Chapter

8：Sex:Evolutionary,HormonalandNeuralBases

Biological

PsychologyInstructor:

xiaohong

PanSchool

of

Psychology

and

Cognitive

xhpan@Contents:一.

SexualBehavior二.

WhyAreThereTwoSexes三.

SexualDifferentiation四.Summary一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStages

Sexualattraction:bringthemaleandfemaletogether.Inmanyspeciesanindividualisattractivetoothersonlywhenitsbodyisreadytoreproduce.Gauge(測(cè)量)howattractiveafemale/maleisbyobservinghowmales/femalesrespondtoher/him—howrapidlytheyapproach,howhardtheyworktogainaccesstoher/him,andsoon.

sexskin—estrogen(雌激素)odor一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStages

Sexualattraction

Appetitivebehaviors:behaviorsthatestablish,maintain,orpromotesexualinteractionfemale—

procetive

(邀配)behaviorsmale—

consist

of

staying

near

the

female,sniffaroundthefemale’sfaceandvagina(mammals),engageinelaboratesongsornest-buildingbehaviors(birds)一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStages

Sexualattraction

Appetitivebehaviors

Copulation(Coitus):intromission,ejaculatesCopulationbringsgametestogetherforreproductionexternalfertilizationinternalfertilizationGametesEgg,ovaSpermfertilizationzygote一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStages

Sexualattraction

Appetitivebehaviors

Copulation(Coitus):intromission,ejaculatesCopulationbringsgametestogetherforreproductionRefractoryphase:Coolidgeeffect:manyanimalsshowashorterrefractoryphaseiftheyareprovidedwithanewpartner一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStages

Sexualattraction

Appetitivebehaviors

Copulation(Coitus):intromission,ejaculatesCopulationbringsgametestogetherforreproductionRefractoryphaseThe

female

often

appears

to

be

the

one

to

choose

whether

or

not

copulation

will

take

placeUsually

females

are

receptive

only

during

periods

when

mating

is

likely

to

result

in

reproduction.一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStages

Sexualattraction

Appetitivebehaviors

Copulation(Coitus)

PostcopulatorybehaviorscopulatorylockparentalbehaviorsStagesofReproductive

BehaviorHormonesplayanimportantroleinanimalsmatingbehaviors.

Testosterone

increases

the

male’s

interest

in

female,

causes

him

to

mount

females

more

rapidly

and

more

often,

and

allows

him

to

achieve

ejaculation.

castrate(閹割)activationaleffect:thehormonetransientlyinfluencesbehavior.AndrogenspermitmalecopulatorybehaviorHormoneleveldoesnotcompletelydeterminetheamountofmalesexualbehaviorHormonesplayanimportantroleinanimalsmatingbehaviors.

thebloodlevelsoftestosteronenotresponsibleforthedifferenceinthevigor(matewithareceptivefemale)

in

fact,

a

very

small

amount

of

testosterone—one-tenth

the

amount

normally

produced

by

the

animals—is

enough

to

maintain

the

mating

behavior

of

stud

rats.Hormonesplayanimportantroleinanimalsmatingbehaviors.

thebloodlevelsoftestosteronenotresponsibleforthedifferenceinthevigor(matewithareceptivefemale)

inducesexualreceptivityinfemalebyinjectingwiththe

sequence

of

estrogens

followed

by

progesterone

Estrogens(雌激素)

Progesterone(孕激素)一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStagesTheNeuralCircuitryoftheBrainRegulatesReproductiveBehavior

Mostofwhatweknowabouttheneuralcircuitryunderlyingreproductivebehaviorscomesfromanimalmodels,especiallyrates.Inawiderangeofvertebratespecies,steroidreceptorsareconsistentlyfoundinmanybrainregions,includingthehypothalamus,medialamygdala,andhippocampus,amongmanyother.Steroideffectsinthehypothalamusplayacentralroleinregulatingcopulatorybehavior,andthese’sreasontobelievethatthisistrueofhumanaswell.一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStagesTheNeuralCircuitryoftheBrainRegulatesReproductiveBehaviorSteroidsActontheHypothalamustoPromoteFemaleReceptivitySteroidautoradiographyindicatedthatthehypothalamuscontainsacenterforestrogen-andprogesterone-sensitiveneuronsTheroleoftheventromedialhypothalamus(VMH)istomonitorsteroidhormoneconcentrationsand,attherighttimeintheovulatorycycle,activateamultisynapticpathwaythatinducesthespinalcordtocontractbackmuscles,producingalordosisresponsethatisotherwiseabsent.VMHTheVMHwasfoundtobecrucialforlordosisbecauselesionsthereabolishtheresponse.Tinyimplantsofestradiol(雌二醇)intheVMHcaninducereceptivity

