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© 2009 International Institute of Anthropology
© 2009 Lolita Nikolova, PhD
Created: 2-23-09 Updated: 2-23-09
Conference at the University of Utah:
The Evolution of Human Aggression: Lessons for Today's
Conflicts, February 25-27, 2009
(Link to the official website of the Conference)
Link to abstracts at the official website of the event:
THE EVOLUTION OF HUMAN AGGRESSION: LESSONS FOR
TODAY'S CONFLICTS
Presentation Abstracts
Keynote 1
Destined to Wage War Forever? The Evolution of Peacemaking Among
Primates.
Frans B. M. de Waal, Living Links, Yerkes Primate Center, Emory University
Following the Second World War, scientists were naturally fascinated with the aggressive "instinct" in humans and
animals. In the 1970s, evolutionary biology added the view of animal social life as an arena of competition. At about
the same time, however, primatologists began to emphasize long-term social relationships. The discovery of
reconciliation behavior came out of this tradition, confirming the impression that societies constitute a balancing act
between cooperation and competition. Reconciliation - defined as a friendly reunion between former opponents - has
since been confirmed in many different species, in both captivity and the field, both experimentally and observationally.
Chimpanzees, for instance, kiss and embrace after a fight. Reconciliation has also been demonstrated in non-primates,
such as dogs and dolphins. This behavior truly serves what its name suggests, i.e. to repair social relationships. The
dominant idea (known as the Valuable Relationship Hypothesis) is that reconciliation will occur whenever parties stand
much to lose if their relationship deteriorates. This means that peacemaking depends on overlapping interests, a
situation common within but rare between primate groups. In our own species, however, interdependencies between
groups or nations are not unusual, and in fact increasing, making for applicability of these models to international
relations.

Panel 1: Conflict and Conflict Resolution among Great Apes.
The imbalance-of-power hypothesis and the evolution of war.
Richard Wrangham, Harvard University
Among vertebrates, lethal intergroup aggression has traditionally been regarded as being unique to humans, and
human warfare has therefore been widely interpreted as an evolutionary aberration due to social construction. The
discovery since the 1970s that chimpanzees kill adult members of neighboring social groups has challenged the social
construction hypothesis. Here I review the imbalance-of-power hypothesis, which states an evolutionary history of
communal territoriality combined with fission-fusion grouping favors the tendency to kill rivals when the costs are
perceptibly low. Current data on chimpanzees, bonobos and other mammals support the imbalance-of-power
hypothesis and suggest that in certain species natural selection has favored a drive to dominate neighboring
communities through attempts to kill. I suggest that the imbalance-of-power hypothesis also provides a useful basis for
understanding intergroup violence in small-scale human societies, but that it needs to be modified to take account of
human-specific attributes such as reward systems and political complexities. The proposal that human intergroup
aggression has its evolutionary origins in an imbalance-of-power system means that violence will emerge predictably
when groups have sufficient power, but that violence is suppressed in conditions without intense power imbalances.

Evolutionary Perspectives on Conflict Resolution
Joan B. Silk, Department of Anthropology and Center for Society and Genetics, University of
California, Los Angeles
Sociality is favored by natural selection because it makes animals safer from predation or enables them to collectively
defend access to resources. At the same time, living in close proximity to conspecifics can lead to conflicts of interest
and competition. In a number of animal taxa, including many nonhuman primate species, evolution has favored an
effective mechanism for resolving conflicts with group members: they engage in peaceful contacts with former
opponents in the minutes that follow conflicts. There is a broad consensus that these reconciliatory interactions relieve
the stressful effects of conflict and permit former opponents to interact peacefully, but less consensus about their
adaptive function. Primates may reconcile to obtain short term objectives, such as access to desirable resources.
Alternatively, reconciliation may preserve valuable relationships damaged by conflict. Some researchers view these
explanations as complementary, but they generate different predictions about the patterning of reconciliation that can
be partially tested with available data. There are good reasons to question the validity of the relationship-repair model,
but it remains firmly entrenched in the reconciliation literature, perhaps because it fits our own folk model of how and
why we resolve conflicts ourselves. It is possible that the function of reconciliation varies across taxa, much as other
aspects of cognitive abilities do.

