2013. Vol.3, No.1, 45-51
Published Online January 2013 in SciRes (http://www.scirp.org/journal/sm) http://dx.doi.org/10.4236/sm.2013.31008
Copyright © 2013 SciRes. 45
Tylor vs. Westermarck: Explaining the Incest Taboo
Gregory C. Leavitt
Idaho State University Pocatello, Pocatello, USA
Received February 11th, 2012; r evised June 9th, 2012; accepted June 29th, 2012
In the late 19th century arguments explaining incest avoidance were framed separately by Edward Tylor
and Edward Westermarck. Tylor offered an environmental theory asserting that people have to marry out-
side of their own kin and communities or die out from the detrimental effects of isolation. Westermarck
turned to Darwin’s theory to explain that harmful inbreeding had been selected against in the human ge-
nome. By the late 20th and early 21st centuries explanations of human behaviors have become increas-
ingly encompassed by natural selection theory. The debate concerning the productiveness of evolutionary
biology for explaining complex human behaviors is highly contentious and continues unabated. Although
human evolutionists repeatedly say that environment is important for understanding human behavior they
often do not develop this part of the equation. Behind the prestige of evolutionary biology selection mod-
els of human behavior have passed into popular science and the public psyche. Often heard today from a
wide range of highly visible media sources is an assortment of topics on human behaviors which are
framed by Darwinian assumptions. Contemplations about incest and inbreeding avoidance fall into this
category and are presented by Darwinian social science as the best case example demonstrating evolu-
tionary suppositions about human behavior. In the article that follows these issues are framed and exam-
ined. The argument is offered that evolutionary approaches are not always the most compelling and that
convincing environmental explanations are overlooked.
Keywords: Explanations of Human Behavior; Environmentalism; Natural Selection; Incest and Inbreeding
Avoidance; Tylor’s Thesis; Westermarck’s Thesis
A keen interest in the incest taboo spans the history of human
studies and bridges such disciplines as cultural anthropology,
sociology, evolutionary biology and psychology. Given the di-
versity of academic interests it is not surprising that explana-
tions of the incest taboo have taken very different directions. In
the late nineteenth century, at almost the same time, Edward
Tylor (1888), an English anthropologist, and Edward Wester-
marck (1891), a Finnish sociologist, proposed alternative and
opposing hypotheses for explaining the incest taboo. These di-
vergent premises laid the foundation in the literature for dis-
tinctive approaches in understanding incest rules.
Tylor looked to the sociocultural environment to understand
what he identified as “the rules of exogamy”1. He proposed that
these marital and sex rules, which required persons to have
relationships outside their consanguineal community, tied dif-
ferent kinship groups and communities together in mutual aid
Exogamy, enabling a growing tribe to keep itself compact
by constant unions between its spreading clans, enables it
to overmatch any number of small intermarrying groups,
isolated and helpless. Again and again in the world’s his-
tory, savage tribes must have had plainly before their
minds the simple practical alternative between marrying
out and being killed out (Tylor, 1888).
Referred to as “alliance theory”, Tylor’s hypothesis has been
developed and expanded by a variety of scholars in anthropo-
logical and sociological studies. Alliance theory has become
one of the most widely excepted explanations of the incest ta-
boo’s position in the intricate web of marriage and sex rules in
human societies. Although modern alliance theory is commonly
credited to Tylor, it nonetheless has a deep historical account-
ing. For example, Plutarch, a Greek philosopher and priest (AD
46-120?) and his Roman contemporary, Tacitus (AD 56-120)
both anticipated alliance theory in their respective writings on
the incest taboo. Somewhat later this same idea appears in the
writings of Augustine (AD 354-430) and Aquinas (AD 1225-
1274) (Honigmann, 1976). More modern presentations of alli-
ance theory can be found in White (1948), Murdock (1949),
Parsons (1954), Cohen (1978) and Leavitt (1989).