EstrogencanincreasethesizeofthedendritictreesofVMHneurons

Estrogeninducetheproductionofprogesteronereceptors,andactivatedthemtheVMHsendsaxonstotheperiaqueductalgray(PAG),wherelesionsgreatlydiminishlordosis.PAGneuronsprojecttothemedullaryreticularformation,whichinturnprojectstothespinalcordviathereticulospinaltract(網(wǎng)狀脊髓束).NeuralCircuitsforReproductioninRodents(Female)Estrogensaffectneurons

at

these

two

sitesEstrogensinduce

production

of

progesterone(孕酮)

receptors

to

respond

so

that

hormone一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStagesTheNeuralCircuitryoftheBrainRegulatesReproductiveBehaviorSteroidsActontheHypothalamustoPromoteFemaleReceptivityAndrogensActivateaNeuralSystemforMaleReproductiveBehaviorHypothalamicmedialpreopticarea(視前區(qū))(mPOA)ischock-fullofsteroid-sensitiveneuronsmPOACastrate&Implant

lesions

of

the

mPOA

abolish

male

copulatory

behavior

in

a

wide

variety

of

vertebrate

species.

mating

can

be

reinstated

in

castrated

males

by

small

implants

of

androgens

in

the

mPOA

but

not

in

other

brain

regions.mPOACastrate&ImplantNeuronsinthemPOAsendtheiraxonstotheventralmidbrainviathemedialforebrainbundle.Fromthere,informationgoestothebasalgangliaand,throughadiffusemultisynapticpathway,tothespinalcord.Thespinalcordmediatesthevariousreflexesofintromissionandejaculation.一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStagesTheNeuralCircuitryoftheBrainRegulatesReproductiveBehaviorSteroidsActontheHypothalamustoPromoteFemaleReceptivityAndrogensActivateaNeuralSystemforMaleReproductiveBehaviorHypothalamicmedialpreopticarea(視前區(qū))(mPOA)ischock-fullofsteroid-sensitiveneurons

Vomeronasalorgan(犁鼻器)VomeronasalorganReceptivefemalereleasepheromonesthatcanbedetectedbythemale’svomeronasalorganandarousethemale,asevidencedbypenileerections.Thevomeronasalinformationfromtoaccessoryolfactorybulbprojectstothemedialamygdala,whereinturnsendsaxonstothemPOA.mPOAappearstointegratehormonalandsensoryinformationsuchaspheromones,andtocoordinatethemotorpatternsofcopulation.NeuralCircuitsforReproductioninRodents(Male)Pheromones

from

receptive

females

are

detected

by

the

vomeronasal

organAndrogens

are

converted

to

estrogen

and

act

on

these

two

sites.Androgens

act

on

spinal

neurons

to

augment

reflexes.一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStagesTheNeuralCircuitryoftheBrainRegulatesReproductiveBehaviorHumanReproductionBringsGametesTogether一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStagesTheNeuralCircuitryoftheBrainRegulatesReproductiveBehaviorHumanReproductionBringsGametesTogetherTheHallmarkofHumanSexualBehaviorIsDiversityI.

TheHallmarkofHumanSexualBehaviorIsDiversityAlfredKinseyconstructastandardizedsetofquestionsI.

TheHallmarkofHumanSexualBehaviorIsDiversityAlfredKinseyThemostcommoncoital(性交的)posturesisface-to-faceposture

TypicalresponsepatternsofhumanasconsistingoffourphasesFourPhases

Increasingexcitement:thephallusbecomesengorgedwithblood,makingiterect.theparagigantocellularnucleus(PGN,旁巨細(xì)胞核)inponssendsserotonergic

