[Paleopsych] Paul R. Ehrlich, Simon A. Levin: The Evolution of Norms
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Paul R. Ehrlich, Simon A. Levin: The Evolution of Norms
PLoS BIOLOGY: a peer-reviewed, open-access journal from the PUBLIC LIBRARY of
Volume 3 | Issue 6 | JUNE 2005
[Here it is again, with all the footnotes. Sorry I wasn't paying attention
when I sent it two days ago.]
Essays articulate a specific perspective on a topic of broad interest
Paul R. Ehrlich is with the Department of Biological Sciences,
Stanford University (Stanford, California, United States of America).
Simon A. Levin is with the Department of Ecology and Evolutionary
Biology, Princeton University (Princeton, New Jersey, United States of
*To whom correspondence should be addressed. E-mail:
slevin at princeton.edu
Published: June 14, 2005
Citation: Ehrlich PR, Levin SA (2005) The Evolution of Norms. PLoS
Biol 3(6): e194
Over the past century and a half, we have made enormous progress in
assembling a coherent picture of genetic evolution--that is, changes
in the pools of genetic information possessed by populations, the
genetic differentiation of populations (speciation) (see summaries in
[1,2]), and the application of that understanding to the physical
evolution of Homo sapiens and its forebears (; e.g., [4,5]). But
human beings, in addition to being products of biological evolution,
are--vastly more than any other organisms--also products of a process
of "cultural evolution." Cultural evolution consists of changes in the
nongenetic information stored in brains, stories, songs, books,
computer disks, and the like. Despite some important first steps, no
integrated picture of the process of cultural evolution that has the
explanatory power of the theory of genetic evolution has yet emerged.
Much of the effort to examine cultural evolution has focused on
interactions of the genetic and cultural processes (e.g., , see
also references in ). This focus, however, provides a sometimes
misleading perspective, since most of the behavior of our species that
is of interest to policy makers is a product of the portion of
cultural evolution  that occurs so rapidly that genetic change is
irrelevant. There is a long-recognized need both to understand the
process of human cultural evolution per se and to find ways of
altering its course (an operation in which institutions as diverse as
schools, prisons, and governments have long been engaged). In a world
threatened by weapons of mass destruction and escalating environmental
deterioration, the need to change our behavior to avoid a global
collapse  has become urgent. A clear understanding of how cultural
changes interact with individual actions is central to informing
democratically and humanely guided efforts to influence cultural
evolution. While most of the effort to understand that evolution has
come from the social sciences, biologists have also struggled with the
issue (e.g., p. 285 of , [11-16], and p. 62 of ). We argue
that biologists and social scientists need one another and must
collectively direct more of their attention to understanding how
social norms develop and change. Therefore, we offer this review of
the challenge in order to emphasize its multidisciplinary dimensions
and thereby to recruit a broader mixture of scientists into a more
integrated effort to develop a theory of change in social norms--and,
eventually, cultural evolution as a whole.
What Are the Relevant Units of Culture?
Norms (within this paper understood to include conventions or customs)
are representative or typical patterns and rules of behavior in a
human group , often supported by legal or other sanctions. Those
sanctions, norms in themselves, have been called "metanorms" when
failure to enforce them is punished [17,19,20]. In our (liberal)
usage, norms are standard or ideal behaviors "typical" of groups.
Whether these indeed represent the average behaviors of individuals in
the groups is an open question, and depends on levels of conformity.
Conformity or nonconformity with these norms are attributes of
individuals, and, of course, heterogeneity in those attributes is
important to how norms evolve. Norms and metanorms provide a cultural
"stickiness" (p. 10 of ) or viscosity that can help sustain
adaptive behavior and retard detrimental changes, but that equally can
inhibit the introduction and spread of beneficial ones. It is in
altering normative attitudes that changes can be implemented.
