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Cognitive Constraints on the Understanding
and Acceptance of Evolution
Andrew Shtulman
and Prassede Calabi
In September 2008, Electronic Arts released a computer game called “Spore,” in
which players control the evolution of a novel organism from microscopic cell to
interstellar explorer. Billed as a potential teaching tool, the game has been sharply
criticized by evolutionary biologists for its lack of scienti c accuracy (Bohannon,
2008). In Spore, “evolution” proceeds by swapping “DNA points,” earned through
nding food and avoiding predators, for new body parts. The collection of select-
able parts is determined not by variation in the population (all members of a given
species are identical) but by the organism’s size and intelligence—attributes that
increase linearly, and deterministically, as the game proceeds. Death and reproduc-
tion occur in Spore but are unaffected by competition or selection, and are thus
unrelated to the organism’s “evolution.” Death actually serves an opportunity to
restart the game from wherever one’s organism last spawned, and reproduction
serves an opportunity to edit the organism, adding and subtracting body parts at
will. Just as Spore makes no role for natural selection, it makes no role for common
descent; every creature on the planet can trace its ancestry back to a different single-
celled organism originally deposited by meteors.
While it is possible that evolutionary misconceptions embodied in Spore’s game
play re ect nothing more than confusion on the part of its designers, these miscon-
ceptions are not random or unique. Science education researchers have documented
similar types of misconceptions among students of every stripe, from middle
school students (Lawson & Thompson, 1988) to high school students (Banet &
Ayuso, 2003; Settlage, 1994) to college undergraduates (Bishop & Anderson, 1990;
Ferrari & Chi, 1998; Greene, 1990; Nehm & Reilly, 2007) to medical school stu-
dents (Brumby, 1984) to preservice teachers (Crawford, Zembal-Saul, Munford, &
Friedrichsen, 2005). These misconceptions include con ating mutation with adap-
tation, con ating species adaptation with individual adaptation, and preferring
teleological explanations of adaptation to mechanistic ones.
From where do these misconceptions arise? Although different researchers have
identi ed different sources (e.g., Jimenez, 1994; Southerland, Abrams, Cummins, &
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48 Folk Theories, Conceptual and Perceptual Constraints
Anzelmo, 2001), here we review evidence suggesting they derive, at least in part,
from an early-emerging tendency to “essentialize” the biological world—that is, to
assume that all members of a species share the same causal potential for growth,
appearance, and behavior (see Gelman, 2003; Gelman & Rhodes, this volume). We
also present evidence that these misconceptions foster skepticism toward evolu-
tion and must be addressed if science educators hope to increase public acceptance
of evolution, especially in the United States, where only 40% of the population
agrees with the statement “Human beings, as we know them, developed from earlier
species of animals” (Miller, Scott, & Okamoto, 2006). The bulk of this chapter is
devoted to describing the results of a teaching-intervention study in which college
undergraduates’ understanding and acceptance of evolution were assessed before
and after an introductory course on evolution and behavior. We begin by describ-
ing the phenomenon of biological essentialism and its consequences for evolution-
ary reasoning, both in the history of evolutionary biology and in the practice of
evolution education.
Biological Essentialism and Its Consequences
The concept of essentialism is well illustrated by Hans Christian Andersen’s (1844)
fairytale “The Ugly Duckling.” The story begins with a mother duck sitting on a
nest of eggs, waiting for her ducklings to hatch. One duckling hatches later than
the others, and he is, to everyone’s dismay, larger and “uglier” than his siblings.
A neighboring duck suggests that he may be a turkey, but that suggestion is soon
refuted by the fact that he can swim. This ability, paired with his unusual looks,
makes him a target of ridicule from both the ducks and the turkeys. Frightened and
upset, the duckling leaves his home in search of animals who will accept him as one
of their own. During his journey, he meets geese, who reject him as an unsuitable
mate; a tom cat, who rejects him for his inability to purr; and a hen, who rejects him
for his inability to lay eggs. Finally, after months of travel, the duckling encounters
a group of graceful white swans, who, to his surprise, accept him into their family.
The reason, he soon discovers, is that he has grown into a graceful white swan
himself. “To be born in a duck’s nest, in a farmyard,” writes Andersen, “is of no
consequence to a bird, if it is hatched from a swan’s egg” (p. 20).
Whatever moral the story was intended to convey, its plot is predicated on the
assumption that an organism’s properties—both current and potential—are deter-
mined by its species kind, which is, in turn, determined by its parentage. Much
developmental research has shown that this assumption appears to be ubiquitous
across cultures (Medin & Atran, 2004) and across ages (Gelman, 2003). Children
and adults from all parts of the world tend to reason about an organism’s outward
appearance and behavior on the basis of an internal causal power, or “essence,”
inherited from the organism’s parents and xed at the organism’s birth. This assump-
tion serves us well in most situations, as an organism’s species is, indeed, a reliable
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Cognitive Constraints on the Understanding and Acceptance of Evolution 49
predictor of its properties. Knowing that an organism is a swan, for instance, allows
us to make accurate predictions about how that organism should look (brown and
fuzzy as an infant, white and sleek as an adult), where that organism should live
(by water), what that organism should eat (vegetation), how that organism should
reproduce (by laying eggs), and many other such properties.
