the evolution of human behaviour

8/9/2019 The Evolution of Human Behaviour

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The Evolution of

Human BehaviourChapter 14 Alcock (Animal Behavior)

Prof. Tom Wenseleers

Ethology &Ethology &BehaviouralBehavioural

EcologyEcology


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The sociobiology controversy

Tests of sociobiological theory

Adoption ate preferences

!oercive se"

Parental care

Plan of lecture


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#. The sociobiologycontroversy$

the clash bet%eenscience and ideology


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!harles &ar%in

Application of evolutionary theory to humanspecies goes bac' to &ar%in

1859: The Origin of Specie

Traits evolve according to la%s of nat. selection

18!1: The "ecent of #an

#()*$ The $%preion of $motion

Applied his theory to the evolution of human

behaviour and society+ e.g. in relation to thehuman psyche+ social behaviour and e"pected

differences in behaviour bet%een the se"es

Emphasi,es our animal descent


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#-/0#-1)$ modern evolutionary synthesis+ e.g.

development of modern population genetics+

proof that natural selection %as compatible %ithgradual evolution+ etc...

2ome big names$ 3onald 4isher+

5.B.2. Haldane+ 2e%all Wright+ etc...

#-/6s0)6s$ application of the modern

evolutionary synthesis to the evolution of animalbehaviour+ caused increased interest in the

application of evolutionary theory to human

behaviour

&illiam 'amilton$ development of a ma7or

e"tension of the modern evolutionary synthesis$inclusive fitness theory

oert Triver$ reciprocal altruism+ parent0

offspring conflict+ se"ual conflict

#a*nar+ Smith$ application of economic thin'ing

to animal behaviour$ evolutionary game theory

The modern evolutionary synthesis


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E.8. Wilson. #-)9.

Socioiolog*: the ne, *nthei

About the evolution of social behaviour

in the animal 'ingdom: last chapter

about evolution of human behaviour.

;ave a popular account of theories

developed by evolutionary biologists

such as W.&. Hamilton+ 3. Trivers and

5.. 2mith

Heavy criticism+ mainly stemming from

various misunderstandings aboutevolution+ or politically motivated


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(i) -,e h.man +on/t +o thing eca.e ,e ,ant to raieo.r incl.ive fitne0 rather ,e .t +o them eca.e it ipart of o.r c.lt.re (-c.lt.ral relativim2)3 Alo0 man* c.lt.recannot e%pre fraction o the* cannot poil* kno, theirincl.ive fitne32 (Am3 Anthropologit #arhall Sahlin)

We don?t need to be a%are of ultimate reasons for behaviorto engage in adaptive behavior.

@t?s enough that pro"imate mechanisms motivate us to dothings that increase our direct or inclusive fitness


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(ii) -ot all h.man ehavior i iologicall* a+aptive an+ ifvario. c.lt.ral practice appear .nlikel* to a+vance

in+ivi+.al fitne3 Socioiological thinking in/t a .ef.l ,a*

of .n+ertan+ing h.man ehavior3

This assumes that all traits of an organism must becurrently adaptive+ %hich is not the case. E.g. evolutionary

psychology ma'es the specific point that many traits may

only have been adaptive in our ancestral environment .Evolutionary biology provides an intellectual toolset to

address interesting uestions in human behavior by

generating and testing plausible evolutionary hypotheses.

!riticisms %e sometimes behave maladaptively


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!riticisms genetic determinism

(iii) -'.man ehavio.r i not entirel* geneticall*+etermine+0 man* ehavio.r are learne+ or

ac6.ire+ via c.lt.ral procee32

sociobiology never claimed that human behaviour

is entirely genetically determined

dual inheritance theory$ loo's e"plicitly at gene0

culture coevolution

the genetic heritability of many traits is also very

high

cultural traits are also often inherited from

parents: in that case an evolutionary approach

can be used irrespective of the fact that the trait

in uestion is culturally determined


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Behavioural genetics

Geneticheritability(own genes)

Sharedenvironment(e.g. parentalinfluence)

Non-sharedenvironment(i.e. uniqueinfluences)