Here, we review the daunting problem of understanding how norms
change, discuss some basic issues, argue that progress will depend on
the development of a comprehensive quantitative theory of the
initiation and spread of norms (and ultimately all elements of
culture), and introduce some preliminary models that examine the
spread of norms in space or on social networks. Most models of complex
systems are meant to extract signal from noise, suppressing extraneous
detail and thereby allowing an examination of the influence of the
dominant forces that drive the dynamics of pattern and process. To
this end, models necessarily introduce some extreme simplifying
Early attempts to model cultural evolution have searched for parallels
of the population genetic models used to analyze genetic evolution. A
popular analogy, both tempting and facile, has been that there are
cultural analogues of genes, termed "memes" [22,23], which function as
replicable cultural units. Memes can be ideas, behaviors, patterns,
units of information, and so on. But the differences between genes and
memes makes the analogy inappropriate, and "memetics" has not led to
real understanding of cultural evolution. Genes are relatively stable,
mutating rarely, and those changes that do occur usually result in
nonfunctional products. In contrast, memes are extremely mutable,
often transforming considerably with each transmission. Among humans,
genes can only pass unidirectionally from one generation to the next
(vertically), normally through intimate contact. But ideas (or
"memes") now regularly pass between individuals distant from each
other in space and time, within generations, and even backwards
through generations. Through mass media or the Internet, a single
individual can influence millions of others within a very short period
of time. Individuals have no choice in what genes they incorporate
into their store of genetic information, and the storage is permanent.
But we are constantly filtering what will be added to our stored
cultural information, and our filters even differentiate according to
the way the same idea is presented [24,25]. People often deliberately
reduce the store of data (for example, when computer disks are erased,
old books and reprints discarded, etc.), or do so involuntarily, as
when unreinforced names or telephone numbers are dropped from memory.
Such qualitative differences, among others, ensure that simple models
of cultural evolution based on the analogy to genetic evolution will
fail to capture a great deal of the relevant dynamics. A model
framework addressed to the specific challenges of cultural evolution
In the models discussed below, the most basic assumption is that the
spread (or not) of norms shares important characteristics with
epidemic diseases. In particular, as with diseases, norms spread
horizontally and obliquely , as well as vertically, through
infectious transfer mediated by webs of contact and influence. As with
infectious diseases, norms may wax and wane, just as the popularity of
norms is subject to sudden transitions . On the other hand, there
are unique features of cultural transmission not adequately captured
by disease models, in particular the issue of "self-constructed"
knowledge, which has long been a source of interest, and the
development of problem-solving models in psychology ([26, 27]; D.
Prentice, personal communication). New syntheses are clearly required.
Substantial progress has been made toward the development of a
mathematical theory of cultural transmission, most notably by
Cavalli-Sforza and Feldman , and Boyd and Richerson .
Cavalli-Sforza and Feldman consider the interplay between heritable
genetic change and cultural change. This is an important question,
addressed to the longer time scale, with a view to understanding the
genetic evolution of characteristics that predispose individuals to
act in certain ways in specified situations. For many of the phenomena
of interest, however, individual behaviors have not evolved
specifically within the limited context of a single kind of challenge,
but in response to a much more general class of problems. Efforts to
provide genetic evolutionary explanations for human decisions today
within the narrow contexts in which they occur may be frustrated
because generalized responses to evolutionary forces in the distant
past have lost optimality, or even adaptive value. Extant human
behaviors for example may be the relics of adaptations to conditions
in the distant past, when populations were smaller and technology less
advanced. Attempts to understand them as adaptive in current contexts
may therefore be futile. Thus, we prefer to take the genetic
determinants of human behavior (that, for example, we react strongly
to visual stimuli) as givens, and to ask rather how those initial
conditions shape individual and social learning . Similar efforts
have been undertaken by others, such as Henrich and Boyd  and
Kendal et al. .