(血清素)fibersdownthespinalcordthatinhibiterection,andsexuallyarousedforebraininhibitsthePGN,permittingerectionFourPhasesIncreasingexcitement:thephallusbecomesengorgedwithblood,makingiterect.PlateauOrgasmResolutionGenderDifferentThegreatervarietyofcommonlyobservedsequencesinwomen.Menhaveonlyonebasicpattern,womenhavethreetypicalpatternsGenderDifferentThegreatervarietyofcommonlyobservedsequencesinwomen.Mostmenhaveanabsoluterefractoryphasefollowingorgasm一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStagesTheNeuralCircuitryoftheBrainRegulatesReproductiveBehaviorHumanReproductionBringsGametesTogetherTheHallmarkofHumanSexualBehaviorIsDiversitySomeDiseasesCanBeTransmittedthroughSexualBehaviorHumanimmunodeficiencyvirus(HIV)Syphilis(梅毒)Gonorrhea(淋病)一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStagesTheNeuralCircuitryoftheBrainRegulatesReproductiveBehaviorHumanReproductionBringsGametesTogetherTheHallmarkofHumanSexualBehaviorIsDiversitySomeDiseasesCanBeTransmittedthroughSexualBehaviorHormonesPlayOnlyaPermissiveRoleinHumanSexualBehaviorIII.

HormonesPlayOnlyaPermissiveRole inHumanSexualBehaviorAndrogensindeedstimulatesexualactivityinmen(double-blindtests)Testosteronelevelsbelownormalaresufficienttorestorebehavior,andthereisnocorrelationbetweensystemicandrogenlevelsandsexualactivityamongmenwhohavesomeandrogenAndrogensmayalsoactivatesexualinterestinwomen.Sexualbehaviorvarieswiththemenstrualcycle?一.

SexualBehaviorReproductiveBehaviorCanBeDividedintoFourStagesTheNeuralCircuitryoftheBrainRegulatesReproductiveBehaviorHumanReproductionBringsGametesTogetherPheromones(信息素)BothPrimeandActivateReproductiveBehaviors4.

PheromonesBothPrimeandActivate ReproductiveBehaviorsSexualactivationcanbethoughtofascommunicationbetweenthegonadsandthebrain(insidetheindividual)

byproducingsteroidstomakegametesthegonadsalsoinformthebrainthatthebodyisreadytomate4.

PheromonesBothPrimeandActivate ReproductiveBehaviorsSexualactivationcanbethoughtofascommunicationbetweenthegonadsandthebrainHormonescanalsoprovideinformationbetweenanimalsthepheromoneactivatesanotherindividual’sbehaviorinarapidandbriefmanner(E.g.TheFprostaglandin(前列腺素)offemalegoldfish)Primeseffect:aggression(male)/blockingofpregnancy(female)Contents:SexualBehavior二.

WhyAreThereTwoSexes三.

SexualDifferentiation四.Summary二.

WhyAreThereTwoSexes

SexualReproductionHelpsCombineBeneficialMutations(突變)SexualReproductionHelpsCombineBeneficialMutationsfission→parthenogenesis

→sexual

reproduction

the

function

of

sexual

reproduction

is

to

bring

many

beneficial

mutations

(突變)

together,

each

of

which

arose

in

different

individuals.

The

mixing

of

beneficial

genes

certainly

generates

biological

diversity.

producing

new

combinations

of

genes

that

may

better

survive

the

new

conditions

Some

animals

can

reproduce

by

either

method.

For

example:

aphid蚜蟲）SexualReproductionHelps CombineBeneficialMutationsfission→parthenogenesis→sexual

reproduction

the

function

of

sexual

reproduction

is

to

bring

many

beneficial

mutations

(突變)

together,

each

of

which

arose

in

different

individuals.

The

mixing

of

beneficial

genes

certainly

generates

biological

diversity.

producing

new

combinations

of

genes

that

may

better

survive

the

new

conditions

Some

animals

can

reproduce

by

either

method.

Hermaphrodites

(雌雄同體)二.

WhyAreThereTwoSexesSexualReproductionHelpsCombineBeneficialMutationsMalesandFemalesOftenAdoptDifferentReproductiveStrategiesMalesandFemalesOftenAdoptDifferentReproductiveStrategiesType

of

reproductionStrategyExamplesAsexualFissionUnicellular

animalsParthenogenesisi(孤雌生殖)

ObligatoryAmazon

mollies

(帆鰭鱸)

OccasionalAphids

(蚜蟲)SexualHermaphroditism(雌雄同體性)

SimultaneousMany

invertebrates

SerialSome

fishSeparate

sexes

Sexually

monomorphicMany

birds

Sexually

dimorphicMost

mammalsGender

Difference

Males

are

not

always

the

larger

sexGender

Difference

Males

are

not

always

the

larger

sex

Nervous

systems

display

sexual

dimorphism

Male

gametes

are

small

and

cheap,

female

gametes

large

and

expensiveGender

Difference

Males

are

not

always

the

larger

sex

Nervous

systems

display

sexual

dimorphism

Male

gametes

are

small

and

cheap,

female

gametes

large

and

expensive

Courtship

is

a

period

during,

which

the

female

indirectly

assesses

the

genetic

makeup

of

a

male

to

judge

his

suitability

as

a

mate.二.

