Barry X. Kuhle Ph.D.

Evolutionary Entertainment

Born Both Ways?

The Alloparenting Hypothesis for Sexual Fluidity in Women.

Posted Apr 09, 2013

Kelly McGillis, Cynthia Nixon, & Carol Leifer have all gone public with their fluid sexuality.

Many a public woman has made headlines because her private life befuddled many. Think Anne Heche, Cynthia Nixon, Meredith Baxter Birney, Gillian Anderson, Kelly McGillis, Carol Leifer, Julie Cypher. Despite being the seeker and object of men’s affections for years, each of these seemingly heterosexual women made news for their late-in-life romances with women. Such sexual fluidity has baffled many, including evolutionary psychologists.   

The Puzzle

As Darwin outlined in The Descent of Man (1871), reproduction is the engine of evolution. Sexual selection favors traits that increase an organism’s ability to reproduce relative to  alternative traits. Given the primacy of reproduction, seemingly counterreproductive traits like same-sex sexual behavior pose an evolutionary puzzle. Why would selection have fashioned motivational mechanisms to engage in sexual behaviors with members of the same sex?

Numerous evolutionary hypotheses have been advanced to account for same-sex sexual behavior in humans and other animals (for reviews see Bailey & Zuk, 2009; Buss, 2003; LeVay, 2011; Mealey, 2000; Sommer & Vasey, 2006; Vasey, 1995). Most of these hypotheses have focused on the potential benefits that same-sex sexual behavior afforded ancestral men. Relatively little attention has been paid to the evolution of same-sex sexual behavior in women. With Ryerson University psychology doctoral student Sarah Radtke, I recently proposed the alloparenting hypothesis which posits that sexual fluidity in women is a contingent adaptation that increased ancestral women’s abilities to form pair bonds with female alloparents who helped them rear children to reproductive age. Before fleshing-out the hypothesis and its predictions, several disclaimers, definitions, and discussions of alloparenting and sexual fluidity are necessary.


Given the sensitive nature of sexuality in many social, political, and religious climates, four important disclaimers are warranted. First, the alloparenting hypothesis is not intended to explain the  evolutionary significance of a homosexual sexual orientation. Rather, it aims to account for same-sex sexual behavior among heterosexually-identified women. Second and third, the hypothesis is not intended to explain all occurrences of same-sex sexual behavior in women, nor does it posit that all same-sex sexual behavior serves to promote alloparenting. Even a cursory view of the sexual landscape suggests otherwise. The hypothesized mechanisms need not always (or ever) serve the adaptive purposes of an ancient time in a modern day world. Fourth, the hypothesis does not imply that same-sex sexual behavior is the only route to promoting alloparenting effort. Rather, the hypothesis posits that selection fashioned adaptations in women to promote alloparenting, one of them being sexually fluid mating mechanisms that facilitated, cemented, and sustained bonds between mothers and allomothers. Fifth, following Diamond (2008), we are not equating a fluid sexuality with a chosen sexuality. That is, although we postulate the existence of mating mechanisms that served to promote flexibility in women’s sexual responsiveness, no conscious choice is invoked or implied.

Sexual fluidity is a hypothesized conditional adaptation designed to promote opposite-sex sexual behavior in certain situations and same-sex sexual behavior in other situations, much like callous-producing mechanisms (Confer et al., 2010) promote calluses in high friction environments but not in less abrasive ones. Much as it would be nonsensical to argue that callous production or absence is a choice, it would be illogical to conclude that the elicitors of women’s sexual responsiveness are chosen.

What is Alloparenting?

Alloparenting occurs when an animal other than a biological parent helps in the rearing of offspring (Hrdy, 2008).  It has been observed in an array of species, including distant and close primates, and among humans across various culture. An example among birds can be found in the Laysan albatross (Phoebastria immutabilis), a large shorebird whose main breeding colonies are in the Hawaiian Islands.  When there is a shortage of paternal investment due to a female-biased sex ratio, females pair with one another for multiple years and cooperate in chick rearing (Young, Zaun, & VanderWerf, 2008). In sum, alloparenting occurs in numerous species, including distant and close primates, and among humans across various cultures. 

What is Sexual Fluidity?