Empirical evidence that young children are biological essentialists comes from at
least three sources. First, many studies have shown that very young children privi-
lege species kind over perceptual similarity when reasoning about the properties
of novel organisms (Gelman & Markman, 1987; Gelman & Coley, 1990; Jaswal &
Markman, 2007). In these studies, children are taught a novel property of a familiar
organism (e.g., “This black-and-white cat can see in the dark”) and asked whether
certain novel organisms possess the same property. Some of the novel organisms
are of the same species as the target organism but differ in appearance (e.g., a cat
with different markings), and others share the same appearance but are of a differ-
ent species (e.g., a skunk with identical markings). Children as young as age 2½ reli-
ably project the target property to the former but not the latter, implying that they
view species identity (conveyed via linguistic labels) as a better predictor of shared
properties than mere appearance.
Second, preschoolers assume that an organism will retain its species identity
across various natural and/or commonplace changes in appearance, like growing
in size (Rosengren, Gelman, Kalish, & McCormick, 1991), growing in complex-
ity (Hickling & Gelman, 1995), or donning a costume (DeVries, 1969). By age 7,
children assume that an organism will retain its species identity even across drastic
and/or unusual changes in appearance, like plastic surgery or chemical injections
(Keil, 1989).
Third, numerous studies have shown that preschoolers assume an organism will
retain its species identity across various changes in upbringing (Gelman & Wellman,
1991; Johnson & Solomon, 1997; Springer, 1996; Waxman, Medin, & Ross, 2007).
In these studies, children are presented with Ugly Duckling–like scenarios, in which
a baby animal is removed from its birth parents (e.g., cows) and raised by members
of a different species (e.g., pigs). The children are then asked to predict which prop-
erties the baby animal would possess as an adult: the biological properties of its
birth parents (e.g., a straight tail and a diet of grass) or those of its adopted parents
(e.g., a curly tail and a diet of slop). Children of all ages tend to predict that the
baby will grow to possess the biological properties of its birth parents. They also
tend to justify their judgments with explicit appeals to the continuity of species
identity (e.g., “It will eat grass because it’s a cow, not a pig”).
In sum, young children appear to construe the biological world in terms of hid-
den essences that give rise to its observable properties and causal regularities. Such
biases are generally a useful constraint on biological induction, as they support the
generally accurate projection of species-speci c properties to individual organisms.
In the absence of such biases, we would be unable to explain or predict the behav-
ior of any organism we had not personally studied. Indeed, virtually all biological
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50 Folk Theories, Conceptual and Perceptual Constraints
sciences operate on the assumption that information gleaned from observing a
subset of a species is applicable to the species as a whole.
Yet, despite its utility for reasoning about the properties of individual organ-
isms, biological essentialism has proven a major impediment for reasoning about
population-level phenomena, such as evolution and natural selection. The problem,
as articulated by historians of science like Gould (1996), Hull (1965), and Mayr
(1982), is that essentialism leads one to treat species as discrete, homogeneous units,
whose aggregate properties are true of all members of the species and only mem-
bers of the species. This view correctly implies that any, and every, baby swan has
the potential to grow into an adult swan—“swanness” is innate, discrete, and dif-
ferent from “duckness”—but it incorrectly implies that differences between swans
are unimportant or inconsequential. Indeed, it positively obscures the fact that
most baby swans will not survive to adulthood, let alone reproduce. Thus, easy
understanding of the similarity among individuals within a species precludes easy
understanding of differences between those individuals, especially differences that
result in differential mortality and differential reproduction. As a result, students
engaged in learning about evolution are likely to adopt what Mayr (2001) terms a
“transformational” theory of evolution, or a theory in which evolution is (incor-
rectly) construed as the cross-generational transformation of a species’ underly-
ing essence, rather than what Mayr terms a “variational” theory of evolution, or
a theory in which evolution is (correctly) construed as the selective propagation of
within-species variation.
The effect of biological essentialism on the development of evolutionary biology
was profound. According to Mayr (1982), Greek scholars had formulated the con-
cept of descent with modi cation as early as 600 BC, but the mechanisms of evolu-
tion remained a mystery for another 25 centuries. Those who attempted to solve
this mystery invariably fell prey to what Gould (1996) calls the “fallacy of rei ed
variation,” or the inclination “to abstract a single ideal or average as the essence of
a system and to devalue or ignore variation among the individuals that constitute
the full population” (p. 40). By focusing on the similarities among members of the
same species rather than their differences, early evolutionary theorists ended up
positing mechanisms of evolution that operated over individuals, not over popu-
lations—namely, mechanisms like the inheritance of acquired traits, the intrinsic
properties of organic matter, or the law of acceleration of growth (see Bowler, 1983,
for a review). Not until Darwin did evolutionary biologists begin eschewing spe-
cies-wide similarities for within-species differences. Indeed, Darwin’s recognition
of the importance of intraspeci c variation allowed him to combine three major
insights: descent with modi cation, competition as a selective force (i.e., applica-
tion of Malthus), and phylogeny as a “tree of life” (i.e., shared ancestry). The result
was a qualitatively different view of evolution—“variationism”—which henceforth
became the backbone of the biological sciences.