2i,e as an adult )( ## ##

Ceuroticity 96 6 96

2ocial responsability 1* * 9

3eligiosity

*# 1/

3eligion #* 9# )

2ilventoinen et al. T,in e3*66: Bouchard D c;ue 73 e.roiol3*66:

&?8nofrio et al. 73 er3#---: endler et al.Am3 73 ch3#--): 3ushton roc3 o 3 Soc3*661

Fariance in traits can be split up$ FtotGFgFseFne

heritabilityGFgIFtotGcorrelation in behaviour bet%een J t%ins reared apart


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&ual inheritance theory

&a%'ins $ suggested that the spread of cultural variants could be analysed by anevolutionary logic

=ater formalisation$ gene0culture coevolution I dual inheritance theory

!ultural transmission can either be vertical or hori,ontal .4itness of a meme$ cultural fitness.

3elatively high mutation rate compared to genetic inheritance

Luca avalli-Sfor!a "arcus #eldman $obert %oyd &ete $icherson Susan %lac'more


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!ultural traits often

inherited from parents

!avalli02for,a et al. Science#-(*

3eligion often inherited from mother

igh cultural heritability

!ultural heritability G regression of culturally determinedtrait in offspring on trait in parents


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!ultural phylogenies$ linguistics

;ray D At'inson at.re*66

2imilarity in vocabulary acrosslanguages also allo%s the construction

of language phylogenies+ shedding

light on their historical relationship.

)nglish

tree

house

Germ

an

bau

mhau

s

*nglo

-Sa+on

treow

hus

Norw

egia

n

trehus

!e

ch

strom

domovn

i

Spanish

arbolc

asa

,talian

albero

casa

$om

ania

n

arbore

cas

a

Latin

arbor domus casa

#rench

arbre

maiso

n


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!ultural phylogenies$ literature

Barbroo' et al3 The phylogeny ofthe !anterbury Tales. at.re -1$ (-

$elationship amongvarious versions ofhistorical manuscripts+ such as The

Canter.r* Tale0 can be determined using

phylogenetic methods from biology


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!ultural phylogenies$ artefacts

Tem'in D Eldredge C.rrent Anthropolog* *66)

Evolutionary trees can also be constructed for

cultural artefacts such as te"tiles+ pottery+music instruments+ arro% points+ etc....+

shedding light on their historical relationship

and on innovation in design over time.

Baltic palter* cornet


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(iv) -$vol.tionar* approache to h.man ehavior are ae+ on apoliticall* reactionar* +octrine that .pport ocial in.tice32

This criticism %as based on fear that sociobiology %ould be

used as scientific cover for legitimising social ineuality+ e.g.

bet%een races or the se"es.2cience has certainly been used to 7ustify some odious practices


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The Caturalistic 4allacy

5ustifying certain ineualities based on

evolutionary theories is a mista'e because

one cannot assume that %hat is natural is

also good+ this is the naturalistic fallacy

;eorge Ed%ard oore

rincipia $thica


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2ocial in7ustice D eugenics

erbert Spencer $ ) years before the publication of The

Origin of Specie$ "evelopment '*pothei @ntroduces the concept of the S.rvival of the ittet0 founder of 2ocial

&ar%inism

Against interference from the state: proponent of laisse,0faire politics

Humanitarian impulses had to be repressed

But$ 8nly vaguely connected to &ar%inism+ some ideas rather based on=amarc'ism

#rancis Galton

4ounder of the eugenics movement

Thought that human race could be improved via artificial selection+ as in

applied animal D plant breeding 8ften based on pseudoscientific notions of racial supremacy