In contrast, Westermarck, noting the universal aversion to
sex among persons raised in intimate proximity, proposed that
incest prevention was a product of natural selection. Recogniz-
ing further that inbreeding had a deleterious effect on the off-
spring of closely related mating pairs, Westermarck hypothe-
sized that nature would select outbreeders for a successful evo-
1The terms “exogamy” and “incest taboo” were often used interchangeably
in early discussions and only became distinct as the scholarship matured
over the twentieth century. In general, exogamy refers to marriage and incest
refers to sex but in practice the rules of exogamy and incest rules are highly
correlated in terms of the persons excluded from marital and sexual rela-
Generally speaking, there is a remarkable absence of ero-
tic feelings between persons living very closely together
from childhood. Nay more, in this, as in many other cases,
sexual indifference is combined with the positive feeling
of aversion when the act is thought of. This I take to be
G. C. LEAVITT
the fundamental cause of the exogamous prohibitions. Per-
sons who have been living closely together from child-
hood are as a rule near relatives. Hence their aversion to
sexual relations with one another displays itself in custom
and law as a prohibition of intercourse between near
kin… We may assume that in this aversion, as in other
cases, natural selection has operated, and by eliminating
destructive tendencies and preserving useful variations
has molded the sexual instinct so as to meet the require-
ments of the species (Westermarck, 1922).
This thesis of genetic inheritance, strongly criticized in the
social sciences, did not become widely accepted until Edward
O. Wilson’s reintroduction of such suppositions in his 1975
publication, Sociobiology. With this publication, Darwin’s na-
tural selection theory was again employed to develop an inheri-
tance model of complex social behaviors in animals (including
humans), incorporating specifically an aversion hypothesis of
incest and inbreeding.
In the following discussion the development of Tylor’s alli-
ance theory will be considered followed by a lengthier and cri-
tical review of the development of Westermarck’s natural se-
lection hypothesis. In general, the argument is forwarded that
the incest taboo is understood in relation to human environ-
mental and sociocultural demands, not an inheritance product
of natural selection.
Tylor’s insight concerning exogamy and the incest taboo is
as simple as it is compelling. He argued that groups of people
must construct lines of cooperation to aggregates outside of
their immediate kinship organization and community or risk
survival. In modern parlance, Tylor’s “savage” societies would
be recognized as nomadic hunters and gathers and simple agri-
cultural peoples. Such societies, organized around family groups
and kinship lineages, enhance their survival and well-being if
they abide rules which require their members to make associa-
tions outside of their immediate kin communities. While other
kinds of cooperative alliances can occur to the same ends, mar-
riage and sexual prohibitions are especially affective in com-
pelling individuals to connect with outside persons and com-
Alliances forged by marriage and sexual relations are regu-
larly confirmed by the birth of children, obligating affinal com-
munities to each other even when marriages fail. And beyond
this the importance of marriage alliances are witnessed in the
related practices of levirate (where a man marries a dead bro-
ther’s wife) or sororate (where a man marries the sister of his
deceased wife) which maintain alliances even after the death of
a spouse. In many societies the exchange of children in adop-
tion is practiced to tie different families together in mutual aid.
Bride-price and dowry are additional practices that enhance
family alliances through the exchange of gifts. In these and
other related cultural practices the incest taboo and rules of
exogamy are revealed as being part of an important and larger
set of rules designed to enhance the web of cooperation among
Through this alliance system, commitments of a political,
military and economic nature are formed. During times of stress
and hardship these coalitions will provide critical aid. It would
seem apparent that groups with firm rules compelling such
associations would fare much better, especially over the long-
run, than groups that remained unattached, self-reliant and in-
In particular, Cohen (1978) expands alliance theory beyond
“savage” cultures to include the full spectrum of human socie-
ties—from the socially and technologically simplest to the most
complex. He hypothesized that the extension of the incest taboo
(how far the incest taboo reached to include distant relatives)
varied with the complexity of the society, with the least com-
plex societies having the greatest extension and coverage. As
societies become more complex, the incest taboo contracts so
that a very limited taboo is found in advanced agrarian and
industrial societies, covering only the immediate family (nuclear
family membe rs, aunts and uncles, grandparents and sometimes
first cousins). Leavitt’s (1989) cross-cultural test of this rela-
tionship provided strong support for Cohen’s hypothesis2.
Cohen argues that in less complex societies kinship alliances
are driven by the need for trade in scarce and essential re-
sources. No society has all of the material goods required to
survive, thus necessitating trade with people outside of their
community and region. Encounters with strangers make trade
an uncertain and even dangerous activity. With incest rules that
compel people to find mates beyond their most distant relatives,
lines of trade are secured and forthcoming. Mapping such trade
alliances reveals a complex network of trade routes moving a
wide variety of resources to even very distant places.
