DOI: 10.1126/science.1155805
, 1844 (2008); 321Science
et al.Charles Efferson,
Favoritism
The Coevolution of Cultural Groups and Ingroup
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were inconspicuous on routine histological in-
spection (Fig. 3B, top left). Staining with anti-
body to K8 facilitated the detection of the ectopic
foci and confirmed their epithelial origin, where-
as in the lungs of uninjected mice only the
bronchial epithelium was stained (Fig. 3B, right).
To determine whether the ectopic foci of
normal epithelial cells persist and grow in the
foreign environment of the lung, we counted the
total number of discrete foci in lung sections at
different times after injection and looked at pro-
liferation markers in these foci. The total number
of foci found in lung sections from C57BL6/J
recipients injected with 4 × 10
5
syngeneic mam-
mary cells was similar in the animals surveyed at
3 weeks (n = 3 mice) and those surveyed at 10
weeks (n = 3 mice) after injection (42 T 7 and 56 T
22 in 10 paraffin lung sections, respectively).
Moreover, the efficiency with which the wild-
type cells were able to form these small epithelial
clusters was similar to the ef ficiency with which
we were able to induce ectopic tumors after inject-
ing cells from doxycycline-naïve TOM;TOR;MTB
donors [1.2 T 0.4 (SD) versus 1.7 T 1.4 (SD) per
10,000 cells injected, n = 6 and 8 mice, respec-
tively; measured as described in (14)]. This result
strongly argues that most or all of the mammary
cells that are capable of surviving in the lung are
able to respond to the initiating oncogene ex-
pression by forming an ectopic mammary tumor .
In both nontransgenic C57BL6/J– and b-actin-
GFP–derived foci, occasional cells displayed
mitotic activity (Fig. 3C). Consistent with this
result, the green foci found under excitation light in
the lungs of animals injected with mammary cells
from b-actin-GFP mice 16 weeks after injection
were larger in size than those found in recipients of
the same preparation 1 week after injection (Fig.
3D). Ectopic epithelial outgrowths contained K8-
and SMA-positive cells, such as observed in intact
mammary glands (fig. S4A), and the outgrowths
occasionally displayed a glandular appearance.
Despite prolonged residenc e in the lun g (up to 4
months), the green cells recovered from the re-
cipients’ lungs were competent to form hollow
acinar structures in three-dimensional morphogen-
esis assays (fig. S4B) and secondary mammary
outgrowths in cleared fat pads of Rag1
−/−
females
(Fig. 3E). These findings establish that the ectopic
cells residing in the lungs are indeed of mammary
origin, that they are viable and mitotically active,
and that at least some of them are multipotent and
able to support full mammary development.
The experiments described here show that, in
the absence of an active oncogene, dissociated
cells from an untransformed mouse mammary
gland can establish residence in the ectopic envi-
ronment of the lung, grow slowly, and remain
clinically undetectable after IV injection. The same
cells can give rise to metastatic malignancies upon
activation of oncogenes that can produce mam-
mary tumors in an intact gland. It is w idely ac-
knowledged that multiple steps are required to
establish metastases, including intravasion of cells
from primary tumors into blood vessels or lym-
phatics; survival in the circulation, extravasation,
and establishment of cells at ectopic sites; and
malignant growth. Because we have injected mam-
mary cells from transgenic mouse donors into tail
veins of recipient mice, we have not examined the
requirements for intravasation. We have, however ,
demonstrated that activated oncogenes and cellular
transformatio n are not required for any of the
subsequent steps, save for malignant growth at
ectopic sites. These findings indicate that properties
inherent in normal cells are sufficient for negotiat-
ing a substantial portion of the metastatic cascade.
Considerable experimenta l and clinical evidence
favors the idea that cells from small cancers may
spread to distant sites early in tumorigenesis and
account for dormancy and late relapse in human
breast cancer (2, 18). Although we do not know
whether premalignant cells can enter the systemic
circulation during these early stages and become
sources of later metastatic tumors, our observations
argue that this hypothesi s sho uld be tested. The
finding that metastatic disease can arise from un-
transformed mammary cells in the circulation re-
fines our conception of cancer progression, and
suggests that each step in the metastatic cascade
should be examined to establish its functional
requirements, including those performed by nor-
mal cells. Such functions might be susceptible to
inhibitory strategies that can ablate disseminated
pre-malignant or malignant cells and thereby
diminish the mortality caused by cancer.
