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often get choosy when outnumbered
by males or required to raise offspring
alone. Males become picky when outnumbered by females or when required
to help raise offspring. In small, spread
out populations with roughly equal numbers of males and females, both sexes tend
to mate indiscriminately.
Brown and her colleagues provide initial evidence that the “astonishing flexibility” of mating strategies also applies
to people, remarks behavioral ecologist
Stephen Emlen of Cornell University.
“A lot of nonhuman animals that are
thought of as innately controlled robots
actually perform sophisticated cost-benefit analyses of different choices
and adjust their behavior based on flexible rules of thumb,” asserts Emlen, who
studies mating decisions in birds. Emlen
says it would be surprising if people
didn’t have even more variable mating
strategies than other animals.
Anthropologist Lee Cronk of Rutgers
University in New Brunswick, N.J., says
that he hopes the study “will have a major
impact on future evolutionary work on
human mating patterns.”
Cronk still thinks that universal mating strategies exist for men and women
and were genetically ingrained during
the Stone Age. But these strategies can
play out in a variety of ways depending
Mating strategies take many forms
New data from a variety of societies question assumptions that sex roles are universal
Ratio of variability in
Population/ reproductive success,
Country ethnic group men to women*
iran yomut Turkmen 1. 14
sweden 1825–1896 genealogies 1. 18
u.s. general social survey 1. 27
central African Republic Aka 1.66
Botswana Dobe !kung 1.77
Paraguay Ache 4. 22
Mali Dogon 4.75
*Higher numbers indicate more variability among men than women within a group
Mating system
Polygyny/monandry
Monogamy
Monogamy
Polygyny/monandry
serial monogamy
serial monogamy
Polygyny/serial monandry
on the cultural, social and demographic
features of modern societies.
Brown sees the situation differently.
Disparities in learned mating strategies
and cultural beliefs that have developed
among groups over the past 50,000 years
or so have also led to genetic differences
among those populations, not to universal
mating tendencies, she suspects.
Brown’s group analyzed information
on reproduction and mating in hunter-gatherer, farming and industrialized
societies from already published studies. Although these studies depended on
women’s reports of who fathered their
children, Brown regards the reliability
of such evidence as comparable between
monogamous and polygynous societies.
Scientists must now try to estimate and
compare numbers of sexual partners, as
well as offspring, for individuals in dif-
ferent societies, she says. Doing so will be
challenging, since attempts to establish
the numbers of sexual partners for men
and women have relied on self-reports
that are unreliable, Brown asserts.
She and her colleagues remain cautious about assuming that men always
have more sexual partners than women
in polygynous societies. In about half of
polygynous societies, most men still take
no more than one wife, they note. In others, women as well as men conceive children with two or more partners.
Thorough cross-cultural studies would
likely reveal a great variety of mating
strategies that respond to local conditions, predicts behavioral ecologist Mhairi
Gibson of the University of Bristol in England. “Unfortunately, few anthropologists
continue to collect such quantitative data
on human behavior,” she says.
Back Story | LET’S TALK ABOUT SEX ROLES, IN THEORY
1871
1948
geneticist Angus Bateman
reports that male fruit flies
that mate with more partners
conceive more offspring, while
female fruit flies show lesser
gains when they mate with more
partners. He concludes that
sexual selection should favor
promiscuous males and discrimi-
nating females.
1972
1981
charles Darwin proposes his
theory of sexual selection, arguing that members of each sex
within a species compete for
resources and display traits most
valued by mate-seeking members
of the opposite sex. The evolution of the male peacock tail is a
classic example.
Evolutionary biologist Robert
Trivers, now of Rutgers university,
elaborates on Bateman’s idea
by predicting that members of
the sex that make the largest
investment in parenthood, usually female, become a relatively
scarce quantity on the mating
scene. Thus males usually compete for females.
Anthropologist sarah Blaffer Hrdy
challenges Trivers’ perspective
by arguing that females of some
species, including the langurs
she studied, benefit from mating
with many males. This strategy
increases uncertainty about who
fathered whom and reduces the
likelihood of infanticide by males,
as well as boosting the probability of getting pregnant.
THE cOMPLETE WORk OF cHARLEs DARWiN ONLiNE ( DARWiN-ONLiNE.ORg.uk); jANEFF/is TOckPHOTO; jOHN iNNEs ARcHivEs, cOuRTEsy OF THE jOHN iNNEs
FOuNDATiON; AjPHOTO/is TOckPHOTO; Nick ROMANENkO/cOuRTEsy Ru TgERs uNiv.; sERENgETi130/is TOckPHOTO