@nterference from the state encouraged

But$ &ar%in distanced himself clearly from eugenics


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ilitarism

E.g. in ;ermany &ar%inism has

been used to rationalise militarism

Hitler in #ein ampf $

-'e ,ho ,ant to live m.t fight an+

he ,ho +oe not ,ant to fight in thi,orl+ ,here eternal tr.ggle i the

la, of life ha no right to e%it32

Sociobiologyho%ever struggles

to e+plain the evolutionary basis

of warfare$ if anything+ everyoneshould be better off deserting+ and

pacifism should be the rule


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3acism

&ar%in$ paternalistic attitude

to%ards other cultures+ but

also emphasi,ed that there

are more commonalities thandifferences among races+ and

also strongly opposed slavery


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2e"ism

N OriginO $ supports certainpatriarchal norms

2ociobiology also emphasi,es

certain e"pected differences in

behaviour bet%een the se"es

But by no means implies that one

se" %ould be in some %ay

superior to the other+ or that they

shouldn?t have eual rights

2teven Pin'er$ men certainly

aren?t depicted any better in

evolutionary psychology than

%oman


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2ociobiology today

Because of the perceived negative connotations of

the term sociobiology researchers started to use

other labels+ e.g. evolutionary psychology+ human

behavioural ecology+ dual inheritance theory+behavioural and e"perimental economics+

evolutionary game theory+ etcQ

@n all of these fields the evolutionary study of humanbehaviour has since proved to be a big success

2ociobiology also e"tensively tries to e"plain social

behaviour+ as opposed to selfish I egoistic behaviour


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Alternative to evolutionary approach$

!ultural relativism ain alternative to an evolutionary approach is

that human behavior is a result of culture+ %hose

development may arise from historical accidentor by arbitrary processes.

Predictions of sociobiology can be compared

%ith those of arbitrary culture theory.E.g. adoption


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*. Test case$ adoption


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Test$ adoption

arshall 2ahlins $ criticised sociobiology.Pointed out that adoption is very freuent on many

islands in the central Pacific . Rp to 6

of all children are adopted. He regarded adoption as

an arbitrary cultural tradition and sa% sociobiologyas irrelevant for understanding human behaviour.

Anthropologist 5oan 2il' $ tested %hether

adopted children %ere related to the adopters in ##cultures in 8ceania. @f they are related+ then

adoption can raise the inclusive fitness of the

adopters.


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Adoption and relatedness

Adopted children are usually nephe%s or nieces

Adoption of relatives confers an inclusive fitness advantage

5. 2il' Adoption and 'inship in 8ceania.American Anthropologit(*$ )--0(*6.


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Evolution of human behavior$

Arbitrary culture theory

But some adopters also adopt children %hose degree

of relatedness is very small or non0e"istent.

!an this be e"plainedS Adopting non0relatives appears to be beneficial in

agricultural societies %here e"tra hands increase the

farm?s productivity and enhance the survival of the

adopter?s genetic offspring.

!onsistent %ith this hypothesis small families in

8ceania are significantly more li'ely to adopt than

larger ones.


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Evolution of human behavior$

Arbitrary culture theory An alternative e"planation is that some decisions may be

maladaptive by0products of other%ise adaptive pro"imate

mechanisms. Adopting non0relatives+ thus+ may be

maladaptive+ but caring for children is usually adaptive.

Pro"imate desires strongly affect

our behavior so that in satisfying those desires %e

sometimes do not ma"imi,e our inclusive fitness.

Also supported by observations of non0humans engagingin similar behaviors+ e.g. penguins that have lost a chic'

may adopt an orphan .


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#1.


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Adaptive mate preferences

Human cultural rules concerning se"ual

behavior and mate selection are very

diverse.

&espite large cultural differences certain

basic aspects of human reproduction are

consistent across societies and parallel

those found in other mammals.


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4emale humans invest very heavily in individual

offspring and their life time reproductive success

is limited by ho% %ell they can care for offspring

rather than ho% many they can give birth to.

ales+ also invest heavily in their offspring+ but

in contrast to %omen+ have the potential to

produce many offspring .


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Adaptive mate preferences of men

;iven that females often reuire

considerable investment by males+ it

%ould be e"pected that males should be

'eenly interested in a female?s potentialfertility.


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Women differ in their li'elihood of conception.

Healthy andIor younger %omen are more li'ely to

conceive than older andIor sic' %omen. 2imilarly+

over%eight or under%eight %omen are less li'elyto become pregnant than %omen of average

%eight.