Chimpanzee Politics: Pacifying Interventions and Reconciliation
Frans B. M. de Waal, Living Links, Yerkes Primate Center, Emory University
Chimpanzee males form coalitions within the group in pursuit of high rank. These coalitions are formed
opportunistically, and may involve high risk, including fatal aggression. High ranking males perform a control role in that
they break up fights among others. This behavior has group-wide repercussions as demonstrated in an experiment on a
different species. Flack et al. (2005) removed control males from a large captive macaque group for brief periods of
time, and each time measured a deterioration of social relationships in the remainder of the group, including a sharp
drop in reconciliation behavior. Reconciliation, which has been demonstrated in a great variety of primates and
other animals, affects stress levels, social tolerance, and long-term social relationships, hence is an essential
component of group harmony.

Sexual dimorphism and aggression in primates: just where do humans fit in?
Michael Plavcan, Anthropology, University of Arkansas
Male primates are often much larger than females, and equipped with large canine teeth (dimorphic). Humans, on the
other hand, show comparatively modest differences in body size, and lack large canine teeth. These characters are
often associated with monogamy and affiliation in humans. However, comparative analyses more closely tie
dimorphism with degrees of intra-sexual aggression and differences in reproductive success among males. The closest
relatives of humans – the great apes – show a gradation of dimorphism that appears to track the degree of relatedness to
humans. Gorillas and orangutans are intensely dimorphic, and chimpanzees much less so. Many models for the
evolution of human behavior use chimpanzees as an analogue for an ancestral condition. But data from the fossil
record strongly contradict this assumption, suggesting that behavioral similarities between chimpanzees and humans
associated with reduced dimorphism evolved in parallel, and that modern humans are derived independently from a
strongly dimorphic ancestor. This has important implications for understanding whether human patterns of aggression
and affiliation represent an inherited condition, or have separately evolved as part of a unique human adaptation.

Keynote 2
Nothing to Lose? Economic Inequality, Poor Life Prospects, and Lethal
Competition.
Martin Daly & Margo Wilson, Department of Psychology, McMaster University
The majority of homicides are the culminations of competitive confrontations between young men, and the immense
variation in homicide rates is primarily due to the variable incidence of such contests. The most successful predictor of
homicide rates has proven to be the intensity of economic competition, as indexed by income inequality. But which
particular men are at risk? In large measure, it is those whose lives are going nowhere unless they escalate their
competitive tactics.
Thinking about homicide in this way has led us to a number of discoveries about its demography and epidemiology,
which we will review. We will also address the questions of why homicide rates declined in much of the developed world
in recent decades although income inequality was on the rise, and whether crosscultural variability in attitudes and
values provides an alternative to economic explanations for the remarkable variability in homicide rates between and
within nations.
We do not suggest that killing per se can be understood as either rational or fitness-promoting. Homicides are relatively
rare dénouements of hostile confrontations, and it is in the modulation of men’s willingness to engage in risky
competition that adaptation should be sought.

Panel 2: Coalitionary Violence and Warfare
A History of Violence
Steven Pinker, Harvard College Professor and Johnstone Family Professor, Department of
Psychology, Harvard University
Contrary to the popular impression view that we are living in extraordinarily violent times, rates of violence at all scales
have been in decline over the course of history. I explore how this decline could have happened despite the existence
of a constant human nature.
Americans at War: Evolutionary Perspectives on an Age Old Story Patricia M. Lambert, Utah State University
The archaeological record of North America is rife with evidence for war, both prehistoric and historic. Ancient palisade
lines, cliff dwellings, towers, entrenchments, burned villages, no-man’s-lands, war weapons, and war dead
attest to a history of conflict extending far back beyond the arrival of Europeans and the establishment of the United
States. These remnants of the past are fascinating, insightful, and historically important—but are they relevant to the
topic of conflict management in the 21st century? The purpose of this paper is to explore the value of this longitudinal
record for revealing the larger causal forces that underlie intergroup conflict, forces that are often masked in the modern
world by proximate triggers such as hotel bombings and suicide attacks, and thus difficult to identify. History has shown
us that conflict resolution is a challenging endeavor, but those efforts that take underlying causation into account may
have a better chance of resolving today’s conflicts and heading off those that threaten our collective future.