References and Notes
1. D. Hanahan, R. A. Weinberg, Cell 100, 57 (2000).
2. Y. Husemann et al., Cancer Cell 13, 58 (2008).
3. G. P. Gupta et al., Nature 446, 765 (2007).
4. P. B. Gupta et al., Nat. Genet. 37, 1047 (2005).
5. K. A. Hartwell et al., Proc. Natl. Acad. Sci. U.S.A. 103,
18969 (2006).
6. M. Jechlinger et al., J. Clin. Invest. 116, 1561 (2006).
7. R. S. Muraoka et al., J. Clin. Invest. 109, 1551 (2002).
8. Y. Kang et al., Cancer Cell 3, 537 (2003).
9. A. Muller et al., Nature 410, 50 (2001).
10. J. Yang et al., Cell 117, 927 (2004).
11. A. E. Karnoub et al., Nature 449, 557 (2007).
12. R. Bernards, R. A. Weinberg, Nature 418, 823 (2002).
13. K. Podsypanina, K. Politi, L. J. Beverly, H. E. Varmus,
Proc. Natl. Acad. Sci. U.S.A. 105, 5242 (2008).
14. Materials and methods are available as supporting
material on Science Online.
15. P. Mombaerts et al., Cell 68, 869 (1992).
16. E. J. Gunther et al., FASEB J. 16, 283 (2002).
17. M. Okabe, M. Ikawa, K. Kominami, T. Nakanishi,
Y. Nishimune, FEBS Lett. 407, 313 (1997).
18. J. A. Aguirre-Ghiso, Nat. Rev. Cancer 7, 834 (2007).
19. We thank M. A. Melnick, G. Sanchez, A. Giannakou, and
J. Demers for expert handling of the mouse colony;
L. Chodosh for providing MMTV-rtTA transgenic mice;
D. Felsher and J. M. Bishop for providing TetO-MYC
transgenic mice; A. Olshen for assistance with statistical
analysis; and L. K. Tan for assistance with histological
analysis. Supported in part by awards from NIH (K01
CA118731 to K.P., P01 CA94060 to H.V., and R24
CA83084 and P30-CA 08748, which provides partial
support for core facilities used in conducting this
investigation), the Martell Foundation (to H.V.), and the
U.S. Department of Defense (W81XWH-05-1-0220 to M.J.).
Supporting Online Material
www.sciencemag.org/cgi/content/full/321/5897/1841/DC1
Materials and Methods
Figs. S1 to S4
Table S1
References
10 June 2008; accepted 11 August 2008
10.1126/science.1161621
The Coevolution of Cultural
Groups and Ingroup Favoritism
Charles Efferson,
1,2
*
Rafael Lalive,
3
Ernst Fehr
1,4
Cultural boundaries have often been the basis for discrimination, nationalism, religious wars,
and genocide. Little is known, however, about how cultural groups form or the evolutionary forces
behind group affiliation and ingroup favoritism. Hence, we examine these forces experimentally and
show that arbitrary symbolic markers, though initially meaningless, evolve to play a key role in cultural
group formation and ingroup favoritism because they enable a population of heterogeneous
individuals to solve important coordination problems. This process requires that individuals differ in
some critical but unobservable way and that their markers be freely and flexibly chosen. If these
conditions are met, markers become accurate predictors of behavior. The resulting social environment
includes strong incentives to bias interactions toward others with the same marker, and subjects
accordingly show strong ingroup favoritism. When markers do not acquire meaning as accurate
predictors of behavior, players show a markedly reduced taste for ingroup favoritism. Our results
support the prominent evolutionary hypothesis that cultural processes can reshape the selective
pressures facing individuals and so favor the evolution of behavioral traits not previously advantaged.
