[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
    to scientists.

    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

    DOI: 10.1371/journal.pbio.0030194

    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 ([3]; 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., [6], see
    also references in [7]). 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 [8] 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 [9] 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 [10], [11-16], and p. 62 of [17]). 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 [18], 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 [21]) 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
    is needed.

    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 [14], 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 [3]. 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.

Microscopic Dynamic

    Substantial progress has been made toward the development of a
    mathematical theory of cultural transmission, most notably by
    Cavalli-Sforza and Feldman [14], and Boyd and Richerson [11].
    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 [3]. Similar efforts
    have been undertaken by others, such as Henrich and Boyd [28] and
    Kendal et al. [20].

    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., [31]. 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 [28] 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
    [20]). 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).

Figure 1.

    (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
    mathematical models.

    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 [35], 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., [12]) are, we argue, neither justified nor necessary
    (see also pp. 221-225 of [38], [39]). 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
    individuals [42].

    Simple models could address such things as the role of contagion in
    cultural evolution, recognized in one of the first works on psychology
    [43] in the context of religious revivals and belief, as what has been
    described as "pious contagion" (p. 10 of [30]). But models must also
    address issues such as the roles of authority or moral entrepreneurs
    (individuals engaged in changing a norm) [32], 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 [44])? 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 [45]. Even so, the conservatism of
    the generals may be overrated [46].

Macroscopic Dynamics

    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, [47]), 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 [48], 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 [3]
    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 [58].

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
    victorious ones.

    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
    vertical transmission.

    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 [14], [59]).

    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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