The sorts of models put forth by Cavalli-Sforza and Feldman, Boyd and
Richerson, and others are a beginning towards the examination of a
colossal problem. To such approaches, we must add efforts to
understand ideation (how an idea for a behavior that becomes a norm
gets invented in first place), and filtering (which ideas are accepted
and which are rejected). How many ideas just pop up in someone's brain
like a mutation? How many are slowly assembled from diverse data in a
single mind? How many are the result of group "brainstorming?" How,
for example, did an idea like the existence of an afterlife first get
generated? Why do ideas spread, and what facilitates or limits that
spread? What determines which ideas make it through transmission
filters? Why are broadly held norms, like religious observance, most
often not universal (why, for instance, has atheism always existed
[29,30])? Ideas may be simply stated, or argued for, but transmission
does not necessarily entail the reception or adoption of behaviors
based on the idea, e.g., . What we accept, and what gets stored in
long-term memory, is but a tiny sample of a bombardment of candidate
ideas, and understanding the nature and origin of filters is obviously
one key to understanding the life spans of ideas and associated
behaviors once generated.
The Emergence of Higher-Level Structure: Some Simple Models
Our filters usually are themselves products of cultural evolution,
just as degrees of resistance of organisms to epidemics are products
of genetic evolution. Filters include the perceived opinions of
others, especially those viewed as members of the same self-defined
social group, which collectively attempt to limit deviance [32-34].
"Conformist transmission," defined as the tendency to imitate the most
frequent behavior in the population, can help stabilize norms  and
indeed can be the principal mechanism underlying the endogenous
emergence of norms. The robustness of norms can arise either from the
slow time scales on which group norms shift, or from the inherent
resistance of individuals to changing their opinions. In the simplest
exploration of this, Durrett and Levin (unpublished data) have
examined the dynamics of the "threshold" voter model, in which
individuals change their views if the proportion of neighbors with a
different opinion exceeds a specified threshold. Where the threshold
is low, individuals are continually changing their opinions, and
groups cannot form (Figure 1A). In contrast, at high thresholds,
stickiness is high--opinions rarely change--and the system quickly
becomes frozen (Figure 1B). Again, groups cannot form. In between,
however, at intermediate thresholds (pure conformist transmission),
groups form and persist (Figure 1C). In the simplest such models in
two dimensions, unanimity of opinions will eventually occur, but only
over much longer time periods than those of group formation (see also
). When the possibility of innovation (mutation) is introduced in
a model that considers linkages among traits and group labels, and
where individuals can shift groups when their views deviate from group
norms sufficiently, multiple opinions and multiple groups can persist,
essentially, indefinitely (Figure 1D).
(A) Long-term patterning in the dynamics of two opinions for the
threshold voter model with a low threshold.
(B) Long-term patterning in the dynamics of two opinions for the
threshold voter model with a high threshold. Note the existence of
small, frozen clusters.
(C) Long-term patterning in the dynamics of two opinions for the
threshold voter model with an intermediate threshold. Note the clear
emergence of group structure.
(D) Long-term patterning in a model of social group formation, in
which individuals imitate the opinions of others in their (two)
groups, and others of similar opinions, and may switch groups when
their views deviate from group norms.
The formation of groups is the first step in the emergence of
normative behavior; the work of Durrett and Levin shows that this can
occur endogenously, caused by no more than a combination of ideation
and imitation. The existence of a threshold helps to stabilize these
groups, and to increase stickiness; furthermore, if threshold
variation is permitted within populations, these thresholds can
coevolve with group dynamics. What will the consequences be for the
size distribution of groups, and for their persistence? Will group
stability increase, while average size shrinks? What will be the
consequences of allowing different individuals to have different
thresholds, or of allowing everyone's thresholds to change with the
size of the group? When payoffs reward individuals who adhere to group
norms, and when individuals have different thresholds, will those
thresholds evolve? The answers to such questions could provide deep
insights into the mechanisms underlying the robustness of norms, and
are ripe for investigation through such simple and transparent
Modeling may also shed light on why some norms (like fashions) change
so easily, while others (like foot binding in imperial China) persist
over centuries, and more generally on how tastes and practices evolve
in societies. Norms in art and music change rapidly and with little
apparent effort at persuasion or coercion. But three-quarters of a
century of communism barely dented the religious beliefs of many
Russians, despite draconian attempts to suppress them , and
several centuries of science have apparently not affected the belief
of a large number of Americans in angels and creationism (e.g.,
[36,37]). Then there are the near-universal norms, such as the rules
against most types of physical assault or theft within groups that,
although they vary in their specifics, are interpreted as necessary to
preserve functional societies. Group-selection explanations for such
phenomena (e.g., ) are, we argue, neither justified nor necessary
(see also pp. 221-225 of , ). Such behaviors can emerge from
individual-based models, simply involving rewards to individuals who
belong to groups.