A
cultural g roup is a group of people who
share a set of beliefs, behavioral norms,
and behavioral expectations that is rec-
ognizably different from those of other groups (1).
Beliefs, norms, and expectations, however, are
often not directly observable, and so by them-
selves they do not provide a practical basis for
identifying cultural groups in everyday social in-
teractions. Nonetheless, cultural groups are fre-
quently identifiable through ethnic markers, which
are arbitrary but observable traits like dress,
dialect, and body modification that symbolically
and conspicuously signal group affiliation (1–5).
Symbolic traits of this sort can be crucial to so -
cial and economic outcomes. When ethnic markers
covary with other cultural traits, individuals can
26 SEPTEMBER 2008 VOL 321 SCIENCE www.sciencemag.org
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potentially use markers to everyone’s mutual ad-
vantage as indicators of what would otherwise be
unobservable variation in beliefs, norms, and ex-
pectations. More nefariously, ethnic markers can
lead to segregation, ethnic discrimination, and per-
sistent inequality, even in the paradoxical cases
when everyone prefers integration (6–8)orwhen
ethnicity indicates nothing about competence in a
given domain (9, 10). Indeed, parochialism and prej-
udice often mar intergroup relations. People show
favoritism toward ingroup members and indiffer -
ence, hostility , or mistrust toward outgroup mem-
bers (11–19). They do so even when groups are
transient and group boundaries rest on the flimsiest
of distinctions among individuals (15, 20–22).
These findings have potentially broad significance
because recent theoretical research has closely and
surprisingly tied outgroup hostility to the evolution
of human prosociality within groups (23, 24).
None of this, however , explains how a group
gets to be a group and why. The long tradition
of empirical research on intergroup relations
(11–22, 25) includes two basic approaches to
defining groups. Studies have either used pre-
existing cultural groups, which formed beyond
the ken of the studies in question, or subjects
were assigned to groups exogenously as part of
an experiment involving the effects of social
categorization. Th ese m ethods can be powerful
for many questions (12, 16), but they cannot
expose the mechanisms behind the formation of
cultural groups. These mechanisms also repre-
sent a gap in evolutionary theories of human
prosociality. Although the initial evolution of
cultural groups may have little to do with co-
operation, much of the theory on the evolution of
human prosociality relies heavily on the obser-
vation that human populations are subdivided
into cultural groups (23, 24, 26). This theoretical
work, however, simply imposes the required pop-
ulation structure exogenously . The endogenous
formation of cultural groups represents a plausi-
ble route to the required population structure that
figures prominently but remains unexplained in
evolutionary theories of human prosociality.
We conducted a set of experiments to identify
the conditions required for cultural groups to form
endogenously and for subjects to show ingroup
favoritism in their subsequent social interactions.
We used neither preexisting cultural groups nor
groups created exogenously by the experimenter.
Our task instead was to see if and when sym-
bolically marked groups form endogenously and
whether their formation can lead to a preference
for interactions with others having the same
symbolic marker. This preference was our oper-
ational measure of ing roup favoritism in the ex-
periment, and more generally such preferences
can limit social interactions across cultural bound-
aries and potentially play a key role in the devel-
opment of ethnocentric attitudes (27). If such a
preference were to emer ge endogenously in our
setting, the result would support a central hypoth-
esis in evolutionary social science (27–31). This
hypothesis posits that a cultural evolutionary pro-
cess can modify the selective environment facing
individuals and so lead to the evolution, whether
cultural o r genetic, of traits that were not previ-
ously advantageous. In our case, the question is
whether the evolution of cultural groups during an
experiment can reconstitute the social environ-
ment to benefit ingroup favoritism in a way that
did not obtain at the beginning of the experiment.
In theory, cultural groups form when variation
in an unobservable but socially critical variable be-
comes manifest. Consider a population of players
pla yi ng a simultaneous, two-person coordination
game with multiple equilibria. Players can choose
behavior A or B. If two players meet and choose
the same behavior, a large payoff results. If they
choose different behaviors (32), a small payoff
results. Some players expect to coordinate on A,
others on B. If players with different expectations
meet, an information problem results. One simply
has to play the odds and risk miscoordinating with
someone who has incompatible expectations. This
kind of problem is general. Variation in behavioral
norms and expectations is widespread (1, 33, 34),
and the mixing of people with different expec-
tations occurs frequently (1, 35, 36). This mixing,
however, creates the potential for people with dis-
cordant social expectations to meet, interact, and
miscoordinate. Variation in expectations, however ,
is not enou g h for the exi st en c e of cult ur al groups
because this variation is not directly observable.