Continuing, Cohen asserts that as modern societies develop
specialized political and economic institutions that establish
formal trade ties, and with better transportation and communi-
cation technology, the need for complex family alliances re-
cedes. In complex societies the incest taboo contracts to include
only the nearest family members. Cohen proposes that this is
evidence that the incest taboo will disappear completely, though
he is not suggesting that immediate family incest would be-
come common as a result. Rather, he proposes that modern
mobility and social organization makes incestuous mating less
While recognizing the alliance model presented above, Par-
sons (1954) offers a version of this theory specific to the imme-
diate family, explaining the continuation of the incest taboo in
developed societies. He argues that “eroticism” (including es-
pecially non-genital physical intimacy) is a necessary tool in
the healthy socialization of children. By giving and withholding
erotic stimulation the family can use affection to reward or
punish the child in the socialization process. But since intimate
affection often bonds people in long-term relationships, a mecha -
nism is required, says Parsons, which both regulates and ulti-
mately separates children from parents. Rules prohibiting inces-
tuous behaviors mark the boundary on the extent of affection
that socializing adults can use with attached children. By deny-
ing dependent children complete erotic satisfaction, the incest
taboo helps to project children out into the greater society to
seek intimacy and sexual partners. This movement of the child
out of the family often culminates in marriage, which ties the
family into the larger cooperating societal network. This expul-
sion of the young from the nuclear family into the greater soci-
ety is healthy for the nuclear family, the child and the society .
2Leavitt’s data supported Cohen’s hypothesis that the incest taboo extension
is negatively related to social complexity with the exception that many
hunting and gathering societies with bilateral kinship organizations have
relatively attenuated incest taboos.
Copyright © 2013 SciRes.
G. C. LEAVITT
The Westermarck Hypothesis
As noted above, Edward Westermarck hypothesizes that
natural selection has provided an incest avoidance mechanism
to avert the deleterious effects of inbreeding. Specifically, Wes-
termarck proposes that this inheritable mechanism operates by
generating sexual disinterest in those individuals raised together.
Since no such genes have been identified, support for Wester-
marck’s hypothesis has typically come indirectly from four
bodies of scholarly literature. This includes observations that
the incest taboo is universal; that inbreeding is deleterious to
offspring; that inbreeding avoidance occurs in many other spe-
cies; and that evidence of such a mechanism has been demon-
strated in Shepher’s (1983) kibbutzim study and Wolf’s (2005)
research on Chinese minor marriage.
Leavitt (1990, 2005) critically examined this support litera-
ture and questions the evidence offered by evolutionary schol-
ars. Although some of that discussion will be covered below,
the focus will be on those central theoretical issues surrounding
the Westermarck hypothesis. Specifically, explored is the no-
tion that a naturally selected incest/inbreeding3 avoidance me-
chanism would be compelled by the conditions forwarded by
The Deleterious Effect of Inbreeding
At the core of Westermarck’s natural selection hypothesis is
the long held scientific and cultural belief that inbreeding and
incestuous matings have a deleterious effect on subsequent of-
fspring. The science behind this perception is assembled on a
number of straightforward and simple facts. As taught in ele-
mentary biology, genes are normally either dominant or reces-
sive. A dominant gene on the loci of a chromosome will nor-
mally manifest itself in the individual’s genotype and will more
often be beneficial or neutral. Recessive genes, which are more
often harmful, will manifest themselves only when paired on
loci with an identical recessive. Since individuals who share a
close ancestor(s) are more likely to inherit the same detrimental
recessive genes, closely related mates will more likely produce
progeny that carry a double measure of the unsafe recessive. A
number of studies on the deleterious effects of inbreeding have
been published and support this commonly accepted fact (see
Leavitt, 1990, 2005).
In addition to the deleterious effects of inbreeding, evolu-
tionists commonly assume that individuals in a population are
competing with one another to produce the most progeny in
order to perpetuate the breeders’ genes in future generations. If
these assumptions are correct, then it is supposed that “out-
breeders” will reproduce more successfully than “inbreeders”4.