There are degrees: the evolution of cooperation is facilitated by
tight interactions, for example when individuals interact primarily
with their nearest neighbors [40,41], and the payoffs that come to
individuals from such cooperation can enhance the tightness of
interactions and the formation of groups. This easily explains why
mutually destructive behaviors, like murder, are almost universally
proscribed. Group benefits can emerge, and can enhance these effects,
but it is neither necessary nor likely that group selection among
groups for these behaviors overrides individual selection within
groups when these groups are not composed of closely related
Simple models could address such things as the role of contagion in
cultural evolution, recognized in one of the first works on psychology
 in the context of religious revivals and belief, as what has been
described as "pious contagion" (p. 10 of ). But models must also
address issues such as the roles of authority or moral entrepreneurs
(individuals engaged in changing a norm) , to say nothing of the
impacts of advertising and the norm-changing efforts of the
entertainment and other industries. In reality, we are intentioned
agents who act with purpose. In maturing, we master the norms that
have been evolved over a long period, but to which we may adapt in
different ways and even (in the case of moral entrepreneurs) strive to
For a moral entrepreneur, a group that is too small may have little
influence and be not worth joining. But large groups may be too
difficult to influence, so also may not be worth joining. For such
individuals, there is likely an optimal group size, depending on the
change the individual wants to effect. Groups also introduce ancillary
benefits of membership that change the equation. Such considerations
influence decisions such as whether to join a third party effort in a
political campaign; understanding the interplay between individual
decisions and the dynamics of party sizes is a deeply important and
fascinating question, with strong ecological analogies. Groups,
collectively, must also wrestle with the costs and benefits of
increasing membership, thereby enhancing influence while potentially
diminishing consensus and hence the perceived benefits to members.
Innovation and Conservatism
Cultural evolution, like biological evolution, contains what we like
to call the "paradox of viscosity." Evolving organisms must balance
the need to change at an appropriate rate in response to varying
environmental conditions against the need to maintain a functioning
phenome. This trade-off between conservatism and adaptability, between
stability and exploration, is one of the central problems in
evolutionary theory. For example, how much change can there be in the
genes required to maintain adaptation in a caterpillar without
lethally affecting the structure and functioning of the butterfly (p.
303 of )? Conservatism in religion might be explained by the lack
of empirical tests of religious ideas. But even in military technology
and tactics, where empirical tests are superabundant, changes are
slower than might be expected. For example, the British high command
in World War I did not react rapidly to the realities of barbed wire,
massed artillery, and machine guns . Even so, the conservatism of
the generals may be overrated .
We have thus far examined the evolution of norms in isolation--as how
the views of individuals (and thus the constituents of a pool of
nongenetic information) change through time. But everywhere in common
discourse and technical literature, it is assumed that norms are
bundled into more or less discrete packages we call cultures, and that
those packages themselves evolve. Recall that everyday notions such as
that American culture of the 1990s was very different from that of the
1960s, that Islamic culture did not undergo the sort of reformation
that convulsed Christian culture (for example, ), and that
Alexander the Great carried Greek culture throughout the Mediterranean
and as far east as Persia. The problem of defining "cultures" in
cultural evolution seems analogous to that of defining "species" (or
other categories) in genetic evolution. There has been a long and
largely fruitless argument among taxonomists over the latter , and
an equally fruitless debate in anthropology (and biology) on the
definition of culture [39, 49-57].
Again, we suggest that the parsing of the various influences that
create and sustain norms and cultures are ripe for theoretical
modeling, but it must begin to incorporate the full richness on
multiple scales of space, time, and complexity. Durrett and Levin 
develop a model integrating the dynamics of clusters of linked
opinions and group membership; appropriate extensions would allow
group characteristics to evolve as well, but on slower time scales.