Male Hierarchies, Parent-Offspring Conflict, and Warfare in Papua New
Guinea
Polly Wiessner, Anthropology, University of Utah
Population growth and the increase of young men in proportion to older men are associated with accentuated
coalitional violence worldwide. Here I will propose that an extension of parent-offspring conflict provides a powerful
framework for understanding the course of coalitional violence. Older men seek to manipulate inter-group competition
to provide optimal resources and security for their offspring and those of their close collaterals. In contrast, young men
seek to demonstrate physical prowess and willingness to sacrifice for the group to reap individual reputation and
rewards. In periods of demographic or technological stability, older men with control of resources, knowledge, and
networks prevail.
With rapid change, younger men are able to disrupt the male power hierarchy, generating chaos.
I will draw on a case study from the Enga of Papua New Guinea to illustrate how parents parent/offspring conflict is
played out in the context of warfare in precolonial and modern times young men in the driver’s seat, and what older men
are doing about it.

Panel 3: Further Discussion of Coalitionary Warfare
Warfare and Human Ultrasociality
Peter Turchin, Ecology and Evolution, University of Connecticut
How did human ultrasociality - extensive cooperation among large numbers of unrelated individuals - evolve? What are
the social forces that hold together
complex societies encompassing hundreds of millions of people? Using theoretical insights from models of multilevel
selection I argue that there is a fundamental connection between human ultrasociality and warfare. It was
intergroup conflict that generated selective pressures for increasing scale and complexity of human societies. I illustrate
this social evolutionary dynamic with two examples. The first is the rise of historical megaempires on the frontiers
between settled farmers and nomadic pastoralists. The second one is the transformative influences of the Indian Wars
on the European settlers in North America.
From Lab to War: The Role of Biology and Psychology in Political Aggression Dominic D. P. Johnson
Politics & International Relations, University of Edinburgh I present results from a series of laboratory experiments
demonstrating that human biology and psychology have significant influences on the probability of aggression. In
interactive war-game experiments over networked computers, we found that: (1) men (not women) were over-confident
about winning, and those who were more over-confident were more likely to attack their opponents; (2) second-to-fourth
finger length ratios (2D:4D), a possible biomarker of pre-natal testosterone exposure, also predicted the probability of
attacking. In our most recent experiments, we found that: (3) behavioral aggression (willingness to inflict harm on others)
was significantly associated with MAO-A (monoamine oxidase A) gene, especially in response to provocation. Finally, in
hypothetical international crisis scenarios, levels of aggression in subject’s chosen policy options (which ranged from
withdrawal, to negotiation, to military attack) were significantly predicted by: (4) political partisanship (Democrat-
Republican affiliation, and a general liberal-conservative scale); and (5) subjects’ confidence that their chosen policy
would succeed. I conclude by arguing that physiological and psychological influences on aggression were adaptive in
our evolutionary past because they promoted survival and reproductive success. However, these same mechanisms are
often costly and maladaptive in today’s very different social and political environment. If we ignore the biological bases
of aggression, we will only make the task of prediction and prevention harder.

Panel 4: Hormones and Human Dominance and Aggression
The challenge of testosterone
John Archer, Department of Psychology,University of Central Lancashire
Chronic high levels of testosterone exert evolutionary costs. A common response to this is in males of many species is to
have a neuroendocrine system that is responsive to situations that require high testosterone levels rather than
maintaining consistent high levels. Evidence from studies of testosterone and behavior in humans is assessed in relation
to whether human males fit this pattern. It is concluded that they do, and also that there are individual differences
associated with testosterone levels indicative of specialization for mating or parental effort.