It is hypothesized that over time this evolutionary process will
be reflected in a population t ha t is almost exclusively composed
of outbreeders. The success of outbreeders is explained by the
fact that they are believed to have acquired through mutation an
inheritable mechanism that makes them sexually adverse to
those to whom they are closely raised. Most often, it is asserted,
these close associates are siblings, parents or other close rela-
Accurately, however, the deleterious thesis is true only in
limited circumstances, which through time has been unusual for
most animal species. Shields (1982a, 1982b, 1983, 1987, 1988,
1993), who has written extensively on the inbreeding issue,
discredits the notion that inbreeding is harmful. Shields (1982a)
concludes that “[t]here is an alternative view of much of that
evidence and additional evidence that is cited less often that
flatly contradicts the view that inbreeding (or even incest) per
se is malada pti ve .”
All breeding populations have a genetic load that reflects an
average number of harmful recessives per individual in the po-
pulation. Large and outbreeding populations, as typically found
in modern complex societies, carry relatively large genetic
loads but avoid the production of significant numbers of ge-
netically diseased offspring because of their outbreeding pattern.
Outbreeding occurs because individuals following exogamous
rules either consciously select mates who have no known com-
mon ancestry and/or because these societies are highly mobile
resulting in individuals selecting mates outside of their com-
munity of origin.
As has been true for most animal species through time,
breeding commonly occurs in small relatively isolated philo-
patric populations5. Only in the past few hundred years have
humans begun to live in large, relatively outbreeding and mo-
bile aggregates. Philopatric populations disperse their offspring
within a known and nearby environment. In most circumstance,
to disperse more widely increases morality because of ignorance
concerning resources and predators. As a result, even when in-
cest rules are quite extended individuals are going to reproduce
with genetically similar mates.
As noted, inbreeding will more readily expose the genetic
load. Such exposure will pair harmful recessive more quickly in
individuals who are then frequently miscarried, still-born or die
before they reproduce. In this manner harmful recessive are
“washed-out” of the breeding population and the genetic load is
reduced resulting in fewer genetically harmed progeny (Living-
stone, 1969; Bengtsson, 1978; Shields, 1982a). As long as a
breeding population maintains a consistent pattern of inbreed-
ing overtime, deleterious effects will be minimized. Outbreed-
ing populations that switch to inbreeding will initially expose
more harmful recessive traits, but in the grand scheme of things
this is not a typical breeding pattern and its limited appearance
cannot account for a naturally selected aversion mechanism. As
Shields (1982a) reveals, most of the research literature demon-
strating the ill effects of inbreeding comes from studies of do-
mestic outbreeding or laboratory populations that have abruptly
been switched to inbreeding.
3As the author has acknowledged at length in other publications (author’s
name removed, 1990, 2005), incest and inbreeding are not precisely or
necessarily the same thing. Incest does not always include all cases of in-
breeding or visa-a-versa.
4“Inbreeding” and “outbreeding” are relative terms that are often used inex-
actly (see Leavitt, 2005). In common parlance inbreeding refers to mating
with known relatives, usually cousins as any closer inbreeding is typically
considered incestuous. “Outbreeding” refers to mating outside of this imme-
diate relative circle. More technically, however, inbreeding refers to mates
who share any common ancestors, thus all matings are to some degree in-
red. In small populations, especially those that are to some degree isolated,
inbreedi ng occurs even wh en rules of exogamy d isallow matin gs with close
relatives simply because the small population size restricts the choice o
mates. For most of human existence (as with most other animal species)
humans lived in small, relativel
isolated and thus inbred communities.
Close Inbreeding and Human Isolates
Cousin marriage is common in human societies as a regular
5Philopatry refers to the widespread tendency of animal species to dis
locally; that is, to remain close to their place of birth and to select mates in
who share a common
Shields, 19 82a
Copyright © 2013 SciRes. 47
G. C. LEAVITT
and even preferred practice (see Leavitt, 2005; Stephens, 1963).
Societies practicing cousin marriage typically have small breed-
ing populations that have been inbreed for long periods of time
yet demonstrate little inbreeding depression. Even more telling
is human isolates that are characteristically small and com-
pletely or virtually cut-off from other human populations. Such
populations are secluded either because of geographical isola-
tion and/or through religious restrictions (see Leavitt, 2005). As
a result of their isolation, these communities become exceed-
ingly inbred even when they are diligent in their exogamous
practices (Messenger, 1971).