Because biological essentialism is seemingly universal (Medin & Atran, 2004),
it is reasonable to suppose that, just as early evolutionary biologists were led astray
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Cognitive Constraints on the Understanding and Acceptance of Evolution 51
by their essentialist intuitions, modern-day students of evolution are led astray as
well, adopting transformational interpretations of evolutionary phenomena prior
to learning correct, variational ones. Shtulman (2006) investigated this hypothesis
by designing an in-depth assessment of evolutionary reasoning intended to distin-
guish between variational and transformational interpretations of six evolution-
ary phenomena: variation, inheritance, adaptation, domestication, speciation, and
extinction. (Sample items from each section of the test are presented below.) The
assessment was administered to 45 high school and college students enrolled in the
Harvard Summer School and found that the majority (53%) held predominantly
transformational views of evolution. Importantly, those who demonstrated trans-
formational conceptions on one section of the assessment tended to do so on several
others, implying that their responses were not isolated misconceptions but rather
the by-products of a qualitatively different way of understanding species change
and species adaptation. Indeed, a factor analysis of students’ responses across the
six different sections of the assessment revealed one, and only one, underlying fac-
tor. Students who scored high on this factor revealed a consistently variational
understanding of evolution, whereas those who scored low revealed a consistently
transformational understanding.
Shtulman and Schulz (2008) extended these ndings by comparing adults’
understanding of evolution, as measured by an abbreviated version of the
Shtulman (2006) assessment tool, to their acceptance of within-species variation,
as measured by a series of questions about whether certain biological proper-
ties can, and do, vary across different members of the same species (e.g., “Do all
giraffes have spotted coats or just most giraffes?” “Do all ants have a tube-shaped
heart or just most ants?”). As expected, adults who demonstrated a variational
view of evolution were signi cantly more likely to accept within-species variation
than those who demonstrated a transformational view. Indeed, the latter group
of adults were no more likely to accept within-species variation than were 4-year-
old children. Taken together, these ndings suggest that deep-seated essentialist
biases lead students to devalue within-species variation, and, as a result, fail to
understand the mechanism of evolution that operates over such variation: natural
selection.
Cognitive Constraints on Understanding
Having described the role of biological essentialism in evolutionary thought, we
now turn to a detailed analysis of how biological essentialism constrains students’
naive theories of evolution. This analysis is presented in the context of a study
assessing the nature of those theories before and after a semester’s worth of college-
level instruction in evolutionary biology. Also assessed was the relation between
students’ understanding of how evolution works and their acceptance of various
evolutionary claims.
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52 Folk Theories, Conceptual and Perceptual Constraints
THE PARTICIPANTS
The participants were 45 college undergraduates enrolled in a one-semester course
on behavior and evolution at a large, public northeastern university. The course
was taught three times, with about 15 students each semester. Because there was
no effect of semester scheduling on any of the variables reported below, we col-
lapsed the three samples into one. Most participants were psychology majors who
had enrolled in the course to ful ll an upper-division requirement of the major.
All had taken at least one high school or college-level biology course prior to the
course in which they were currently enrolled, and some had taken as many as
three. Although it is unclear how much of that coursework entailed evolutionary
concepts/phenomena, most participants (76%) claimed to have taken a class or
read a book that suf ciently explained the concept of natural selection prior to
instruction.
THE TEACHING INTERVENTION
The main objective of the teaching intervention was to help participants derive, for
themselves, the concepts of evolution and natural selection from basic principles
of biology, natural history, and population dynamics. The intervention was based
on Mayr’s (1982) analysis of how Darwin initially derived the concepts of evolu-
tion and natural selection from four basic phenomena (superfecundity, resource
limitation, trait variation, and trait heritability) and two intermediate inferences
(differential survival and differential reproduction). Five clusters of activities pro-
duced opportunities for participants to reproduce this chain of inferences, either by
generating their own data or by analyzing preexistent data from real populations
of organisms. These activities also produced opportunities for participants to con-
front, articulate, and question their nongenetic, nonvariational assumptions about
species adaptation.
The opening activity was designed to introduce the concept of superfecundity
(the potential inherent in all species to grow exponentially) and to set the stage for the
other three phenomena. That activity began with the instructor posing the question,
“Why is the earth not covered in dogs?” Participants were asked to estimate the num-
ber of offspring a single pair of dogs would produce over six generations of breeding
if each pair in each generation produced ten offspring per year (the so-called “over-
lapping generations model”). After making an estimation, participants calculated
and graphed the actual number of offspring produced. This value typically exceeded
their estimates by several orders of magnitude and raised questions that served as
entry points for discussing the other phenomena and inferences. For instance, the
follow-up question “Why do so few individuals survive?” raised issues of resource
limitation, intraspecies competition, predation, disease, and bad weather. The
follow-up question “Who dies?” raised issues of trait variability, differential repro-
duction, and chance (for more on the intervention, see Calabi, 1998, 2005).