Ontogeny of hormonal mechanisms for coalitionary aggression
Mark Flinn, University of Missouri, Columbia
Humans have an unusual suite of traits, including: (1) extensive male parental effort, (2) relatively exclusive, long term
mating relationships, (3) mutual respect for other males’ mating relationships, (4) communities composed of many males
from multiple kin groups, (5) inter-community aggression, and (6) a long period of juvenile dependence. The
neurological and hormonal mechanisms that underpin this unique suite of behavioral traits are uncertain, but may
provide important clues about the selective pressures that guided human evolution.
Here I present data from a 20-year study of a rural community on the island of Dominica. Testosterone and cortisol
response to competitive events among adult males within a coalition are different than responses among males from
different coalitions. Similarly, adult males have different hormonal responses to females that are attached to close
friends than to unattached females, or females attached to males that are not close friends. We are currently studying
the ontogeny of these distinctive hormonal responses. During middle childhood, boys and girls show behavioral
differences in play and social interactions: boys tend to invest more time in organizing groups of peers, among which
they form hierarchies, and compete with other groups. Conversely, girls usually invest more time in dyadic interactions
with similar age girls, caring for siblings, and doing domestic chores. How the onset of male coalitional and female
dyadic psychobiology and life history trajectories are related to social events is yet an open question. We are
examining the onset of adrenarche, pubarche, and individual differences in DHEA (Dehydroepiandrosterone)
production using semi-structured, long interviews and a competitive enzymatic immunoassay of saliva samples. Peer
network density is assessed by multidimensional scaling (MDS), with the hypothesis that it is denser for boys than for girls.
Everyday social interactions are coded from observations and video. Analyses suggest that middle childhood
and the unusual temporal patterning of adrenarche are important components in the ontogeny of coalitionary behavior.

The role of physical strength in anger and anger expressions
Aaron Sell
Center for Evolutionary Psychology, University of California, Santa Barbara
Anger can be understood as a cognitive mechanism designed by natural selection to negotiate conflicts of interest in
ways similar to, but distinct from, non-human animal conflict. Using an evolutionary biological framework, one can ask
under what conditions aggression is mobilized by the anger system, and predict individual differences in thresholds for
aggression. For example, because physical aggression was frequently used by men during our evolutionary history
to negotiate conflicts of interest, it was predicted and found across different cultures that physically stronger men were
more prone to anger. Similarly, physical changes to the face, body, and voice preceding aggression can be
understood as displays designed by natural selection to enhance signals of physical strength and fighting ability.

Panel 5: Domestic Violence, with Emphasis on Spousal/Partner Relationships
An evolutionary perspective on family violence
John Archer, School of Psychology, University of Central Lancashire
The aim of this presentation is to evaluate the application of evolutionary principles to the understanding of family
violence. The following relevant evolutionary principles will be outlined: kinship and inclusive fitness; paternity
uncertainty and mate guarding; reproductive value; parent-offspring conflict; resource holding power. The motivational
mechanisms underlying these principles are then discussed, specifically discriminative parental solicitude and
kin resemblance. The following forms of family violence are presented in the light of these principles and mechanisms,
with relevant empirical research: (1) parental violence to unrelated children; (2) parental violence to biological
children; (3) offspring’s violence to parents; (4) violence between siblings.
Violence between sexual partners is considered in relation to (1) conflicts of interest and power relations between males
and females; (2) spousal abuse as mate guarding; (3) male sexual jealousy as a mediator of partner violence; (4)
reproductive value. It is concluded that an evolutionary approach has a number of strengths in terms of providing a
comprehensive theoretical framework and specific principles underlying many aspects of family violence, although the
current emphasis on male mate guarding is too narrow to explain current findings in relation to partner violence.

Men’s Proprietary View of Their Romantic Partners is Specific to Sexuality:
An Experimental Study
Aaron T. Goetz, California State University, Fullerton
Even across disciplines and theoretical perspectives, most agree that men take a proprietary view of their romantic
partners; men view their partners as an entity that they privately own and control. Disagreement, however, arises over the
extent of this proprietary view. Some theorists have argued that men attempt to control and dominate all aspects of their
partners’ lives, while others—particularly those taking an evolutionary approach—have argued that men’s proprietary
view of their romantic partners is specific to sexuality. Here, I describe the results of a recent experimental study in
which I demonstrated that men are less likely to tolerate their partner’s participation in activities that more likely to lead
the opportunity for infidelity and that men become more tolerant of their partner’s participation as the activities become
less related to the opportunity for infidelity. These results suggest that men afford their partners many freedoms with the
exception of those related to their sexual behavior. Discussion addresses how the adaptive problem of paternity
uncertainty plays a central role in intimate partner violence.