One of the most studied human isolates is the Samaritan com-
munity currently residing in Israel (Jamieson, 1982; Bonne-
Tamir, 1980; Talmon, 1977). The Samaritans are a small en-
dogamous religious group that broke completely from the larger
Jewish society about BC 200. The Samaritans have declined in
size from several hundred thousand during the late Roman pe-
riod to about 700 people today. Their historical and rather dra-
matic decline in population during the past millennium and a
half has been due primarily to forced conversions to Islam, but
also because of plagues, physical and economic repression. The
Samaritan population reached its smallest number of 146 peo-
ple at the end of World War II.
Because of their religious beliefs and cloistered way of life,
the Samaritans have seldom married outside of their own com-
munity, resulting in a substantially inbred population. This in-
bred condition is further amplified by their customs of marrying
within extended family lineages and by their preference for
cousin marriage. About 85 percent of all Samaritan marriages
are to first or second cousins. Not only does the Samaritans si-
tuation challenge the notion that close inbreeding is detrimental,
but it also disputes Westermarck’s hypothesis that children
raised together trigger an inherent mechanism of sexual apathy.
As described by Talmon (1977),
The Samaritans live in virtual seclusion… restricted to a
special quarter of Nablus that was a small scale replica of
a typical medieval European Jewish ghetto… Its isolation
guarded the community against alien inroads and streng-
the n ed i nte r nal cohesio n. The physical concentration of the
entire group within a comparatively small area enabled its
members to maintain their social and religious identity for
century after century even though their number was small.
Under these circumstances it seems likely that a particular
cohort of children, being small in number, is raised close to-
gether from birth.
Incest in the Immediate Family
Confronted with the considerable evidence that inbreeding in
human communities is not harmful, many evolutionary social
scientists have changed their Westermarckian claims to include
only the immediate (grandparents, aunts and uncles, and first
cousins) and nuclear families. Testing this claim is problematic
because with a strong incest taboo in the society we would not
expect to find many cases of incest. How cultural rules can be
convincingly detached from genetic factors to determine which
is actually operating is unclear, though the kibbutzim and Chi-
nese minor marriage studies examined in the next section have
been engaged in this attempt.
Even if there was no incest taboo, and even if immediate and
nuclear family members were inclined to have sexua l relations,
a number of difficulties arise that make any sustained pattern of
family incest difficult and in many cases impossible. As Slater
(1959) and Case (1969) had argued, a number of demographic
factors make an intergenerational pattern of family incest un-
likely. For example, age differences between generations, spac-
ing of siblings, and the difference in gender numbers and order
among siblings would moderate the number of incest cases.
Father-daughter or mother-son incest would be unlikely to con-
tinue into the second generation because of age differences and
would certainly not continue into the third generation.
Most telling are those incest cases that directly challenge
Westermarck’s hypothesis. Certainly there are numerous cases
of incest, though evolutionists dismiss these as rare individual
instances, often cases of pathological child abuse. Since good
data on the frequency of adult-partnered incest are absent, this
part of the discussion will go unresolved until further research
is done. More notable are cases of socially institutionalized in-
cest practices. These cases cannot be dismissed as individually
exceptional because they are instances of sociocultural practice.
Best known are the cases of royal incest such as those found
among the Inca, Hawaiian and Egyptian societies (see Goggin
& Sturtevant, 1964). Again, evolutionists dismiss these occur-
rences as rare and exceptional, though Goggin and Sturtevant
found 34 cases in the Human Relations Area Files.
A revealing circumstance is that of the Roman Egyptians
who for at least three well documented centuries practiced bro-
ther-sister incest and marriage. This was not a practice of roy-
alty but of common people. As Middleton (1962) observed,
During the period of Roman rule in Egypt there is, for the
first time, an abundance of papyrus documents and re-
cords which give evidence that commoners often prac-
ticed brother-sister marriage. These documents are of se-
veral kinds: personal letters, marriage contracts, ot her types
of contracts, petitions and documents addressed to the
administrative authorities, and census documents carrying
genealogical information. Unlike some of the earlier types
of evidence which may be subject to differing interpreta-
tions, these documents of a technical character have an
“indisputable precision” (also see Hopkins, 1980).