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Cognitive Constraints on the Understanding and Acceptance of Evolution 53
This way of teaching evolution contrasts with typical instruction in that it allows
students to derive relevant outcomes from rst principles and real data, while also
accounting for their preinstructional misconceptions. This approach has proven
successful in a variety of domains (e.g., Moss & Case, 1999; Smith, 2007; Wiser &
Amin, 2001), though it has not been used in the domain of evolution (to our
knowledge). Our hope was that, by helping students derive the relevant concepts
themselves, they would more likely understand those concepts and would also more
likely accept them as valid, thereby side-stepping the typical “dualistic” outcome of
science education, in which students maintain their intuitive beliefs alongside those
explicitly required by the instructor (Bloom & Weisberg, 2007).
THE COMPREHENSION ASSESSMENT
At the beginning and end of the 15-week semester, participants’ were administered
a 30-question assessment of their understanding of variation, inheritance, adapta-
tion, domestication, speciation, and extinction. The same questions were used at
both pretest and posttest and were never discussed during the teaching interven-
tion itself. Each question was designed to differentiate inaccurate, transformational
interpretations of the target phenomenon from accurate, variational ones, and par-
ticipants were instructed to answer each question based on their best understanding
of evolution regardless of whether they believe that evolution actually occurs. This
assessment can be found, in its entirety, in the appendix of Shtulman (2006). Below,
we discuss sample questions from each section of assessment to illustrate the nature
of the instrument as a whole.
Variation
The evolution of the peppered moth, Biston betularia, was used as a vehicle for
eliciting participants’ reasoning about the prevalence and importance of within-
species variation. On the rst question of this section, participants were told that
nineteenth-century England underwent an industrial revolution with the unfortu-
nate side effect of covering the English countryside in soot and ash and that during
the same time period England’s native moth species, Biston betularia, became, on
average, darker in color. Participants were then asked to speculate how a change
in the moths’ environment might have brought about a change in the moths’ color.
Responses that referenced individual differences in tness (e.g., “predators lunched
on the lighter ones, leaving the darker ones to reproduce”) were coded as varia-
tional, and responses that referenced the needs of the population as a whole (e.g.,
“the moths needed to blend into their environment in order to survive”) were coded
as transformational. Prior to instruction, 44% of participants provided variational
responses, 44% provided transformational responses, and 12% provided ambigu-
ous responses (e.g., “evolution”). Following instruction, 51% provided variational
responses, 38% provided transformational responses, and 11% provided ambiguous
responses.
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54 Folk Theories, Conceptual and Perceptual Constraints
Inheritance
A ctitious species of woodpeckers was used as a vehicle for eliciting participants’
reasoning about the heritability of various traits. Participants were told that two such
woodpeckers migrated to a windier environment and, in consequence, developed
stronger wing muscles. Participants were then asked to decide whether offspring of
these two woodpeckers would be born with (a) stronger wing muscles than the par-
ents had at birth, (b) weaker wing muscles than the parents had at birth, or (c) either
stronger wing muscles or weaker wing muscles, neither being more likely. Participants
who chose (c) and justi ed their response by referencing the randomness of muta-
tions (e.g., “phenotypic differences occur by random chance”) or the phenotype-
genotype distinction (e.g., “things that develop or are learned during the lifetime of
an animal cannot be passed down to its offspring”) were coded as having provided
a variational response. Participants who chose (a) and justi ed their responses by
referencing the necessity of adaptation (e.g., “the offspring need to have strong wing
muscles to survive in a windier environment”) were coded as having provided a trans-
formational response. At pretest, 38% of participants provided variational responses,
42% provided transformational responses, and 20% provided ambiguous responses.
At posttest, 53% provided variational responses, 33% provided transformational
responses, and 14% provided ambiguous responses.
Adaptation
Five analogical-reasoning questions were used to assess participants’ interpretation
of the mechanism of adaptation. On each question, participants were shown four
explanations for why a group of individuals had improved their performance along
some particular dimension and asked to select the explanation that was most analo-
gous to Darwin’s explanation for the adaptation of species. For example, partici-
pants were shown the following four explanations for why a youth basketball team
had won more games in the current season than in the previous season: (a) each
returning team member grew taller over the summer; (b) any athlete who partici-
pates in a sport for more than one season will improve at that sport; (c) more people
tried out for the same number of spots this season; or (d) each team member prac-
ticed harder this season than he did last season. Whereas choice (c) attributes the
improvement to changes in group membership (a variational analogy), choices (a),
(b), and (d) attribute the improvement to the transformation of each group member
(a transformational analogy). Prior to instruction, 40% of participants chose the
variational analogy, and 60% chose one of the three transformational analogies.