;iven that females differ in fertility+ evolutionary

biologists %ould e"pect that males have beenselected to evaluate female fertility.

@s there a relationship bet%een %hat males

consider Kgood loo'sL and fertilityS


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A cultural alternative is that males and

females have been culturally indoctrinated

to perceive KbeautyL in relation to a nearly

impossible standard designed to maintainfemale insecurity.

2o are standards of beauty arbitraryS


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ales in %estern society generally prefer

females %ho possess$ full lips+ thin noses+

large breasts+ a %aist that is narro%er than

the hips+ and an intermediate %eight.


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The listed traits are associated %ithdevelopmental homeostasis+ a strong immune

system+ good health+ high estrogen levels and

youth.

!irculating levels of estrogen+ for e"ample+ are

related to body shape in Polish %omen.


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any physical features in %omen that

men find attractive are lin'ed to fertility.

Women can only become pregnant at the

time of ovulation. Thus+ %e %ould e"pect

males to be attuned to cues that signal

ovulation.


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ales do e"hibit these traits.

en find the scent of a T0shirt %orn by an ovulating%omen se"ier than that %orn by a non0ovulating

%oman.

2imilarly+ men rate as more attractive+ facial photosof %omen ta'en during their period of ovulation thanpictures ta'en %hen the same %oman %as notovulating.

2ingh D Bronstad 4emale body odour is a potential cue to ovulation. roc3o*3 Soc3 ;on+3 B*/($ )-)0(6#.3oberts et al. 4emale facial attractiveness increases during the fertilephase of the menstrual cycle. Biolog* ;etter*)#$ *)60*)*.


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3oberts et al. 4emale facial attractiveness increases during the fertile phase of the menstrual cycle.

Biolo ;etter*)#$ *)60*)*.

follicular phase luteal phase d d


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follicular phase

luteal phase

3oberts et al. 4emale facial attractiveness increases during the fertile phase of the menstrual cycle.

Biolo ;etter*)#$ *)60*)*.

udged

most attractive


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!onclusion$ cues males use in assessing

attractiveness of partners appear to have

a significant selective advantage

associated %ith them+ %hich is notconsistent %ith the cultural e"planation.


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Adaptive mate preferences of %omen

Women also e"hibit adaptive mate

preferences.

Human females appear to rate capacity of

males to supply good genes and to

provide resources highly %hen evaluating

potential mates.


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Cumerous studies have reported that females e"press a

preference for men %ith KmasculineL features$ a

prominent chin and strong chee'bones+ athletic and

muscular upper body and facial symmetry.

These features have been lin'ed to high testosterone

levels+ good health and developmental homeostasis.

Evolutionary basis for these preferences may be that

males can pass their genes for attractiveness to their

children or that these traits are correlated %ith ability to

secure resources.



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2tudies have documented females have

preferences for male physical appearance

but most researchers have concluded that

loo's are much less important for singlemen than single %omen.

@nstead+ the difficult tas' of rearing

offspring to maturity has selected femalesto focus on securing a mate %illing and

able to assist %ith child rearing.



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!onsistent %ith this idea many studieshave sho%n that %omen prefer males %hocan supply resources and that this

preference enhances fitness.

E.g. %omen %ith %ealthier husbands in

societies %here there is no birth controlhave higher lifetime fitness than %omen%ith poorer husbands.



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Women %hose husbands are good hunters

produce more surviving offspring among the Ache

of Paraguay.

2tudies of %omen in Africa and @ran sho% that a

%oman?s reproductive success is related to her

husband?s %ealth.

Even in modern %estern societies income is

correlated %ith children?s health and chronic

illness in childhood can reduce earning po%er in

adulthood+ perpetuating the cycle of poverty.



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;iven the importance of resources to %omen?s

prospects of enhanced reproductive success it

%ould ma'e evolutionary sense for females to

favour %ealth+ status and po%er over good loo'sin a partner.

&ata from personal ads provide relevant evidence of the features

most valued in a mate$ males across culturesconsistently see' younger partners and females see' older

partners.