Symbolic markers can change matters greatly,
but only if they covary with expectations and by
extension behavior. To illustrate, let players in
our coordination game wear shirts with either
triangles or circles. The shape on one’s shirt does
not af fect payoffs, and so it fills the theoretical
role of a symbolic marker . Consider a hypothetical
population of 100 people, 50 of whom expect to
coordinate on A and 50 on B. In addition, the 50
players who expect to coordinate on A have
triangles on their shirts, and the 50 players who
expect to coordinate on B have circles. The dis-
tribution of behavior -marker types in the popu-
lation is consequently 50 (A, ▲) ind ividuals, 0
(A, ●) individuals, 0 (B, ▲) individuals, and 50
(B, ●) individuals. The covariation between be-
havior and marker is at its maximum possible
value in this example, and the markers perfectly
reveal expectations and their associated behav-
iors in the coordination game. More generally,
when covariation characterizes the distribution of
behavior-marker types, the observable markers
allow one to draw statistical inferences about
what is unobservable but really important, namely ,
behavioral expectations in a social setting with
multiple equilibria. When this is true, interacting
preferentially with others having the same marker
reduces the probability of miscoordination and
1
Institute for Empirical Research in Economics, University of
Zürich, Blümlisalpstrasse 10, 8006 Zürich, Switzerland.
2
Santa
Fe Institute, NM 87501, USA.
3
Department of Economics,
University of Lausanne, 1015 Lausanne, Switzerland.
4
Colle-
gium Helveticum, 8092 Zürich, Switzerland.
*To whom correspondence should be addressed. E-mail:
efferson@iew.uzh.ch
Fig. 1. Summary of linked choices for the marker-
randomized (gray) and marker-maintained (black)
treatments. The behavior and marker chosen in
stage 1 are coded as either linked or unlinked rel-
ative to the behavior and marker chosen in stage
1 of the previous period. Proportions are plotted for
the cases in which the player miscoordinated (M) in
the previous period, coordinated on the suboptimal
(C/S) behavior (i.e., A i n subpopulation 2 or B in
subpopulation 1), and coordinated on the optimal
(C/O) behavior (i.e., A in subpopulation 1 or B in
subpopulation 2).
Fig. 2. (A) The informational content of the
marker. The graph shows the mean magnitude of
the covariance between behavior and marker in a
subpopulation relative to the theoretical maximum
for the marker-randomized (line with filled circles)
and the marker-maintained (solid line) treatments.
The period trend for marker-randomized is not
significant [Newey-West (40) regression, maximal
lag of 10, t test, P =0.368],whereasitishighly
significant for the marker-maintained treatment
(Newey-West, lag of 10, t test, P <0.001).(B)
Ingroup favoritism, as indicated by the proportion
of players requesting a partner with the same
shape. The marker-randomized period trend is not
significant (Newey-West, lag of 10, t test, P = 0.868).
The marker-maintained period trend i s highly
significant (Newey-West, lag of 10, t test, P < 0.001),
leading to large differences in ingroup favoritism
across treatments.
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increases expected payoffs. The puzzle, however,
is how to get strong covariation endogenously in
decentralized societies under limited information
about the distribution of behavior-marker combi-
nations. How does symbolic meaning emerge in
the absence of fiat? Interestingly , mixing players
with different expectations, which creates the orig-
inal problem, also creates a potential solution. It
does so by producing small amounts of covariation
(37) that can feed back into the system and accu-
mulate dynamically (38, 39).