Following instruction, 53% chose the variational analogy and 47% chose one of the
three transformational analogies.
Domestication
The domestication of corn from Teosinte, a wild grass native to Central America,
was used as a vehicle for assessing participants’ interpretation of the role of human
intervention in the domestication process. On one set of questions, participants
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Cognitive Constraints on the Understanding and Acceptance of Evolution 55
were asked to rank six factors in order of their relevance to the domestication of
corn: (a) the degree of similarity among plants of the same generation, (b) the aver-
age amount of time each plant was exposed to direct sunlight, (c) the preferences
of those who decided which kernels to plant, (d) the fertility of the soil in which the
kernels were planted, (e) the average rainfall per year, and (f) the percentage of each
crop used to breed the next generation. Whereas factors (a), (c), and (f) are relevant
to the modi cation of an entire species, factors (b), (d), and (e) are relevant only to
the modi cation of individual organisms. At pretest, 53% of participants assigned
higher rankings to the species-relevant factors than to the organism-relevant factors
(a variational response), and 47% did the reverse (a transformational response). At
posttest, 87% assigned higher rankings to the species-relevant factors than to the
organism-relevant factors, and 13% did the reverse.
Speciation
Primate evolution was used as a vehicle for eliciting participants’ reasoning about
common ancestry and species individuation. On one question, participants were
shown a list of nine species—lemurs, elephants, salamanders, sparrows, bees, jelly-
sh, algae, daffodils, and brontosauruses—and asked to place a check next to each
species that shares a common ancestor with humans. Common descent is perfectly
consistent with variationism, as variationists interpret divergence as a product of
prolonged geographic isolation or assortative mating. It is not, however, consis-
tent with transformationism, for transformationists assume that all members of the
same species share a common essence unaffected by geographical constraints and
thus impervious to divergence. Transformationists must therefore assume either
that all extant species evolved independently of one another or that some extant
species are actually “preevolved” versions of others (e.g., viewing chimpanzees as
a parent species, not a sister species, to humans). Consistent with this view, 56%
of participants claimed, at pretest, that humans share a common ancestor with
fewer than half of the nine species; only 36% claimed that humans share a com-
mon ancestor with all nine. At posttest, 47% claimed that humans share a common
ancestor with fewer than half of the nine species, and 40% claimed that humans
share a common ancestor with all nine.
Extinction
The evolution of bacteria was used as a vehicle for eliciting participants’ beliefs about
the prevalence of extinction. On the rst question of the section, participants were
asked to decide whether the number of extinct bacteria species is (a) greater than the
number of living bacteria species, (b) smaller than the number of living bacteria spe-
cies, or (c) either greater or smaller than the number of living bacteria species, neither
option being more likely. Participants who chose (a) and justi ed their response by
referencing the unlikelihood of adaptation (e.g., “evolution is about trial and error;
many fail and few succeed”) or the scope of a geological timescale (e.g., “the extinct
species have accumulated over billions of years”) were coded as having provided a
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56 Folk Theories, Conceptual and Perceptual Constraints
variational response. Participants who chose (b) or (c) and justi ed their responses
by referencing the likelihood of adaptation (e.g., “although some bacteria went
extinct, most adapted to their environment”) were coded as having provided a trans-
formational response. Prior to instruction, 40% of participants provided variational
responses, 40% provided transformational responses, and 20% provided ambiguous
responses. Following instruction, 44% provided variational responses, 33% provided
transformational responses, and 22% provided ambiguous responses.
DIFFERENCES FROM PRETEST TO POSTTEST
Participants’ responses were analyzed quantitatively by scoring them along a three-
point scale. Responses consistent with variationism were scored +1; responses con-
sistent with transformationism were scored −1; and responses consistent with both
theories (i.e., ambiguous responses) were scored 0. Summed across 30 questions,
participants’ assessment scores could range from −30 to +30. In actuality, they
ranged from −24 to +27 at pretest and −22 to +28 at posttest. The actual distribution
of participants’ scores is displayed in Figure 3.1. At pretest, most participants (47%)
scored between −30 and −11, and only a small minority (24%) scored between +11
and +30. At posttest, most participants (44%) scored between −10 and +10, with
approximately the same number (36%) scoring between +11 and +30. The increase in
the proportion of participants who scored between +11 and +30 was not statistically
signi cant, but the decrease in the proportion of participants who scored between
−11 and −30 was. Thus, the intervention appeared to be more effective at eliminating
strong transformational reasoning than at fostering strong variational reasoning.
Overall, the mean pretest score was −3.5 (SD = 14.8), and the mean posttest score
was 2.3 (SD = 13.3). This difference was highly signi cant, yet the difference itself
is ambiguous as to the nature of participants’ conceptual progress. On one hand,
participants who began the course with a predominantly transformational view of
Pretest
Posttest
Number of Participants
0
10
5
15
20
Comprehension Assessment Score
−30 −20 −10 0 10 20 30 −30 −20 −10 0 10 20 30
FIGURE 3.1 Frequency distributions of participants’ comprehension assessment scores at
pretest and posttest (range = −30 to +30). Negative scores are indicative of transformational
reasoning and positive scores are indicative of variational reasoning.
