Basis for Male Promiscuity Questioned

Sally Law, LiveScience's Science of Sex Columnist,

LiveScience.com

Males are promiscuous and females are selective when

choosing a mate, biologists have said for decades. But a

new study finds it might not be that simple.

The study, published in this month's issue of the journal

Trends in Ecology and Evolution, evaluated data on 18

populations - from Pitcairn Islanders to the Dogon of Mali

- and found that on average, the variance in the number of

children is greater for men than for women. This is about

what you'd expect on the basis of long-time theory.

However, Gillian R. Brown, a professor at the School of

Psychology at the University of St Andrews and the

study's lead researcher, says that the research also

found big differences among populations on the patterns

of reproductive success for men and women.

For example, the study cites societies in Botswana,

Paraguay and Tanzania in which women - not just men -

conceive children with multiple partners.

Brown's study challenges the research of Angus J.

Bateman, who in 1948 conducted a study on the mating

habits of fruit flies. Bateman found that male flies had

greater variance, and success, in both their number of

sexual partners and their number of offspring than did

females.

Because eggs are harder to come by than sperm,

Bateman said, a female fly's offspring were restricted by

her ability to produce eggs; meanwhile, a male fly's

reproductive success was limited only by the number of

females he inseminated.

In subsequent years, the fruit-fly finds were applied to

other species, including humans. But Brown's research

shows that in some populations, Bateman's work - in

particular, his findings regarding offspring - doesn't

necessarily hold true.

"Evidence for sex differences in variation in reproductive

success alone does not allow us to make generalizations

about sex roles, as numerous variables will influence

[Bateman's findings] for men and women," Brown writes.

Population size is one such variable: both men and

women will be selective about mates when there are lots

of options - in a large city, for example. Conversely,

neither gender will be choosy in low-population areas. In

such a scenario, both men and women will take what they

can get.

"We argue that the more flexible and variable human

behavior is, the less powerful their explanation in terms of

universal sex roles," Brown tells LiveScience. "Males and

females should perhaps not be characterized in the way

normally presented by evolutionary psychologists ... the

idea that we can predict everything about human sex

roles on the basis of the differential costs of producing

eggs and sperm is simplistic."

Brown's research also addresses the issue of

reproduction within a monogamous partnership; while only

16 percent of societies have monogamous marriage

systems, they make up a large percentage of

relationships in the developed world. In such societies,

variances in male and female reproductive success were

similar. Furthermore, in half of the world's polygamous

marriages - which account for 83 percent of the world's

societies - less than 5 percent of men take more than one

wife.

Brown is careful to point out that the research didn't

include data on the actual number of people's sex

partners. Here is why: the studies reporting these

statistics are scientifically unsound, she said, which helps

explain the mathematical difficulties in research that finds

that men have more sex partners than women. (One such

study, conducted by the National Center for Health

Statistics, claims that men have an average of seven sex

partners during their lifetime, while women have four.)

"[The reported numbers are] logically impossible if we're

assuming these are heterosexual interactions and that all

individuals have been questioned," Brown says. "We

were particularly interested in asking whether the

variance (not average) in mating success differs between

men and women, but questionnaire studies don't seem to

be a sufficiently reliable source of evidence."

Indeed, past research has shown that not only do people

lie on such surveys, but that they admit their lies minutes

later.

Hard statistical evidence is easy to gather from fruit flies,

but the application of the resulting data to the more

complex sexual dynamic of humans does not necessarily,

er, bear fruit.

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