"Long before it's in the papers"
May 29, 2012

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“Sexual revolution” may have paved way for invention of marriage

May 29, 2012
Courtesy of National Institute for Mathematical 
and Biological Synthesis
and World Science staff

The typ­i­cal hu­man family struc­ture arose as the re­sult of a “sex­ual rev­o­lu­tion” fea­tur­ing a new al­li­ance be­tween wom­en and low-ranking ma­les, ac­cord­ing to new cal­cula­t­ions by a bi­ol­o­gist.

His study pro­poses that faced with fre­quent de­feat by high-ranking males in com­pe­ti­tion for mates, some low-rank­ing males re­acted by of­fer­ing fe­males some­thing those oth­er males would not: long-term pro­vi­sion­ing. And wo­men liked it. Faith­ful fe­males be­gan to choose good providers as mates, and pair-bonding re­placed promiscu­ity, ac­cord­ing to the re­search, pub­lished in this week’s on­line is­sue of the jour­nal Pro­ceed­ings of the Na­t­ional Acad­e­my of Sci­ences.

The study ad­dresses long-stand­ing ques­tions about how the mod­ern, rela­tiv­ely mono­gam­ous fam­i­ly emerged fol­low­ing ear­li­er times of promiscu­ity, said au­thor Sergey Gav­ri­lets of the Uni­vers­ity of Tennessee-Knox­ville. 

In ad­di­tion to the es­tab­lish­ment of sta­ble, long-lasting rela­t­ion­ships, the tran­si­tion to pair-bonding was al­so char­ac­ter­ized by a re­duc­tion in ma­le-to-male com­pe­ti­tion in fa­vor of pro­vid­ing for fe­males and pro­vid­ing close pa­ren­tal in­volve­ment, sci­en­tists say.

Cal­cula­t­ions show the most pop­u­lar the­o­ries for the tran­si­tion to hu­man pair-bonding are un­work­a­ble, said Gav­ri­lets. He in­stead pro­poses that the tran­si­tion can oc­cur when fe­male choice and faith­ful­ness, among oth­er fac­tors, are tak­en in­to ac­count. The re­sult is an in­creased em­pha­sis on pro­vi­sion­ing fe­males over male com­pe­ti­tion for mat­ing.

The ef­fect is most pro­nounced in low-ranked males who have a low chance of win­ning a mate in com­pe­ti­tion with a high-ranked ma­le, said Gav­ri­lets, who is as­so­ci­ate di­rec­tor for sci­en­tif­ic ac­ti­vi­ties at the Na­t­ional In­sti­tute for Math­e­mat­i­cal and Bi­o­log­i­cal Syn­the­sis, al­so in Knox­ville. Thus, he said, the low-ranked male at­tempts to buy mat­ing by pro­vid­ing for the fe­ma­le, which in turn is then re­in­forced by fe­males who show pref­er­ence for the low-ranked, “pro­vi­sion­ing” ma­le.

“Once fe­males beg­in to show pref­er­ence for be­ing pro­vi­sioned, the low-ranked ma­les’ in­vest­ment in fe­male pro­vi­sion­ing over ma­le-to-male com­pe­ti­tion pays-off,” Gav­ri­lets said. He added that the stu­dy’s re­sults de­scribe a “sex­ual rev­o­lu­tion” in­i­ti­at­ed by low-ranking males who be­gan pro­vid­ing in or­der to get mat­ings. “Once the pro­cess was un­der­way, it led to a kind of self-domestica­t­ion, re­sulting in a group-living spe­cies of pro­vi­sion­ing males and faith­ful fe­ma­les,” he said.

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The typical human family structure arose as the result of a “sexual revolution” featuring a new alliance between women and low-ranking males, according to new calculations by a biologist. His study proposes that low-ranking males—often defeated by high-ranking ones in competition for mates—responded by offering females something many higher-ranked males would not: long-term provisioning. And it worked. Faithful females began to choose good providers as mates, and pair-bonding replaced promiscuity, according to the research, published in this week’s early online issue of the journal Proceedings of the National Academy of Sciences. The study addresses long-standing questions about how the modern family, characterized by intense, social attachments with exclusive mates, emerged following earlier times of promiscuity, said study author Sergey Gavrilets of the University of Tennessee-Knoxville. In addition to the establishment of stable, long-lasting relationships, the transition to pair-bonding was also characterized by a reduction in male-to-male competition in favor of providing for females and providing close parental involvement, scientists say. Calculations show the most popular theories for the transition to human pair-bonding are unworkable, said Gavrilets. He instead proposes that the transition can occur when female choice and faithfulness, among other factors, are taken into account. The result is an increased emphasis on provisioning females over male competition for mating. The effect is most pronounced in low-ranked males who have a low chance of winning a mate in competition with a high-ranked male, said Gavrilets, who is associate director for scientific activities at the National Institute for Mathematical and Biological Synthesis, also in Knoxville. Thus, he said, the low-ranked male attempts to buy mating by providing for the female, which in turn is then reinforced by females who show preference for the low-ranked, “provisioning” male. “Once females begin to show preference for being provisioned, the low-ranked males’ investment in female provisioning over male-to-male competition pays-off,” Gavrilets said. He added that the study’s results describe a “sexual revolution” initiated by low-ranking males who began providing in order to get matings. “Once the process was underway, it led to a kind of self-domestication, resulting in a group-living species of provisioning males and faithful females,” he said. The study reveals that female choice played a crucial role in human evolution and that future studies should include between-individual variation to help explain social dilemmas and behaviors, Gavrilets added.