The accumulation of covariation requires more
than mixing, however, because mixing by itself
often creates only a small amount of covariation
between behavior and marker (38). During our
experiment, individuals did not have information
about the aggregate distribution of behavior-marker
combinations, and thus it would have been difficult
or impossible to recognize an initially weak rela-
tion between behavior and marker . Covariation
can increase, however, if individuals link behav-
iors and markers in specific ways. Linkage refers
to a tendency for an individual either to retain both
her current behavior and marker or to change both
her behavior and marker; what an individual does
not do is change one trait but not the other. Link-
age is crucial because it preserves the covariation
created by earlier mixing, while continued mixing
creates additional covariation that feeds back into
the system and gets added to existing covariation.
The result is that the total covariation accumulates,
and this increases the economic incentives to
interact with others having the same marker . For
covariation to accumulate, however, linkage should
not be ind iscriminate. Rather , theory suggests it
should be more prevalent in specific situations
like those in which individuals acquire information
about economically successful behavior-marker
combinations (38, 39). If individuals, however,
never link under any circumstances because they
choose behaviors and markers independently,
covariation is constantly destroyed, and markers
cannot become strongly associated with behavior .
We conducted the following experiment to see
if players would show a preference for (i) linking
behaviors and markers and for (ii) interacting with
partners displaying the same marker . In addition,
we wanted to know (iii) whether linkage, if present,
would generate sizable covariation between behav-
ior and marker , which would then enable subjects
to increase coo rdination via ingroup favoritism.
Players were assigned to one of multiple populations
of 10. We randomly subdivided these 10 players
into two subpopulations of 5. Players within a
subpopulation played one of two coordination
games (table S1). Each game had two pure-strategy
equilibria, and thus players had to solve a co-
ordination problem. Both games had two behaviors
to choose from, A and B, but in subpopulation 1,
coordinating on A (41 points for each player
paired with another playing A) was better than
coordinating on B (21 points for each player
paired with another playing B), whereas in sub-
population 2, coordinating on B (41 points) was
better than coordinatin g on A (21 points). Mis-
coordinating in either subpopulation brought a
small payoff (1 point). Payoffs were designed to
draw players in different subpopulations toward
different behaviors and so mimic the variation in
norms, preferences, and expectations that often
exists because of historical separation or impor-
tant but unobservable environmental differences.
To create a persistent coordination problem,
players from the different subpopulations were
mixed, and they were never told to which sub-
population they were assigned. If players had re-
mained in their initial subpopulations, the game
would have posed little problem. Players would
have soon figured out their respective situations,
and presumably players in subpopulation 1 would
have only chosen A, whereas players in subpopu-
lation 2 would have only chosen B. Each period,
however, a randomly selected player from subpop-
ulation 1 and a randomly selected player from sub-
population 2 switched subpopulations. All players
knew this would happen, but no one knew which
two players had switched. In sum, each player had
a strong incentive to develop accurate expe ctations
about her current subpopulation, but from time to
time she found herself in a new situation where
her social expectations ran askew of local norms.
Players could also condition social interactions
on symbolic markers. In each period, each player
chose one of two shapes, ▲ or ●.Aplayer’s payoff
did not directly depend on her shape, but players
could use shapes to influence with whom they would
play the coordination game (39). The experiment
lasted 80 periods. Each period proceeded as follows.
Stage 1. Each player chose a payoff-relevant
behavior , A or B, for the coordination game and
a payoff-irrelevant shape, ▲ or ●.
Stage 2. An unidentified player from each
subpopulation switched subpopulations.
Fig. 3. Payoff proportions in the marker-
maintained treatment (A)andthemarker-
randomized treatment (B). The graphs
show the distribution of players by period
coordinating on the optimal behavior (black)
given the subpopulation (A in 1, B in 2),
coordinating on the suboptimal behavior
(gray) given the subpopulation (A in 2, B
in 1), or miscoordinating (white). See
supporting online text for a multinomial
regression analysis.
Fig. 4. Ingroup favoritism for the
modified marker-maintained (solid
line), payoff-equivalent (dashed line),
and fixed-marker (line with open
circles) treatments. Newey-West (40)
regressions indicate that the modified
marker-maintained treatment began
with more assortment than the other
two treatments, and the differences
across treatments increased through
time. Comparison of regression results
for the modified marker-maintained
and payoff-equivalent treatments shows
that the intercepts are significantly
different (z test, P <0.001),asare
the period trends (z test, P <0.001).
