Anthro 101: Human Biological Evolution

Lecture 8: Human Mating Behaviors

Human Mating Behavior

• How do we select our mates?

• What makes a good mate?

• How has this process changed in a modern world?

One of the most important adaptive problems that humans face is: Who is a good mate?

• Choosing a good mate is closely linked to fitness• Avoid mating with kin• Prefer mates who are likely to reproduce successfully

• Evidence of special design to fulfill these functions

• Avoid inbreeding• Choose someone of high quality• Compatibility is important

In choosing “good” mates, individuals have to avoid kin

Nonhuman primates• one sex leaves the original troop• Avoid mating with close kin

• Chimp females avoid brothers• Baboons avoid age-mates (likely half

brothers through father)• Rhesus macaques avoid kin

Human Societies• Lots of different mating systems

• None allow regular mating between siblings or parents

• Rules, customs to prevent close kin from mating

Beyond the incest taboo (cultural rules), we have evolved mechanisms for avoiding inbreeding

• Must solve the problem of kin recognition• Mothers easily know offspring• Fathers is harder in many cases• Siblings…

• Westermarck effect• Phenotypic similarity

Westermarck effect: a mechanism identifying kin

• Two people who live in close domestic proximity during the first few years in the life of either one are desensitized to later sexual attraction

• cohabitation as a cue• Critical period in the first 5 years of life

• turns “off” attraction between same age siblings, caregiver and child

• higher reproduction / survival rate than those that where attraction not “off”

Evidence for Westermarck effect: Sim-pua

• sim-pua marriages in Taiwan• Poor families would arrange

marriage for daughters at birth• Girl is raised in future

husband’s household• Bride and groom raised like

siblings• Sim-pua marriages were

problematic• Lack of sexual interest• Low fertility• More infidelity• More divorce

When bride adopted into family later, couples end up with more children on average

Kibbutzim in Israel (J. Shepher):• Agricultural collectives• Children raised communally

by childcare specialists• Unrelated individuals raised

together in Kibbutzim show diminished sexual attraction to each other

• Of 2,769 marriages in 211 kibbutzim, only 14 between members of same peer group, and all but one joined after age 6

Evidence for Westermarck: Kibbutzim

Evidence for Westermarck Effect: Lebanon

In Lebanon, cousin marriage is practiced• Brothers often live in same household• Families of brothers are very close• Offspring of brothers grow up much like siblings• Ideal marriage = patrilateral parallel cousins

Patrilateral parallel cousin Patrilateral cross cousin

male female

Patrilateral parallel cousin marriages are less successful than other marriages

• Patrilateral parallel cousins are raised in close association• Patrilateral cross cousins are NOT raised in as close

association

• Marriages between patrilateral parallel cousins are less successful than marriages between patrilateral cross cousins• 23% fewer children• 4 times as likely to divorce• Less sexual attraction

Cues ≠ Facts

• Mechanism does not detect actual kinship

• Uses childhood cohabitation as a cue to kinship

• In the past, cue was probably right often enough

• Can be wrong

• siblings raised apart are more likely to have a relationship when

they meet, even though they are genetically related

• Adopted siblings, step parents not interested in each other

Another cue: Humans may be able to detect genetic similarity through olfactory cues

• MHC = major histocompatability complex• Part of immune system• Linked to disease resistance

• MHC is hypervariable genetic marker• Kin share similar MHC markers

• Mice prefer mates with MHC unlike themselves• Could be adaptation to enhance pathogen resistance• Could be adaptation to prevent inbreeding

• Humans can distinguish (by smell) mouse strains that differ only in MHC

Experimental evidence suggests humans respond to MHC “odor” cues when choosing mates

• Assess MHC type for male and female subjects• Males asked to wear T-shirt for 2 nights

• Asked not to smoke, use deodorant or cologne• Try to maintain “odor neutral” environment

• Women were asked to rate smell of T-shirts worn by men • Pleasantness• Sexiness• Intensity

Women’s perceptions of male smells depend on MHC and hormonal status

• Women not on hormonal birth control

• find men with dissimilar MHC more attractive

• Oral contraceptives (pill) simulate hormonal effects of pregnancy

• Women on hormonal birth control

• find men with similar MHC more attractive

• Avoid kin while sexually receptive, seek kin while pregnant (yes, new data suggests they do!)

Studies of Hutterite population also suggest that MHC affects mate choice• Hutterites are a small

Anabaptist community in US

• Marry within their community

• Hutterite Marriages • mates with different

MHC types more common

• Mates with similar MHC types less common

The relationship between inbreeding avoidance (biology) and incest rules (culture) is not always clear• If aversions to inbreeding are part of human psychology,

why do we need incest rules?• Why are first cousin marriages allowed in some societies,

but not others?

Þ Reflects complex interaction between biology & culture

Þ Complex interaction between biology, psychology, & cultureÞ True for ALL human behavior

Behavioral Ecology & Mate Choice: The Kipsigis

• Farmers & herders in Kenya• Wealth measured as:

• Amount of land owned• Number of livestock owned• Household possessions

• Men marry first in late 20’s• Pay bridewealth to marry a

woman• Wealthier men have more wives• Wealthier men have more kids• Women marry in late teens• Father’s arrange marriage

• But daughters give consent

What makes a desirable Kipsigis husband?