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Cognitive Constraints on the Understanding and Acceptance of Evolution 57
evolution might have ended the course with signi cantly fewer transformational
conceptions and signi cantly more variational conceptions. On the other, partici-
pants who began the course with a predominantly transformational view might
have ended the course more confused, producing signi cantly more ambiguous
responses (scored 0) but not signi cantly more variational responses (scored +1).
A closer analysis of participants’ responses revealed that the former scenario was
actually more typical than the latter. Prior to instruction, participants provided
an average of 11.2 variational responses, 15.4 transformational responses, and 3.4
ambiguous responses. Following instruction, they provided an average of 14.2 vari-
ational responses, 13.8 transformational responses, and 2.8 ambiguous responses.
The increase in variational responses from pretest to posttest was statistically sig-
ni cant, but the decrease in ambiguous responses was not. Thus, the decrease in
participants’ transformational reasoning was accompanied not by confusion but by
a corresponding increase in variational reasoning.
Participants’ pretest and posttest scores are broken down by section in Fig ure 3.2.
These data show that the effects of instruction were widespread, as participants
increased their score on all six sections (though the increase on the Extinction
section was not statistically reliable). Interestingly, there was a strong correlation
between the mean pretest score for each section and its corresponding pre-post
gain (r = 0.73), implying that participants made greater conceptual progress on the
sections they understood better from the start.
Postinstructional gains in understanding were widespread not only across sec-
tions but across participants as well. Seventy-six percent of participants increased
their score by at least 1 point, 49% increased their score by at least 5 points, and
27% increased their score by at least 10 points. Overall, there was a negative cor-
relation between a participant’s pretest score and his/her pre-post gain (r = −0.41)
Posttest
Pretest
−2.0 −1.5 −1.0 −0.5 0 0.5 1.0 1.5
Mean Section Score
2.0
Variation
Inheritance
Adaptation
Speciation
Domestication
Extinction
FIGURE 3.2 Mean scores on the individual sections of the comprehension assessment
(range = −5 to +5), ordered from smallest pre-post gain to largest. Pre-post gains were
statistically signi cant for all sections except Extinction.
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58 Folk Theories, Conceptual and Perceptual Constraints
such that participants with low pretest scores gained more points (or lost fewer
points) than participants with high pretest scores. This correlation was not due to a
ceiling effect among participants with high pretest scores, as all but one participant
with a positive pretest score could have improved his or her score by at least ve
points. Rather, instruction appears to have been more effective for participants who
entered the classroom with moderate to strong transformational misconceptions.
WITHIN-PARTICIPANT CONSISTENCY
One of the hallmarks of conceptual change is the degree to which an individual’s
beliefs about various domain-speci c phenomena cohere both before and after
the change (e.g., Au, Chan, Chan, et al., 2008; Smith, Solomon, & Carey, 2005;
Vosniadou & Brewer, 1992). To measure the coherence in participants’ beliefs, we
looked for within-participant consistency across different sections of the same
assessment. The average correlation among participants’ section scores was high
at both pretest (r = 0.44) and posttest (r = 0.39), with nearly all such correlations
proving statistically reliable. Furthermore, a factor analysis of participants’ scores
on the six different sections revealed one, and only one, factor capable of explaining
the majority of variance in those scores at both pretest (54%) and posttest (51%).
Thus, participants’ understanding of a diversity of evolutionary phenomena was
well described by a single factor; those who scored high on this factor demonstrated
consistently variational reasoning, whereas those who scored low demonstrated
consistently transformational reasoning (replicating Shtulman, 2006).
That said, participants who began the course with strong transformational beliefs
tended to end the course with a less coherent view of evolution, as evidenced both
by a signi cant decrease in number of participants who scored between −11 and −30
on the comprehension assessment (observable in Figure 3.1) and by a signi cant
decrease in the strength of the intercorrelations among participants’ section scores.
These participants apparently held “mixed” or “synthetic” theories of evolution,
reasoning about some phenomena on the basis of transformational principles and
others on the basis of variational principles. This outcome, though perhaps less than
ideal from an instructional point of view, mirrors that documented in domains like
cosmology (Vosniadou & Brewer, 1992; Samarapungavan, Vosniadou, & Brewer,
1996) and physiology (Astuti & Harris, 2008; Legare & Gelman, 2008), where knowl-
edge derived from intuition frequently con icts with that derived from testimony.
Cognitive Constraints on Acceptance
Both before and after the instruction participants were asked to rate their agree-
ment with ve statements of belief: (1) “Species have changed over time”; (2) “The
species in existence today have not always existed”; (3) “Natural selection is the best
explanation for how species adapt to their environment”; (4) “Natural selection is
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Cognitive Constraints on the Understanding and Acceptance of Evolution 59
the best explanation for the origin of new species”; and (5) “The origin of human
beings does not require a different explanation than the origin of other species.”