Comparison of results for modified
marker-maintained and fixed-marker shows that both the intercepts (z test, P <0.001)andperiodtrends
(z test, P = 0.019) are significantly different.
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Stage 3. Each player indicated whether she
wanted to play the coordination game with (i) a
randomly selected player with the same shape
from her subpopulation or (ii) any randomly
selected player from her subpopulation.
Stage 4. Each player was paired using her choice
in stage 3 and received a payoff based on her behav-
ior, her partner’s behavior, and their subpopulation.
To clarify our discussion of the results, when
there was little or no covariation between behav-
ior and marker, we will call a set of individuals
whosharedthesamemarkera“trivial” group.
These groups were trivial in the sense that the
markers partitioned the population into circles
and triangles, but these markers did not reliably
reflect any underlying variables affecting pay-
offs. We will call a group “cultural,” in contrast,
only when a set of individuals shared the same
marker after a sustained increase in the aggregate
covariation between behavior and marker . Groups
were cultural in this case because markers did not
simply partition the population into circles and
triangles; they also, on average, partitioned the pop-
ulation into those who expected to coordinate on
A versus those who expected to coordinate on B.
The experiment consisted of two treatments. In
the marker-randomized treatment, each player
was randomly assigned a shape after stage 2 re-
gardless of the shape chosen in stage 1. In the
marker-maintained treatment, each player retained
her chosen shape. The marker-randomized treat-
ment was a control treatment in which marker
randomization precluded the possibility of the
marker becoming an accurate predictor of behavior .
The comparison between the two treatments shows
(i) how much informational content the marker
acquired in the marker-maintained treatment
beyond the baseline when markers were randomly
assigned and (ii) whether any differences in
informational content translated into differences
in the preference for ingroup favoritism.
Subpopulations were not equiv alent to sym-
bolically marked groups, whether trivial or cultural.
In a given period, a player’s subpopulation was the
pool of players available for social interaction. A
symbolically marked group, in contrast, was the set
of players from the entire population with the same
marker. In short, the division of players into two
subpopulations, one favoring behavior A and the
other behavior B, sustained variation in norms and
expectations. This variation, however, was not ob-
servable, and so it could not by itself serve as a
means of distinguish ing one group from a nother .
Symbolic markers, in contrast, were observable
traits, and they could serve as a means of distin-
guishing one group from another. Markers, how-
ever , did not bear any necessary relation to behavior
and subpopulation. The significance of markers, in
essence, could only emerge during the experiment
as a result of player choices. Markers had the poten-
tial to become the basis for determining cultural
group affiliation ex post, and indeed that was our
question, but they were devoid of content ex ante.
For a sustained increase in covariation, indi-
viduals have to link the behavioral and marker
dimensions. We coded behavior-marker choices
fromstage1ofperiods2to80as“linked” or
“unlinked.” A linked choice was one in which a
player either retained her behavior and chosen
marker from the previous period or changed
both. An unlinked choice was when she changed
her behavior or marker but not both. A strong
preference toward linked choices was present in
general (Fig. 1), but it was significantly stronger
in the specific case when a player received the
optimal coordination payoff in the previous
period (conditional logit, P < 0.001, table S3).
These linked choices consisted almost exclusively
of choices in which the player retained her be-
havior and marker from the previous period (figs.
S1 and S2). In addition, although the preference
for linked choices after coordinating on the opti-
mal behavior was present in both treatments, it
was significantly stronger in the marker-maintained
treatment (conditional logit, P <0.001,tableS3).
These results indicate that players showed a gen-
eral tendency to couple behaviors and markers.
This tendency , however, was strongest when a
player hit upon a successful behavior-marker com-
bination, and it was further reinforced and ampli-
fied in the marker-maintained treatment when the
marker was not prevented from acquiring meaning.
Substantial linkage at the individual level
produces covariation between behavior and marker
at the aggregate level. If strong enough and
specific enough, the linkage exhibited in the
experiment should have produced a significant
increase in covariation in the marker-maintained
treatment, when it was possible, but not in the
marker-randomized treatment, w hen it was not.
