, _____ ant, _____ truck, _____ arm, the word to be gen-
erated and remembered was fire. The design was similar to the
design of Experiment 1a—that is, half of the sets that defined a
to-be-remembered word were presented massed, whereas the
other half were presented spaced—and 20 undergraduate par-
ticipants were tested. In this case, spacing made it nearly im-
possible to solve the problems, and, thus, later memory for the
target words was significantly better in the massed condition
than in the spaced condition (.34 vs. .22), t(19) 52.78, p<.05,
p
rep
5.94, d50.65.
Admittedly, this simple experiment was contrived to be a
situation in which massing, not spacing, would enhance the
0
0.2
0.4
0.6
0.8
1
Studied Artists
(Hits)
Nonstudied
Artists (FAs)
Proportion 'Familiar'
Massed
Spaced
Fig. 5. Results from the recognition test in Experiment 2: proportion of
paintings judged to be painted by a studied artist as a function of whether
the artist had been studied (hits) or had not been studied (false alarms, or
FAs), separately for the spaced and massed conditions. Only data from
the first test block were analyzed and plotted here. Error bars represent
standard errors.
590 Volume 19—Number 6
Spacing and Induction
generation and memory of the critical words. The experiment
demonstrates, however, that whether spacing is the friend or
enemy of induction is a matter for sophisticated theorizing, be-
cause induction is a product of conceptual and memory processes
that are open to multiple situational influences. The important
point, though, is that in less contrived and more complex real-
world learning situations, spacing appears to facilitate induction.
Practical Implications
Inductive learning—that is, learning from examples—is a key
element of formal education, and of how humans (and other
animals) informally learn about the world. There are many in-
ductive-learning situations that would seem, from an intuitive
standpoint, to lend themselves to massed study, but may not.
Examples include a baby learning what chair means by ob-
serving people talking about chairs; an older child learning the
rules of a language, such as that most plural English words end
in s, by listening to people speak the language; a student in
school learning how words are spelled by reading them (as well
as through more direct instruction); a quarterback learning to
recognize a complex pattern of motion that predicts an inter-
ception by gaining experience in practice and during games; a
monkey learning to recognize the warning signs that another
monkey is acting threateningly by observing other monkeys’
behavior; and a medical student learning to recognize warning
signs of lung cancer by reading x-rays under an expert’s su-
pervision. Our results cannot necessarily be generalized to all of
these situations, of course, but they do suggest that in inductive-
learning situations, spacing may often be more effective than
massing, even when intuition suggests the opposite.
Our results also suggest that individuals responsible for the
design and evaluation of instruction that involves induction are
susceptible to being very misled by their own intuitions and
subjective experiences. Although prior experiments (Baddeley &
Longman, 1978; Simon & Bjork, 2001; Zechmeister & Shaugh-
nessy, 1980) have shown that people can experience an illusion
that massing is effective, we know of no experiment that can
match the current findings in terms of sheer inaccuracy of
judgments. In Experiments 1a and 2 combined, 85% of the
participants did at least as well in the spaced condition as in the
massed condition, but 83% of the participants rated the massed
condition as equally effective as or more effective than the
spaced condition. The illusion of effective learning in the massed
condition, based, apparently, on a sense of fluency of induction,
was clearly powerful in the experiments presented here. In real-
world inductive-learning tasks, therefore, it seems likely that
people will be heavily influenced by the illusory benefits of
massing when making decisions about their own learning or the
learning of their students or children. That is, most people are
likely to prefer massing in inductive-learning situations, but our
results suggest that they may do so at their own (and their stu-
dents’ and children’s) peril from a learning standpoint.
CONCLUSION
Looking back at our own inability to foresee the benefits of
spacing, perhaps we fell victim to the same illusion that we have
railed against (e.g., Bjork, 1994, 1999; Kornell & Bjork, 2007),
namely, the illusion that a sense of ease or fluency accompanies
effective learning, whereas a sense of difficulty signifies in-
effective learning. In the case of induction, as in many other
types of learning, spacing appears to be sometimes, if not al-
ways, a desirable difficulty (Bjork, 1994).
Acknowledgments—We thank Makah Leal and Timothy Wong
for their invaluable contributions to all facets of the experiments,
Elizabeth Bjork for her insights, and Katherine Huang and Jeri
Little for their help carrying out the experiments. Grant 29192G
from the McDonnell Foundation supported this research.
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