Participants used a 5-point scale, with 1 indicating “strongly disagree;” 2, “dis-
agree;” 3, “neutral;” 4, “agree,” and 5, “strongly agree.” The ordering of the state-
ments was determined by their controversiality, as public opinion polls have shown
that Americans are more accepting of microevolution than macroevolution and
more accepting of nonhuman evolution than human evolution (Scott, 2005).
Because the focus of our study was on measuring understanding, not acceptance,
our instrument for measuring acceptance was less comprehensive than those devel-
oped by other researchers (e.g., Rutledge & Warden, 1999). Nevertheless, participants’
agreement ratings were highly correlated across the ve statements of belief at both
pretest (r = 0.51) and posttest (r = 0.49), implying that the various ratings re ected a
single attitude or disposition toward the endorsement of evolutionary claims. Before
instruction, participants’ ratings averaged 4.2 across the ve statements of belief
(SD = 0.7); after instruction, they averaged 4.4 (SD = 0.6). This increase was statisti-
cally signi cant, though the magnitude of change (0.2) was small. Closer inspection
of the data revealed 14 participants whose preinstructional agreement ratings were at
ceiling and could not therefore have increased. Removing those participants from the
sample yielded a mean pre-post difference of 0.4, which is equivalent to a 10% increase
in agreement ratings. The magnitude of this increase is virtually identical to that docu-
mented by Ingram and Nelson (2006), which is noteworthy given that these authors
used a different instrument for measuring acceptance, a different curriculum for teach-
ing evolution, and a different participant sample (upper-level biology majors).
The number of participants who selected “agree” or “strongly agree” for each
statement of belief are displayed in Figure 3.3, at both pretest and posttest. Two
effects are observable. First, the number of participants who agreed with each
statement decreased from statement 1 (about species change) to statement 5 (about
human evolution), as predicted by the controversiality of statement content. Second,
FIGURE 3.3 Percent of participants who “agreed” or “strongly agreed” with the ve
statements of belief at pretest and posttest. Pre-post gains in percent agreement were
statistically signi cant for S2, S3, and S5.
S1 S2 S3 S4 S5
Statement of Belief
% Who “Agreed” or “Strongly Agreed”
100
90
80
70
60
50
Posttest
Pretest
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60 Folk Theories, Conceptual and Perceptual Constraints
the number of participants who agreed with each statement increased as a function
of instruction (though the increase was statistically reliable only for statements 2, 3,
and 5). The fact that instruction increased acceptance of statement 5 (“The origin
of human beings does not require a different explanation than the origin of other
species”) is particularly noteworthy, as it is this claim that Americans are least likely
to endorse (Miller et al., 2006; Newport, 2004).
Although most participants agreed with most statements, there was still suf cient
variation in participants’ agreement ratings (summed across the ve statements)
to compare them to their comprehension assessment scores. In contrast to previ-
ous studies that have found no correlation between understanding evolution and
accepting evolution (Bishop & Anderson, 1990; Brem, Ranney, & Schindel, 2003;
Demastes, Settlage, & Good, 1995; Lawson & Worsnop, 1992; Sinatra, Southerland,
McConaughy, & Demastes, 2003), the present study found strong correlations
between these two measures at both pretest (r = 0.55) and posttest (r = 0.46). In
other words, variationists were more likely than transformationists to endorse the
ve statements of belief at both assessment periods. These correlations may have
gone undetected in prior studies due to differences in how understanding was mea-
sured, how acceptance was measured, or both. They may also have gone undetected
due to insuf cient variation in those measures within the particular populations
under investigation. That said, the present study is not the rst, or the only, study
to have documented correlations between understanding and acceptance. Similar
ndings have been obtained by Nadelson and Sinatra (2009), Nehm, Kim, and
Sheppard (2009), and Rutledge and Warden (2000), all of which used different mea-
sures of understanding and different measures of acceptance than those used here.
Theoretical and Pedagogical Implications
Consistent with previous research (Shtulman, 2006; Shtulman & Schulz, 2008), par-
ticipants in the present study demonstrated pervasive, preinstructional misconcep-
tions of a transformational nature. Some of these misconceptions were corrected by
instruction, and some were not. Although pre-post gains in assessment scores were
modest in size, they were frequent in occurrence. A full 76% of participants increased
their score by 1 or more points, 49% increased their score by 5 or more points, and
27% increased their score by 10 or more points. This rate of change is unprecedented
in the evolution education literature (e.g., Bishop & Anderson, 1990; Demastes et al.,
1995; Jensen & Finley, 1995), which implies that teaching interventions targeted at
transformational misconceptions may be more successful than those that trace his-
torical changes in evolutionary thought (Jensen & Finley, 1995) or those that focus
strictly on natural selection (Demastes et al., 1995). This nding complements nd-
ings from other science education studies demonstrating that students’ misconcep-
tions must be adequately addressed before they can be replaced by new, accurate
conceptions (e.g., Moss & Case, 1999; Slotta & Chi, 2006; Smith, 2007; Vosniadou,
Ioannides, Dimitrakopoulou, & Papademetriou, 2001; Wiser & Amin, 2001).