• Wives are given a plot of the husband’s land• Women with larger plots

• Can grow more food• Have healthier kids• Have higher reproductive success

Acres per wife increases as

man’s landholdings increase Men with more land have more wives

What makes a desirable Kipsigis wife?

• Men have many wives• some men get none =

competition for women• Bridewealth payment goes

to bride’s father• father of bride & the

groom negotiate bridewealth

• Bridewealth = value of the marriage partner

What makes a desirable Kipsigis wife?

• Early maturing women bring higher bridewealth

What makes a desirable Kipsigis wife?• Early maturing women have higher reproductive success

What makes a desirable Kipsigis wife?

• Plump women - with plenty of nutrition & no disease - also bring higher bridewealth

Evolutionary psychologists also explore factors that affect mate choice

• Adapted to choose mates who enhance our reproductive success: Sex differences

• Males• Increase number of matings• Prefer mates who can produce more offspring

• Particularly important when pair-bonding common

• Females• Get more resources to provide to offspring• Reduce mortality• Improve quality of genes in offspring (by

selecting them in mate)

• Don’t commit the Naturalistic Fallacy!

As a pair-bonding species, we also predict similarity in mating strategies & preferences

• Able to produce many healthy children

• Able to provide good care for children

• Willing to invest in children

• Not likely to be unfaithful

Charles DarwinEmma Wedgewood

Evolutionary psychologists predict that women’s mate choice criteria will be related to resources

• Human infants require extended care

• In foraging societies, men contribute critical resources for

women and children

• Women’s fitness enhanced by male provisioning

• Females will prefer mates who can and will provide

resources for offspring

• In societies in which individuals can acquire property and

resources, females will prefer wealthy males

Male mate preferences are expected to be based on female fertility

• Male mating success depends on reproductive success of female• Women’s reproductive potential

• peaks in early adulthood• declines until menopause

• Men are expected to prefer young women• Standards of beauty are correlated with youth and health

• Smooth skin• Good muscle tone• Shiny hair

• Men will prefer youthful mates• Men will value good looks

Aishwarya Rai - Bollywood actress

Both males and females must be concerned about fidelity, but this may matter more to males than females

• For females• If males have multiple mates,

they may divert resources to other offspring

• For males• If females have multiple mates,

males may be providing resources for other man’s children

• Expect men to value fidelity more than women

Campaign for Aids prevention in Uganda:

A-abstinence, B-be faithful, C-use condoms

Evolutionary psychologists predict that similar preferences will exist in all societies

Sample mate choice criteria in 37 countries around the worldAsk people to rate importance of various traits

Problems with the sample 1. mainly university students 2. mainly developed, Westernized nations

Preference for good financial prospects

Indispensable

Unimportant

CSUCI

Preference for good looks

Unimportant

Indispensable

CSUCI

Males Females1. Good health

2. Mutual attraction and love

3. Good looks

4. Emotional stability and maturity

5. Dependable character

6. Sociability

7. Ambition and industrious

8. Good cook and house keeper

9. Education and intelligence

10. Refinement and neatness

11. Similar education

12. Good financial prospects

13. Desire for home and children

14. Chastity

15. Similar political background

16. Favorable social status

17. Similar religious background

1. Mutual attraction and love

2. Emotional stability and maturity

3. Dependable character

4. Good financial prospects

5. Education and intelligence

6. Good health

7. Ambition and industrialness

8. Desire for home and children

9. Good cook and housekeeper

10. Refinement and neatness

11. Sociability

12. Favorable social status

13. Good looks

14. Similar religious background

15. Similar education

16. Similar political background

17. Chastity

However, the cross-cultural data also show that men and women care most about character

Trait Males Females

Mutual attraction, love 1 1

Dependable 2 2

Emotionally stable, mature 3 3

Pleasing disposition 4 4

Good health 5 7

Education, intelligence 6 5

Sociable 7 6

Desire for home, children 8 8

Refinement, neatness 9 10

Good looks 10 13

Good financial prospects 13 12

Chastity 16 18

1 1

1 3

4 6

4 7

4 4

1 2

9 7

11 10

11 13

11 11

7 9

18 15

CSUCI

Men and women BOTH value the traits that would contribute to long-term unions

• Human children require extended period of investment• women play primary role in

childcare• men make big investment in

children • Stable marriage/bond increases

reproductive success• Stability of marriage may be

enhanced • choose a loving, dependable,

stable, clever, sociable partner

There is also considerable cross-cultural variation in preferences

Preferences about chastity are most variablePreferences about financial prospects are least variableSex differences hold across cultures

Conclusions

• In general, BOTH men and women want:• Mutual attraction, Love• Kindness, pleasing disposition, humor• Emotional Stability, dependability

• Men emphasize youth, attractiveness, chastity more• Women emphasize resources, hardworkingness, age, status

more

• Differences in male and female mate preferences make sense in light of sexual selection theory

• While there are big cultural differences in magnitude of preferences, reversals are rare (e.g., no reversals in age preferences)

After a long evolutionary history…where are we as a species?

• We are adapted to be flexible in our behavior

• We are adapted to rely on our brains more than our bodies

• We are adapted to be social and cooperative

• We are adapted to be cultural

• We have evolved a lot of phenotypic variability