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#1.)

ales generally see' younger partners and femalessee' older partners.


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2chmitt et al. 73 er3 Soc3 *ch3*66


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!onditional mate preferences

What people %ant in an ideal mate and

%hat they can obtain are not al%ays the

same.

The supply of the most desirable mates is

limited and people employ conditional

strategies in mate selection that reflecttheir o%n attractiveness as mates.


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4or e"ample+ females %ho rate their o%n attractiveness

highly sho% a stronger preference for both relatively

masculine and symmetrical faces


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2imilarly+ both males and females %ho consider

themselves high ran'ing mate prospects e"press a

preference for similarly highly ran'ed mates.


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2e"ual conflict

Because male and female fitness interests

do not necessarily coincide se"ual conflict

is li'ely.

8ne significant area of conflict is male?s

on average greater interest in multiple

se"ual partners.


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2e"ual conflict

en consistently e"press a greater interest in having

multiple se"ual partners over a given time period than

%omen.


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2e"ual conflict

en and %omen also differ in ho% li'ely they %ould be to

have se" %ith a partner after 'no%ing the individual for

different lengths of time.


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2e"ual conflict

en are less selective in the choice of a partner

!lar' D Hatfield 73 *ch3 '.m3 Se%3#-(-

ales 4emales

KWould you go out %ith me tonightSL

KWould you come to my appartmentSL

KWould you go to bed %ith me tonightSL


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en are less selective in the choice of a partner

!lar' D Hatfield 73 *ch3 '.m3 Se%3#-(-

ales 4emales

KWould you go out %ith me tonightSL 96 9/KWould you come to my appartmentSL /- /

KWould you go to bed %ith me tonightSL )9 6

2e"ual conflict


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en are less selective in the choice of a partner+ particularly

in the conte"t of short0term relationships

&avid Buss en team van 96 %etenschappers bestudeerden#6+666 mensen

in ) culturen

op / continenten

%esluit se'se verschillen in selectiviteit partner'eu!e !in universeel

2e"ual conflict

2h t t l ti hi D


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2hort0term relationships D

e"tra0pair paternity 2hort0term relationships$

in men+ %e might e"pect

these$ small cost+ potentially

large fitness advantage

But also %oman engage inshort0term relationships$ ca.

*09 of all children are

e"tra0marital

%hyS;enetic father %ith good

genes a good parent


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2e"ual conflict

Polygynous men can potentially achieve substantially

higher reproductive success than monogamous men and

polygyny has been %idespread in human history.

Women+ ho%ever+ are li'ely to do %orse in a polygynous

system.

4or e"ample+ in #-thcentury Rtah monogamous ormon

%omen married to relatively poor men had more

surviving children on average than %omen married

to rich polygynous men . The polygynous men+ of course+ did much better than

the monogamous men.


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2e"ual conflict

Potential benefits of polygyny to males

increase potential for conflict bet%een

mates.

Ho%ever+ females may also opt for e"tra0

pair matings if these offer better genes+

more resources or the possibility oftrading0up to a better partner.


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2e"ual conflict

2e"ual 7ealousy is li'ely an evolutionary

conseuence of the conflict bet%een the se"es.

The conditions that cause the most intense

e"pression of 7ealousy appear to differ bet%eenthe se"es.

ales respond most to a potential loss of

paternity and se"ual infidelity %hereas females

e"press greater concern %hen their matesdevelop deep emotional relationships %ith other

%omen+ %hich threaten the long0term

partnership.


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1. !ontroversy$ coercive se"


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!oercive se"

Thornhill D Palmer$ evolutionary biologists %ho

%rote a boo' on the natural history of rape


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!oercive se"

Ues+ raped %oman are most often in their early *6s+ %hen

they are ma"imally fertile. urder victims+ by contrast are

usually %ell into their 6s.


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!oercive se"

re6.enc* of tar cl.ter < chromoome haplot*pe3

Sha+e+ area=e%tent of >enghi han? empire at the time

of hi +eath.

!a. ( of all men in Asia carry a U chromosome haplotype that originates from male0

line descendants of ;enghis han .