Hurting the ones we love:
The features and functions of aggressive punishment in close relationships
Julie Fitness, Macquarie University
Human beings are born with a fundamental need for attachment, intimacy, and the love and esteem of valued others.
Close relationships, then, are the source of our most intense positive emotions, including love and joy. However, close
relationships are also the source of intense pain and anger when relationship partners reject or hurt one another, or fail
to meet one another’s needs, desires, or expectations. Further, the experience of emotional pain may generate a
powerful impulse to punish, or inflict pain upon, the person who appears to have caused the distress. In this paper I will
argue that the urge to retaliate in response to partner-triggered emotional pain is, to an extent, hard-wired and serves a
variety of potentially adaptive functions, though it may also have destructive and tragic consequences. Following a
discussion of the features and functions of punishment in close relationship contexts from an evolutionary,
socialpsychological perspective, I will discuss the roles of emotional pain and punishment as it relates to domestic
violence. I will then present the findings of an empirical study of aggressive punishment in marriage and suggest some
implications of this work for both enhancing our understanding of aggression in close relationships, and preventing its
occurrence.

Panel 6: Further Discussion of Domestic Violence, with Emphasis on Parent-
Child Relationships
Violence against Stepchildren. The Evidence and its Discontents.
Martin Daly and Margo Wilson, Department of Psychology, McMaster University
Parental investment is costly and evolves to be allocated where it is most likely to promote parental fitness. While it is
implausible that abusing or killing stepchildren would have promoted the assailants’ fitness in ancestral human
social environments, a general preference for their own offspring surely would have. Elevated risks to stepchildren are a
likely byproduct of such discriminative parental solicitude.
It is now almost 30 years since we first demonstrated that children living with one genetic parent and one stepparent
were indeed mistreated more than children in intact birth families. Further research has shown that such “Cinderella
effects” are widespread, perhaps even universal, are often substantial, and cannot be explained away as artifacts of any
correlated factor yet suggested.
The disproportionate victimization of stepchildren is now the most extensively documented generalization in the family
violence literature, raising further questions, such as what explains variability in risk differentials between maltreatment
types and locales, and whether the individual-level predictors of abuse are the same for genetic and stepparents.
Unfortunately, progress on these important issues has been hindered by a relentless distraction: the manufacture
of “controversy” about whether Cinderella effects exist at all. A motivation for this nay-saying appears to be antipathy to
the Darwinian worldview and/or to its application to Homo sapiens.

Hormonal responses to domestic violence
Mark Flinn, University of Missouri, Columbia
Exposure to stressful experiences increases vulnerability to adverse health outcomes. A potential endocrine mechanism
mediating the link between stress and health is the hypothalamic-pituitary-adrenal (HPA) system, with a key role
attributed to the glucocorticoid hormone cortisol. Retrospective clinical studies indicate that traumatic experiences
during childhood such as exposure to domestic violence can have a permanent influence on HPA regulation. Here I
present analyses of naturalistic, longitudinal data on cortisol levels, social stressors including domestic violence, and
health among children to assess developmental trajectories of HPA functioning. Saliva samples (N=32,219) were
collected and assayed for cortisol in concert with monitoring of growth, morbidity, and social environment for children
(N=317) in a rural Dominican community each year over a 20-year period (1988-2008). Several measures of
individual cortisol (C) profiles are analyzed: (1) average C, (2) average wake-up C, (3) average ratio of AM/PM C, (4)
variability of AM and PM C, and (5) reactivity of C in response to stressors. A majority of children exhibit moderate
stability of all five measures over multiple year periods. Children exposed to domestic violence exhibit significant
changes in some of these measures.
Changes in HPA response, however, appear to be context-specific, with increased reactivity to some types of social
stressors, but normal or reduced reactivity to physical stressors.
Link to the poster
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