The oversimplicity of models of symmetric imitation on regular grids,
as represented in our simple models, must give way to those that
incorporate fitnesses and feedbacks, as well as asymmetries and power
brokers, on more complex networks of interaction .
Challenges and Hypotheses
One of the major challenges for those interested in the evolution of
norms is, at the most elementary level, defining a norm. This is
related to another general problem of defining exactly what is
changing in cultural evolution--which we might call the "meme dilemma"
in honor of Dawkins' regrettably infertile notion. A second major
challenge is discovering the mechanism(s) by which truly novel ideas
and behaviors are generated and spread. A third is discovering the
most effective ways of changing norms.
We've got a long way to go before being able to meet those challenges.
One place to start is to begin formulating hypotheses about the
evolution of norms that can be tested with historical data, modeling,
or even (in some cases) experiments. Some hypotheses we believe worth
testing (and some of which may well be rejected) are given in Box 1.
Box 1. Sample Hypotheses about the Evolution of Norms
Hypothesis 1. Evolution of technological norms will generally be more
rapid than that of ethical norms.
Technological changes are generally tested promptly against
environmental conditions--a round wheel wins against a hexagonal one
every time, and the advantages of adopting it are clear to all.
Ethical systems, on the other hand cannot often be tested against one
another, and the standards of success are not only generally
undetermined, they often vary from observer to observer and are the
subject of ongoing controversy among philosophers.
Hypothesis 2. In societies with nonreligious art, the evolution of
norms in art will be more rapid than those in religion.
We hypothesize that art is less important to the average individual
than his or her basic system of relating to the world, and
conservatism in the latter would be culturally adaptive (leading to
success within a culture).
Hypothesis 3. Military norms will change more in defeated nations than
Was the Maginot Line and the generally disastrous performance of the
French army in 1940 an example of a more general rule? Does success
generally breed conservatism?
Hypothesis 4. The spread of a norm is not independent of the spread of
others, but depends on the spread of other norms (norm clusters).
Does, for example, empathy decrease with social stratification?
Hypothesis 5. Susceptibility to the spread of norms is negatively
correlated with level of education.
Are the less educated generally more conformist, or does the spread of
norms depend almost entirely on the character of the norm?
Hypothesis 6. Horizontal transmission will show less stickiness than
This conjecture is based on anecdotal observations that norms like
using hula hoops come and go and are primarily horizontally
transmitted, and religious values and other high-viscosity points of
view are mostly vertically transmitted (p. 129 of , ).
In this essay we have tried to be provocative rather than exhaustive.
There is a welter of issues we have not even attempted to address,
including: (1) asymmetries of power in the spread of norms, (2) the
role of networks, (3) the efficacy of persuasion as opposed to
imitation, (4) the cause of thresholds in the change of norms, (5) the
genesis of norms during child development, (6) the connection between
attitudes and actions, (8) competition among norms from different
cultures; and (9) the question, can norms exist "free of people" in
institutions? Institutions certainly may emerge as independent
structures, stabilized by laws and customs that are enforced to
varying degrees through formal punishment or social pressure. Can such
norms persist long even when adherence to them is disappearing? The
interplay between the dynamics of individual behaviors and normative
rules, operating on different time (and other) scales, may be the key,
we argue, to understanding sudden phase transitions that can transform
the cultural landscape.
We hope that, by being provocative, we can interest more
evolutionists, behavioral biologists, and ecologists in tackling the
daunting but crucial problems of cultural evolution. Few issues in
science would seem to be more pressing if civilization is to survive.
We have received helpful critical comments from Kenneth Arrow, John
Bonner, Samuel Bowles, Kai Chan, Gretchen Daily, Partha Dasgupta,
Adrian deFroment, Anne Ehrlich, Marcus Feldman, Michelle Girvan, Ann
Kinzig, Deborah Prentice, and Will Provine. Amy Bordvik provided
invaluable assistance in preparing the manuscript for publication.
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