WhyAreThereTwoSexesSexualReproductionHelpsCombineBeneficialMutationsMalesandFemalesOftenAdoptDifferentReproductiveStrategiesSexualSelectionAccentuatesDifferencesbetweentheSexesSexualSelectionAccentuatesDifferencesbetweentheSexes

Sexual

selection:

a

special

type

of

natural

selection—feature

二.

WhyAreThereTwoSexesSexualReproductionHelpsCombineBeneficialMutationsMalesandFemalesOftenAdoptDifferentReproductiveStrategiesSexualSelectionAccentuatesDifferencesbetweentheSexesMatingSystemsCanBeClassifiedintoFourBasicTypes4.

MatingSystemsCanBeClassifiedintoFourBasicTypes

Promiscuity:

with

several

partners

and

do

not

establish

long-lasting

associations

Polygyny:

one

male

mating

with

a

group

females

Polyandry:

each

female

of

the

species

mates

with

several

males

Monogamy:MatingSystemsContents:SexualBehaviorWhyAreThereTwoSexesSexualDifferentiation四.Summary三.

SexualDifferentiation

TheSexofanIndividualIsDeterminedEarlyinLifeThepathofsexualdifferentiationissetbythechromosomeMale—heterogametic(異形配子),theyhavetwodifferentsexchromosome(XY)Female—homogametic(同配子),theyhavetwosamesexchromosome(XX)三.

SexualDifferentiation

TheSexofanIndividualIsDeterminedEarlyinLifeThepathofsexualdifferentiationissetbythechromosomeSexChromosomesDirectSexualDifferentiationoftheGonadsMaleIn

mammals

the

Y

chromosome

contains

a

gene

called

the

SRY

geneIf

an

individual

has

a

Y,

the

cells

in

the

core

of

indifferent

gonad

begin

making

the

SRY

protein.（性相關(guān)蛋白~）The

SRY

protein

causes

the

cells

in

the

core

of

thedifferentgonad(themedulla)toproliferate(增生)attheexpenseoftheouterlayers(thecortex),andtheindifferentgonaddevelopsintoatestis.三.

SexualDifferentiation

TheSexofanIndividualIsDeterminedEarlyinLifeThepathofsexualdifferentiationissetbythechromosomeSexChromosomesDirectSexualDifferentiationoftheGonadsMaleFemaleIftheindividualhasnoY(orhasaY,buttheSRPgeneisdefective),noSRYproteinisproduced,andtheindifferentgonadtakesadifferentcourse:Cellsofthecorticallayersproliferatemorethanthoseofthemedullarylayers,andanovaryforms.（自然傾向于女性）

三.

SexualDifferentiation

TheSexofanIndividualIsDeterminedEarlyinLifeThepathofsexualdifferentiationissetbythechromosomeSexChromosomesDirectSexualDifferentiationoftheGonadsGonadalHormonesDirectSexualDifferentiationoftheRestoftheBodyDevelopingtestesproduceseveralhormones,whileearlyovariesproduceverylittlehormone.Theembryo

(胎兒),receivethetesticularhormones,willbegindevelopingmasculinecharacters;otherwise,developfemininecharacters.副睪前列腺陰囊膨脹陰蒂大陰唇陰道精囊SexualDifferentiationandGenderIdentity(1)SexualDifferentiationandGenderIdentity(2)SexualDifferentiationandGenderIdentity(3)Congenitaladrenalhyperplasia(CAH)istheresultoffemaleexposuretoandrogensbeforebirth.Androgensareproducedinsteadofcorticosteroidsandthenewbornhasanintersexappearance.Notestesarepresentandovariesarenormal,despiteappearanceofexternalgenitalia.AnIntersexPhenotypeThesegenitaliaarecharacteristicofnewborngirlswithcongenitaladrenalhyperplasia(CAH)Congenitaladrenalhyperplasia(CAH)istheresultoffemaleexposuretoandrogensbeforebirth.Cloacalexstrophy（泄殖腔外翻）referstogeneticboysbornwithtestesbutwithoutpenises.Congenitaladrenalhyperplasia(CAH)istheresultoffemaleexposuretoandrogensbeforebirth.Cloacalexstrophy（泄殖腔外翻）referstogeneticboysbornwithtestesbutwithoutpenises.Androgeninsensitivitysyndrome(AIS)occursinXYpeoplewhoseandrogenreceptorsdonotrespondtotestosterone.Testesremaininternalandtheydevelopmainlyaswomen.Androgen-InsensitiveWomen三.