Even though linkage was present, however,
covariation should have been similar in the two
treatments at the beginning of the experiment,
before covariation had time to accumulate. Only
in later periods should the covariation have been
significantly higher in the marker-maintained
treatment. The aggregate covariation between
behavior and marker in deed followed this dy-
namical pattern. During the first five periods, the
covariation was not different in the two treatments
(Welch two-sample t test, df = 7.01, two-sided P =
0.68, Fig. 2A), whereas in the final five periods,
the covariation was significantly higher in the
marker-maintained case (Welch two-sample t test,
df = 12.107, two-sided P < 0.001, Fig. 2A).
Covariation thus strongly and significantly increased
in the marker-maintained treatment but not in the
marker-randomized case. This led to a strong
overall treatment dif ference in the accumulation
of the markers’ predictive power [Newey-West
regression (40), P < 0.001, Fig. 2A].
The presence of covariation does not mean
that players will exploit it by assorting into groups
characterized by shared markers. Players could
simply fail to recognize the association between
behavior and marker as it developed, or they could
fail to recognize its usefulness. Nonetheless, play-
ers exhibited an increasing inclination to request
partners with the same shape as covariation accu-
mulated. Throughout the marker-randomized treat-
ment, players requested same-shape partners roughly
50% of the time (Fig. 2B), a result consistent with
indifference concerning the two intera ct i o n pol i -
cies. In the marker-maintained treatment, however,
players increasingly requested partners having the
same shape as time passed. This increase was
highly significant (Newey-West regression, P <
0.001), and the vast majority of players (87%) re-
quested partners with the same shape i n the final
five periods (Fig. 2B), indicating that ingroup favor-
itism became an almost universal phenomenon.
In the presence of covariation, this kind of
ingroup favoritism should lead to more coordina-
tion and improved payoffs, but the strength of the
effect will vary with the degree of covariation and
preferential assortment. A calculation of the mean
payoff over periods for each subject shows that
payoffs were significantly different across the
two treatments. The mean payoff in the marker-
randomized treatment was 20.819 points, and it
was 27.454 in the marker-maintained treatment
(Welch two-sample t-tes t, df = 88.912, two-sided
P < 0.001). This difference, however, depended spe-
cifically on the dynamical increase in the markers’
predictive content in the marker-maintained treat-
ment, and this fact is central to our finding that the
evolution of cultural groups changed the incentives
associated with ingroup favoritism. Specifically,
for those players who requested a partner with the
same shape, the mean payoff per period was not
significantly different between the two treatments
in the first five periods (Welch two-sample t-test,
df = 123.139, two-sided P = 0.1638), whereas it
was highly significant in the final five periods
(Welch two-sample t test, df = 105.733, two-sided
P < 0.001). The higher overall payoffs in the
marker-maintained treatment stemmed from an
increase in coordinating on the optimal behavior
in each of the two subpopulations (Fig. 3). A de-
tailed analysis formally confirms the substantial and
robust payoff effect that resulted from assorting on
markers in the marker-maintained treatment (37).
These results show how the evolution of cul-
tural groups can reconstitute the social environment
and produce selection for an ingroup bias that
was not initially advantageous. If selective pressures
of this sort were common in past human societies,
a plausible outcome would arguab ly be a rel-
atively inflexible bias leading individuals to prefer
others similar in some symbolic dimension. This
idea is consistent with much research showing an
astonishing willingness for subjects to exhibit in-
group favoritism when groups are based on trivial,
short-lived distinctions (12, 15, 16, 21, 22). Fo r
our study , this could mean that the marker-based
assortment we documented largely reflected a
readiness to favor the ingroup that was already in
place when the subjects came to the lab, and it did
not stem from the endogenous formation of cul-
tural groups during the experiment. In particular,
although we found a pronounced difference in
assortment dynamics in our two treatments, we
still found a strong tendency to assort in the marker-
randomized treatment. This assortment was rel-
atively meaningless with respect to payoffs, but
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