Probably lin'ed to the slaughter+ pillage and rape e"ercised

during the many conuests of ;enghis han and his male0

line descendants .

#e+ian@oining net,ork of


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!oercive se"

!oercive se" also occurs in the animal 'ingdom+

e.g. in the @ron cross blister beetle.

=eft$ normal courtship pattern

3ight$ coercive se"


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9. Test case$ parental care


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Adaptive parental care

a'ing decisions that increase the

li'elihood that genetic offspring %ill

reproduce successfully and directing

resources so as to ma"imi,e the numberof grandchildren produced by offspring are

strategies li'ely to be favored by selection.


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Humans appear to apply such strategies.

@t is %ell established that parents of genetically related and

stepchildren discriminate in resource allocation.

4or e"ample+ the odds that a man %ill give a child money for college

is much higher if he is the genetic father rather than the stepfather.


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2imilarly+ stepmothers are less li'ely tocare for stepchildren than their o%noffspring.

Households in %hich a %oman cares forstepchildren+ foster children or adopted

children spend less on food thanhouseholds in %hich the mother cares forgenetic offspring.


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&aly+ D Wilson+ . *66#. An assessment of some proposed e"ceptions to the

phenomenon of nepotistic discrimination against stepchildren.Annale oologici ennici

($ *()0*-/.

/ype of father

Genetic father Stepfather

V murders per million I yr *./ **

#+//9+666 #6+666

2tepchildren are also more li'ely to suffer abuse thangenetic children.

@n one !anadian study the relative ris' of abuse %asmuch higher for stepchildren than for genetic children.

Adaptive parental care$ assisting


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children?s reproduction

Parents not only discriminate bet%een

genetically related children and step

children+ but also discriminate among

genetically related children.

@n many societies parent commit

resources to enable their offspring tomarry.



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@n some societies the grooms family must contributeresources to the bride?s family+ %hereas in

others the bride?s family contributes a do%ry.

@f these payments are purely cultural %e %ould e"pect the t%o

forms to be eually common. Ho%ever+ they are not. 2e"ual selection theory suggests that because males usually

compete for females that bride%ealth payments should be

more common than do%ries.

@n // of societies studied bride%ealth payments occur+ butdo%ries in only . Bride%ealth payments are especially

common in cultures %here men can have more than one %ife.

@n these societies marriageable females are in demand and

so demand a price.


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asai fatherand his soon0to0be

0married daughter.

The father %ill have

received a

bride%ealth

payment beforegiving permission

for the marriage.



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Even in supposedly monogamous %esternsocieties rich men may have greateropportunities for reproductive success.

Parents in modern societies appear to retain anancestrally selected bias that favors investingmore in the offspring %ith the highestreproductive potential.

This sho%s in inheritance decisions. Wealthy!anadians bias their legacies to%ards sons.


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marriageable females in demand and so demand a price

sons have much greater

reproductive potential

Adaptive advantage of menopause


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01th-02th century

#inland anada

Age of having #st child *9.1 **.(

Age of having last child -. (.)

Cumber of children /.( -.#

Cumber of children that

survive to #9 years

.( 9.#

Cumber of grandchildren ##. (.*

=ife e"pectancy for %omenaged 96

/).9 )1.6


=ahdenper et al. *661. at.re

Why stay alive past menopause given that %omen are no longer fertile thenS

Theory$ help rear grandchildren .



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0

10

20

30

40

50

40 50 60 70 80 90 100

Total

number

ofgran

dchildren

=ife e"pectancy

!anada+ n G */*

4inland+ n G -

03 years of post-menopausal life results in

ca. 4 e+tra grandchildren

=ahdenper et al. *661. at.re


Economic game theory


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co o c ga e t eo y

0conomic game theory$ aimed at determining %hat people

should do in conflict situations if they behaved rationally


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The future

E.8. Wilson proposed in #-)9 in his boo'

2ociobiology$ The Ce% 2ynthesis that

evolutionary theory %ould transform the

social sciences Was he rightSS

8nly the future %ill tell

the evolution of human behaviour

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