SexualDifferentiationTheSexofanIndividualIsDeterminedEarlyinLifeHowShouldWeDefineGender—byGenes,Gonads,Genitals,ortheBrain?BehaviorismuchmoredifficultthanphysicalfeaturestodefineasfeminineormasculineFeaturesmayconfusing(e.g.,androgen-insensitive)Partsofbrainaretypicallydifferentbetweenthesexesinhumanandotheranimals.三.

SexualDifferentiationTheSexofanIndividualIsDeterminedEarlyinLifeHowShouldWeDefineGender—byGenes,Gonads,Genitals,ortheBrain?GonadalHormonesDirectSexualDifferentiationoftheBrainandBehaviorEarlyTesticularSecretionsResultinMasculineBehaviorinAdulthoodI.

EarlyTesticularSecretionsResultin MasculineBehaviorinAdulthoodOrganizationalhypothesis(1959,WilliamC.Young)Steroidshaveanorganizationalinfluenceonlyduringearlydevelopment(—sensitiveperiod)andtheeffectispermanent,inadulthoodtheyexertanactivationaleffect,temproarilyinfluencingbehavior.Thenervoussystemisatypeoftissuelisteningfortheandrogenicsignal;otherwise,willorganizeitselfinafemininefashion三.

Sexual

DifferentiationTheSexofanIndividualIsDeterminedEarlyinLifeHowShouldWeDefineGender—byGenes,Gonads,Genitals,ortheBrain?GonadalHormonesDirectSexualDifferentiationoftheBrainandBehaviorEarlyTesticularSecretionsResultinMasculineBehaviorinAdulthoodTheEstrogenicMetabolitesofTestosteroneMasculinizetheNervousSystemandBehaviorofRodentsTheEstrogenicMetabolitesofTestosteroneMasculinizetheNervousSystemandBehaviorofRodentsWhennewbornfemaleratsweretreatedwithverysmalldoseofestrogenscouldpermanentlymasculinizebehaviors;whereasachievingthesameeffectwithtestosteronerequiredahigherdose.Themajorandrogen(testosterone)andthemajorestrogen(estradiol)differbyonlyonemolecule.Aromatizationhypothesisα-fetoprotein

（胎蛋白）Aromatizationhypothesisrosen_08.dcr三.

SexualDifferentiationTheSexofanIndividualIsDeterminedEarlyinLifeHowShouldWeDefineGender—byGenes,Gonads,Genitals,ortheBrain?GonadalHormonesDirectSexualDifferentiationoftheBrainandBehaviorEarlyTesticularSecretionsResultinMasculineBehaviorinAdulthoodTheEstrogenicMetabolitesofTestosteroneMasculinizetheNervousSystemandBehaviorofRodentsSeveralRegionsoftheNervousSystemDisplayProminentSexualDimorphism

SeveralRegionsoftheNervousSystemDisplayProminentSexualDimorphismSong

control

regions

in

male

songbirds

(

male

larger

than

female)

Syrinx

(咽鼓管)—XII

cranial

nerve

SeveralRegionsoftheNervousSystemDisplayProminentSexualDimorphismSong

control

regions

in

male

songbirds

(

male

larger

than

female)The

preoptic

area

(POA)

of

rats

(sexually

dimorphic

nucleus

of

the

POA,

SDN-POA)SexuallyDimorphicNucleusofthePreopticArea(SDN-POA)

SeveralRegionsoftheNervousSystemDisplayProminentSexualDimorphismSong

control

regions

in

male

songbirds

(

male

larger

than

female)The

preoptic

area

(POA)

of

rats

(sexually

dimorphic

nucleus

of

the

POA,

SDN-POA)The

spinal

cord

in

mammals

(spinal

nucleus

of

the

bulbocavernosus

(球海綿體),

SNB)Inrats,thespinalnucleusofthebulbocavernosus(SNB)islargerinmales.Motoneuronsinthisnucleusinnervatebulbocavernosus(BC)musclesthatsurroundthebaseofthepenis.SexualDifferentiationoftheSpinalNucleusoftheBulbocavernosus(SNB)三.