The instances of incestuous marriages were not rare but com-
mon and culturally condoned. In one thoroughly documented
location, Scheidel (2005) reported that “… 37 percent of the
marriages are between full siblings. Owing to the limited avai-
lability of suitable sibling-spouses in any particular family and
a strong preference for younger wives, the observed incidence
approaches the feasible maximum.” Scheidel concluded that
these sibling marriages represent a cultural custom not just a to-
Brother-sister marriages were not arranged or forced by par-
ents, nor were they ceremonial in nature. Rather Egyptian sib-
lings freely entered socially sanctioned marriages that were
[W]hat is important for our present argument is that bro-
ther-sister marriages were obviously fertile, not formal,
and were declared openly, not only in family matters but
also in business [records]; for example, the sale of a
crop… in a lawsuit… in a petition to an official… the par-
ticipators made it clear that a husband was also a brother
and that a wife was also a sister… I come to the tentative
conclusion that Egyptian brothers and sisters married each
Copyright © 2013 SciRes.
G. C. LEAVITT
other because they themselves wanted to (Hopkins, 1980).
Other culturally institutionalized cases of immediate family
incest are expectedly unusual as reflected in the literature. Sch-
roeder (1915) described incest among the Mormons, Slotkin
(1947) offered a similar example with historical Iran and Ev-
ans-Pritchard (1974) reported customary incest between broth-
ers and sisters among the Azande.
The Shepher and Wolf Marriage Studies
Shepher’s (1983) study of the Israeli kibbutzim and Wolf’s
(1995) examination of the Chinese minor marriage form have
been repeatedly forwarded by the evolutionary community as
representing unambiguous examples of Westermarck’s avoid-
ance mechanism operating in children raised in intimate cir-
cumstances. In these instances the children involved are not
siblings, thus presumably not subject to the influences of the
incest taboo. These cases are said to represent clear occurrences
where Westermarck’s mechanism has operated.
The easiest of these studies to dismiss is Shepher’s conclu-
sion that children raised together in kibbutzim peer-groups
never marry (Leavitt, 2005). These children, who take up resi-
dence in a nursery when only a few weeks old, are raised by
professionals in an intimate environment until they finish high
school. Sharing sleeping and bathroom areas, and raised in a
sexually tolerant atmosphere, these children, according to She-
pher, never marry or have sex with one another thus supporting
Westermarck’s hy pothesis. To arri ve at this conclusion Shepher
went through mathematical and definitional contortions and
made many errors. In a re-analysis of Shepher’s data, Hartung
(1985) not only found that some peer-group members married,
but did so at a rate higher than would be expected by chance.
These marriages occurred even though there are many social
and structural barrier, some deliberate, that discourage peer
mates from marrying (Talmon, 1964).
Wolf (2005) has for many years studied a Chinese marriage
form referred to as “minor marriage”. In this situation a married
couple adopts an infant or very young girl and raises her to be
their son’s wife. Being raised as brother and sister, Wolf found
that these marriages are less successful then “major marriages”
(arranged marriage), the form most common and socially de-
sirable. The lack of success in minor marriage compared to the
major form is attributed by Wolf to Westermarck’s mechanism.
Without invoking genetic inheritance, it is much easier to
understand the higher failure rate of minor marriage by simply
accepting Wolf’s account of the persuasive and destructive cul-
tural context that surrounded this minor marriage form. The
marriage records employed by Wolf were taken from a time in
Taiwan history when the island community was westernizing
and minor marriage was becoming more unpopular. In addition,
major marriages were culturally more desirable and came with
prestigious public ritual and economic advantages that were
missed if a couple was joined in the minor form. Minor mar-
riage was openly ridiculed and the butt of jokes. The minor
marriage bride is described as a cultural symbol of misery cus-
tomarily being raised in a household that treated her as a ser-
vant and which often abused her. That this marriage form was
less successful does not need inherited mechanisms to under-
stand. Indeed, we might wonder why under these social condi-
tions more minor marriage did not fail.
Inbreeding Vigor and Inclusive Fitness
The common appearance of inbreeding in human societies,
and in the animal kingdom more generally (Shields, 1982a),
suggests that inbreeding is often an adaptive strategy. In other
words, it is not just that inbreeding is usually harmless but that
it provides a distinct advantage over outbreeding under com-
mon circumstances. The breakup of successful gene combina-
tions through outbreeding in a population that overtime has
adapted to a particular environment is referred to as outbreed-
ing depression. In the extreme case we find hybrids, individuals
who come from genetically dissimilar parents or even parents
of different species. Outbred and hybrid individuals will not be
well adapted to either parental environment and thus are com-
monly selected against in nature.