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Cognitive Constraints on the Understanding and Acceptance of Evolution 61
One intriguing aspect of the change in participants’ understanding of evolu-
tion from before instruction to after is that participants made signi cant conceptual
progress in ve of the six areas tested. This nding was unexpected, as the teaching
intervention focused primarily on microevolution and only brie y touched on such
macroevolutionary topics as speciation and domestication. Apparently, participants
were able to apply some of their newfound understanding of microevolutionary
principles to their prior (inaccurate) understanding of macroevolutionary phenom-
ena. This spontaneous transfer of information from one set of beliefs to another
may have been a byproduct of their interrelatedness. In other words, the coherence
in participants’ preinstructional beliefs may have actually facilitated their revision
(see Au et al., 2008, and Slaughter & Lyons, 2003, for a similar pattern of results).
Another intriguing aspect of the change in participants’ understanding of evolution
is that it was accompanied by a change in their acceptance of evolutionary claims—
namely, increased understanding led to increased acceptance. This nding implies that
Americans’ skepticism toward evolution is rooted, at least in part, in a misunderstand-
ing of what evolution is. Rather than construe evolution as the selective propagation
of within-species variation, many appear to construe evolution as the uniform adapta-
tion of all individuals within a species. This construal is not only incorrect but is also
highly problematic for appreciating how biological phenomena bear on evolutionary
claims and how evolutionary claims make sense of biological phenomena.
As an illustration, consider the recent discovery that humans share over 80% of
their genes with mice (Waterston et al., 2002). This discovery is easily assimilated
by a variationist, who sees species as continuums of variation related by common
ancestry, but is not easily assimilated by a transformationist, who sees species as
discrete entities characterized by unique, nonoverlapping essences. A transforma-
tionist must either recast mice as the evolutionary forbearers of humans (as done
by Russell [2002], the San Francisco Chronicle reporter who asserted that “scientists
have found a wealth of common chemistry between human beings and our tiny,
four-legged ancestors”) or downplay the importance of genes in determining a spe-
cies’ identity (as done by McKie [2001], the London Observer reporter who asserted
that “environmental in uences are vastly more powerful [than genetic in uences] in
shaping the way humans act”).
This example highlights a particular means by which understanding might in u-
ence acceptance: evidential reasoning (Chinn & Brewer, 2001; Kuhn, 1991; Sa, Kelley,
Ho, & Stanovich, 2005). Changes in how students understand a particular theory can
lead to changes in how they evaluate data relevant to the theory, which, in turn, can
lead to changes in how well they think the theory is supported by evidence. Because
much of the evidence for evolution cannot be interpreted, let alone appreciated, with-
out an understanding of natural selection, we suspect that critics of evolution are
incapable of engaging with the very evidence they nd “unconvincing.” That said, we
did not assess our participants’ ability to evaluate novel evolutionary data or novel
evolutionary claims, so it remains an empirical question whether the relationship
between understanding and acceptance is indeed mediated by evidential reasoning. It
has been shown, however, that acceptance of evolution is signi cantly correlated with
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62 Folk Theories, Conceptual and Perceptual Constraints
an understanding of the nature of science, even when controlling for general interest
in science and past science education (Lombrozo, Thanukos, & Weisberg, 2008).
In a similar vein, it should be noted that our ndings, while implicating under-
standing evolution as an important in uence on accepting evolution, do not impli-
cate understanding as the only in uence. Religious commitments are certainly an
important in uence as well (see Brem, et al., 2003; Evans, 2001; Miller et al., 2006;
Poling & Evans, 2004). The in uence of such commitments was evident in the pres-
ent study from participants’ agreement ratings for two statements of belief regarding
explicitly religious matters: (1) “I believe in the existence of God” and (2) “I believe in
the existence of souls.” These ratings did not change as a function of instruction and,
when averaged together, were negatively correlated with participants’ mean agreement
ratings for the ve statements about evolution, both before instruction (r = −.33) and
after (r = −.32). They were also negatively correlated with participants’ assessment
scores before instruction (r = −.25) and after (r = −.33), indicating that, throughout
the duration of the study, religious participants were less likely than nonreligious
participants to understand evolutionary concepts and accept them as valid.
Clearly, multiple factors in uence an individual’s acceptance of evolution. An
understanding of evolution is, however, the main factor that science educators are
charged with changing. Although different researchers hold different opinions on
the question of whether science educators should advocate for evolution rather
than merely explain it (Smith, Siegel, & McInerney, 1995), our own opinion is that
fostering an acceptance of evolution is crucial to the long-term advancement of sci-
enti c literacy and scienti c reasoning. Accordingly, we see the correlation between
understanding evolution and accepting evolution as highly informative to those
goals. Although more research needs to be done to determine why and how this
relationship obtains, one straightforward implication of our ndings is that improv-
ing evolution education in the United States could help to increase the U.S. public’s
acceptance of evolution to a level more typical of other rst-world nations.
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