Tylor vs. Westermarck: Explaining the Incest Taboo

doi:10.4236/sm.2013.31008

Paper Menu >>

Journal Menu >>

Sociology Mind

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

Tylor vs. Westermarck: Explaining the Incest Taboo

Gregory C. Leavitt

Idaho State University Pocatello, Pocatello, USA

Email: [email protected]

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

Introduction

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

for survival.

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-

lutionary outcome.

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-

tionships.

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.

Alliance Theory

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-

munities.

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

communities.

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-

wardly directed.

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

likely.

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.

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

evolutionary scholars.

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-

tives.

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

erse

locally; that is, to remain close to their place of birth and to select mates in

that localit

who share a common

enetic herita

(

Shields, 19 82a

)

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-

lerated alternative.

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

affectionately motivated.

[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

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-

tent.

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

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

young.

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.

REFERENCES

Bengtsson, B. O. (1978). Avoiding inbreeding: At what cost? Journal

of Theoretical Biology, 73, 439-444.

doi:10.1016/0022-5193(78)90151-0

Bonne-Tamir, B. (1980). The Samaritans: A living anci e nt i so l at e. In A.

W. Eriksson, H. R. Forsius, H. R. Nevanlinna, P. L. Workman, & R.

K. Norio (Eds.), Population structure and genetic disorders (pp. 27-

41). London: Academic Press.

Caldecott, J. (1984). Coming of age in macaca. New Scientist, 101, 10-

12. doi:10.1016/0003-3472(86)90025-4

Caldecott, J. (1986a). Mating patterns, societies and the ecogeography

of macaques. Animal Behavior, 34, 208-220.

Caldecott, J. (1986b). An ecological and behavioural study of the pig-

tailed macaque. New York: Karger.

Case, C. (1969). Comments in Current Anthropology, 10, 50-51.

Cohen, Y. (1978). The disappearance of the incest taboo. Human Na-

ture, 1, 72-78.

Evans-Pritchard, E. E. (1974). Man and woman among the Azande.

New York: Free Press.

Goggin, J. M., & Sturtevant, W. C. (1964). The calusa: A stratified,

nonagricultural society (with notes on sibling marriage). In W. H.

Goodenough (Ed.), Explorations in cultural anthropology: Essays in

honor of George Peter Murdock (pp. 179-219). New York: McGraw-

Hill.

Harris, M. (1977). Cannibals and kings. The origins of cultures. New

York: Random House.

Hartung, J. (1985). Incest. A biosocial view. American Journal of Phy-

sical Anthropology, 67, 169-171. doi:10.1002/ajpa.1330670213

Honigmann, J. J. (1976). The development of anthropological ideas.

Homewood, IL: The Dorse y Press.

Hopkins, K. (1980). Brother-sister marriage in Roman Egypt. Com-

parative Studies in Society a n d H i s t or y , 22, 303-354.

doi:10.1017/S0010417500009385

Jamieson, J. W. (1982). The Samaritans. Mankind Quarterly, 23, 141-

148.

Leavitt, G. C. (1989). Disappearance of the incest taboo: A cross-cul-

tural test of general evolutionary hypotheses. American Anthropolo-

gist, 91, 116-131. doi:10.1525/aa.1989.91.1.02a00070

Leavitt, G. C. (1990). Sociobiological explanations of incest avoidance:

A critical review of evidential claims. American Anthropologists, 92,

971-993.

Leavitt, G. C. (2005). Incest and inbreeding avoidance. A critique of

darwinian social sci e n c e. Lewiston, NY: The Edwin Mellen Press.

Livingstone, F. B. (1969). Genetics, ecology and the origins of incest

and exogamy. C u rrent A nthr op o logy, 10, 45-61. doi:10.1086/201009

Messenger, J. C. (1993). Sex and repression in an Irish folk community.

In D. N. Suggs, & A. W. Miracle (Eds.), Culture and human sexual-

ity (pp. 240-261). Pacific Grove, CA: Brooks/Cole Publishing.

Middleton, R. (1962). Brother-sister and father-daughter marriage in

ancient Egypt. American So ciological Review, 27, 603-611.

doi:10.2307/2089618

Murdock, G. P. (1949). Social s t r u c ture. New York: Free Press.

Parsons, T. (1954). The incest taboo in relation to social structure and

the socialization of the child. British Journal of Sociology, 5, 101-

117. doi:10.2307/587649

Patterson, C. (1978). Evolution. Ithaca, NY: Cornell University Press.

Scheidel, W. (2005). Ancient Egyptian sibling marriage and the Wester-

marck effect. In A . P. Wo lf , & W. H. Durham (Eds.), Inbreeding, in-

cest, and the incest taboo (pp. 93-108). Palo Alto, CA: Stanford Uni-

versity Press.

Schroeder, T. (1915). Incest in mormonism. American Journal of Uro-

logy and Sexology, 11, 409-416.

Shepher, J. (1983). Incest. A biosocial view. New York: Academic Press.

Shields, W. M. (1982a). Philopatry, inbreeding, and the evolution of

sex. Albany, NY: State University of New York Press.

Shields, W. M. (1982b). Inbreeding and the paradox of sex: A resolu-

tion? Evolutionary Theory, 5, 245- 279.

Shields, W. M. (1983). Optimal inbreeding and the evolution of philo-

patry. In I. R. Swingland, & P. J. Greenwood (Eds.), The ecology of

animal movement (pp. 132- 159). Oxford: Clarendon Press.

Shields, W. M. (1987). Dispersal and mating system: Investigating their

causal connections. In D. B. Chepko-Sade, & Z. T. Halpin (Eds.),

Mammalian dispersal patterns: The effects of social structure on po-

pulation genetics (pp. 3 -24). Chicago, IL: Chicago University Press.

Shields, W. M. (1988). Sex and adaptation. In R. E. Michod, & B. R.

Levin (Eds.), The evolution of sex: An examination of current ideas

(pp. 253-332). Sunderland, MA: Sinauer Associates.

Shields, W. M. (1993). The natural and unnatural history of in breeding

and outbreeding. In N. Wilmsen (Ed.), The natural history of in-

breeding and outbreeding (pp. 143-169). Chicago, IL: University of

Chicago Press.

Slater, M. K. (1959). Ecological factors in the origin of incest. Ameri-

can Anthropologist, 61, 1042-1059.

doi:10.1525/aa.1959.61.6.02a00090

Slotkin, J. S. (1947). On a possible lack of incest regulation in old Iran.

American Anthropologist, 49, 612-617.

doi:10.1525/aa.1947.49.4.02a00080

Stephens, W. N. (1963). The family in cross-cultural perspective. Wa-

shington DC: University Press of America.

G. C. LEAVITT

Talmon, S. (1964). Mate selection in collective settlements. American

Sociological Review, 29, 491-508.

Talmon, S. (1977). The Samaritans. Scientific American, 236, 100-108.

doi:10.1038/scientificamerican0177-100

Tylor, E. B. (1888). On a method of investigating the development of

institutions; applied to laws of marriage and descent. Journal of the

Royal Anthropological Institute, 18, 245-269.

Westermarck, E. (1922). The history of human marriage. London:

Macmillan.

White, L. A. (1948). The definition and prohibition of incest. American

Anthropologist, 5, 416-435. doi:10.1525/aa.1948.50.3.02a00020

Wilson, E. O. (1975). Sociobiology. The new synthesis. Cambridge, MA:

Belknap Press.

Wolf, A. (1995). Sexual attraction and childhood association. A Chi-

nese brief for Edward Westermarck. Stanford, CA: Stanford Univer-

sity Press.