SexualDifferentiationTheSexofanIndividualIsDeterminedEarlyinLifeHowShouldWeDefineGender—byGenes,Gonads,Genitals,ortheBrain?GonadalHormonesDirectSexualDifferentiationoftheBrainandBehaviorSocialInfluencesAffectSexualDifferentiationoftheNervousSystemSocialInfluencesAffectSexualDifferentiationoftheNervousSystem

The

SNB

system

offers

an

illuminating

example

of

how

social

factors

can

mediate

the

masculinization

produced

by

steroids.

the

dam

treats

a

pup

differently

because

he’s

a

male

and

thereby

masculinzes

his

developing

nervous

system

what

about

humans?

in

every

culture

most

people

treat

boys

and

girls

differently——societyaffectssexualdifferentiationofthebrainandbehavior

what

determines

a

person’s

sexual

orientation?what

determines

a

person’s

sexual

orientation?

Somepeopledevelopromanticattachmentstoandyearntohavesexwithapersonofthesamesex.

Why?what

determines

a

person’s

sexual

orientation?View1:Thissexorientationisbasedoninfantsobservingtheadultsaroundthemandmodelingtheirbehavioronthebehavioroftheoppositesex,ratherthantheirown.View2:somethinghappenstothefetalbraintodeterminelaterorientation——”somepeopleareborngay”View2Ifwedepriveamaleratofandrogensatbirth,wecanbesurethathewilldisplaylordosis(afemalebehavior)inresponsetoanothermaleinadulthood.Postmortemexaminationsonthebrainofhomosexualmen,heterosexualmen,andwomen,andfoundthatthePOAcontainsanucleus(INAH-3)thatislargerinmenthaninwomenandlargerinheterosexualmenthaninhomosexualmen.View1It’spossiblethatearlysocialexperienceaffectsthedevelopmentofINAH-3todeterminelatersexualorientation.SexualexperiencesasanadultcouldaffectINAH-3structure,sothesmallernucleusinsomehomosexualmenmaybetheresultoftheirhomosexuality.Homosexualmenandwomenaremorelikelytobeleft-handedthanheterosexualmenandwomen—androgenscannotbethewholestory?FemalehomosexualPrenatal(出生以前的)androgensmayormaynotactonthebrain,buttheycertainlyactontheperiphery(外表面).Somepeopleseemtochangesexatpuberty(青春期)Whysomepeopleraisedasgirlslaterbehaveasmen?Prenataltestosteronemayhavemasculinizedtheirbrain.ThesocialinfluencesofgrowingupView1&View2Thereportsthatsexualorientationisheritable.Estimatesarethatabouthalfthevariabilityinsexualorientationisduetodifferencesinthegenome(comefromtheirmother’sXchromosome).BothgeneticandenvironmentalinfluencesaffectsexualorientationinhumansDespiteextensiveefforts,noonehascomeupwithareliablewaytochangeaman’ssexualorientationContents:SexualBehaviorWhyAreThereTwoSexesSexualDifferentiationSummarySummary—SexualBehavior

Reproductivebehaviorsaredividedintofourstages:sexualattraction,appetitivebehaviors,copulation,andpostcopulatorybehaviors,includingparentalbehaviorsinsomespecies.

Thebraingovernsthetimeatwhichtheorganismshouldreproduceandusesproteinhormonestoinducethegonadstoproducegametes.Thegonadsinturnproducesteroidhormonesthatactivatethebraintoincreasetheprobabilityofreproductivebehaviors.

Inhumans,verylowlevelsoftestosterone

arerequiredforeithermanorwomentodisplayafullinterestinmating,butadditionaltestosteronehasnoadditionaleffect.Thereforethereisnocorrelationbetweencirculatingandrogenlevelsandreproductivebehaviorsinmen.Noristhereanystrongcorrelationbetweencopulatorybehaviorandstageofthemenstrualcycleinwomen.

Humancopulatorybehaviorisremarkablyvaried.Attemptsatclassificationofthestagesofcopulationinhumanssuggestthatmostmenshowasinglepattern,whilemostwomendisplayoneofthreebasicpatternsofsexualresponse.Forbothsexes,thefourbasicstagesofthesexualresponsepatternare(1)increa