Inbreeding populations preserve successful genomes and
more rapidly adapt to the residential environment. Because pa-
rents in a particular breeding population and environment rep-
resent successful genotypes/phenotypes, the offspring’s best
chances for survival are to duplicate the parental genome as
closely as possible rather than chance that a new mix of genes
will be equally or more adaptive.
Even though evolutionary scholars have routinely argued that
inbreeding should be avoided in nature, their own theoretical
concept of inclusive fitness (or kin selection) suggests other-
wise (see Leavitt, 2005). The idea of inclusive fitness proposes
that individuals will act altruistically only toward genetically
related individuals and in proportion to the strength of that ge-
netic relationship. Altruism, it is supposed by evolutionists, is a
nature induced method whereby individuals can perpetuate
their genes in the competition of life with the aid of related
individuals. If the supposition is true that individuals compete
to leave the most genes in later generations, inbreeding indi-
viduals will have a greater chance of gene survival than out-
breeding competitors because they will be more closely related
to those around them. This means they will gain more aid and
leave more genes than outbreeders. Perfect outbreeders will
discount their genome by 50% each generation and thus will
not only quickly destroy their adapted genotype but discount
the aid they will receive from others.
Discussion: The Ecology of Mating Patterns
The above reasoning does not conclude with the notion that
“more inbreeding is better.” Rather, observing the wide range
of breeding patterns in different populations through time and
space would suggest that different environments produce dif-
ferent optimal breeding strategies. In general, the breeding con-
tinuum is ultimately an inbreeding continuum since all mem-
bers of a species are related. Examining modern human “out-
breeding” populations Patterson (1978) observes that “[e]ven
today, when international travel and social mobility are rela-
tively easy, an individual is most likely to marry a close neigh-
bor, if not the girl or boy next door.” In other words, mo st popu-
lations being to a degree philopatric are inbreeding to some ex-
The great variation in human breeding patterns strongly sug-
gests that there is no Darwinian induced Westermarckian me-
chanism at work. A thorough understanding of the ecology of
human mating patterns yet eludes us, in part because we are
often looking in the wrong places. Nonetheless, some general
patterns can be observed. For example, it seems reasonable to
Copyright © 2013 SciRes. 49
G. C. LEAVITT
assume that the closeness of inbreeding in a population gener-
ally declines with greater social complexity, the increasing size
of the breeding populations, and with geographical and social
mobility. On the other hand, we can observe inbreeding in-
duced by social class, religion, ethnicity and race in the sense
that these groupings tend to be exclusive.
A more exacting ecology of breeding patterns is offered by
Caldecott (1984, 1986a, 1986b) in his study of macaque mon-
keys. Dividing macaques species into two groups, Caldecott
(1986a) found that the availability of food in the environment is
related to the degree of inbreeding. Where food is scarce in the
environment females compete with males for limited food re-
sources. Consequently, the female foraging group will keep the
male population at a distance by encouraging males to compete
with one another for breeding females. The result is that fe-
males will share sex and food with only one dominant male.
The female competition with males is further reduced because
rivalry among males leads to a high male mortality. The re-
maining males will seek mates outside of their foraging group
of origin thus encouraging outbreeding.
Where food is abundant in the macaque environment males
are not compelled by females to compete for mates and they
remain part of the foraging group in more or less equal numbers
with females. During estrous, females in these groups mate
promiscuously engaging most, if not all of the adult males in
the group. Because there is no competition among males for
mates, males do not leave the group, which results in an inbred
population. Keeping males in the foraging group also provides
greater protection against predators and increases what Calde-
cott (1986a, 1986b) calls “paternalism”—because all the males
of the group are highly related to all other members of the troop,
adult males spend a great deal of their time caring for the
Darwinian evolutionists who deal with complex social and
cultural behaviors commonly underestimate the influence that
the environment has on explaining behavior. We are so accus-
tomed to thinking of ourselves as free agents that it seldom
occurs to us that much of our behavior is often unconsciously
molded by the context within which we live. It did not occur to
Paleolithic hunters and gatherers, as they made the slow change
to agriculture, that the changing environment was dictating a
new way of life that would engulf most humans (Harris, 1977).
Though nearly universal, we would not imagine that agriculture
is an expression of genes.
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