In her groundbreaking book on sexual fluidity, Lisa Diamond defines sexual fluidity as “situation-dependent flexibility in women’s sexual responsiveness…that makes it possible for some women to experience desires for either men or women under certain circumstances, regardless of their overall sexual orientation” (Diamond, 2008, p. 3).  Diamond argues that “the hypothesis that female sexuality is fundamentally fluid provides the most robust, comprehensive, and scientifically supported explanation for the research data” on women’s sexuality (Diamond, 2008, p. 9).  This data indicates that relative to men, women are more likely to report bisexual attractions than exclusive same-sex attractions (for reviews see Baumeister, 2000; Diamond, 2006, 2007, 2008; Peplau, 2001; Peplau & Garnets, 2000).  Additionally, U.S. women aged 18-44 years are more than twice as likely as men to report being attracted to and having had sexual contact with members of the same sex (Chandra, Mosher, and Copen, 2011).  Women’s fluid sexuality is also evidenced physiologically (Chivers, 2005, 2010).  In contrast to men, women’s genital arousal is not significantly greater for stimuli of partners of their preferred versus nonpreferred sex (Chivers and Bailey, 2005; Chivers, Rieger, Latty, & Bailey, 2004).  Among heterosexuals, women are more likely than men to exhibit genital arousal (Chivers & Bailey, 2005; Chivers et al., 2004) and pupil dilation (Rieger & Savin-Williams, 2012) to erotic stimuli of both sexes.  In sum, a growing body of evidence suggests that “non-specific sexual arousal best describes heterosexual but not homosexual women’s genital sexual response” (Chivers, 2010, p. 411).  Given women’s flexible psychological & physiological sexual responsiveness to both sexes, a growing notion among relationship researchers is that women’s sexuality is best described as being plastic (Baumeister, 2000) or fluid (Diamond, 2007, 2008; Peplau, 2001, 2003; Peplau & Garnets, 2000).

The Alloparenting Hypothesis

Under the alloparenting hypothesis, sexual fluidity in women is a contingent (facultative) adaptation that increased ancestral women’s ability to form pair bonds with female alloparents who helped them rear children to reproductive age. Ancestral women recurrently faced the adaptive problems of securing resources and care for their offspring, but were frequently confronted with either a dearth of paternal resources due to their mates’ death, an absence of paternal investment due to rape, or a divestment of paternal resources due to their mates’ extra-pair mating efforts. A fluid sexuality would have helped ancestral women secure resources and care for their offspring by promoting the acquisition of allomothering investment from unrelated women. Under this view, most heterosexual women are born with the capacity to form romantic bonds with both sexes. Sexual fluidity is a conditional reproductive strategy with pursuit of men as the default strategy and same-sex sexual responsiveness triggered when inadequate paternal investment occurs or when women with alloparenting capabilities are encountered. 


Sexual selection is hypothesized to have designed sexual responsiveness mechanisms in women that are sensitive to the situations and experiences that were recurrently associated with the availability of paternal and allomothering investment over evolutionary history.  Situations and experiences indicative of low paternal investment or the potential for high allomothering investment are hypothesized to shunt some women into forming same-sex romantic bonds that facilitate alloparenting.  The alloparenting hypothesis makes at least fourteen testable predictions, five of which I include here (see article for complete list).

  1. Relative to women who have never been abused by their male mates, women who have experienced abuse by male mates will be more likely to have subsequently engaged in same-sex sexual behavior.
  2. Relative to women who have never been raped by men, women who have been raped by men are more likely to have subsequently engaged in same-sex sexual behavior.
  3. Relative to women who were never abused as children, women who experienced physical or sexual abuse by men during childhood or adolescence will be more likely to have subsequently engaged in same-sex sexual behavior.  As with research on the consequences of father absence (Belsky, Steinberg, & Draper, 1991; Ellis, 2004; Ellis, Schlomer, Tilley, & Butler, 2012; James, Ellis, Schlomer, & Garber, 2012), this prediction is predicated on the notion that formative childhood experiences with men shape women’s future mating psychology. 
  4. Women whose husbands divested in them for the sake of other women are more likely to have subsequently engaged in same-sex sexual behavior (especially if they have children) relative to women whose husbands’ investment did not diminish due to being diluted among other women. 
  5. Women whose husbands deserted them are more likely to have subsequently engaged in same-sex sexual behavior (especially if they have children) relative to women whose husbands remain mated to them.

But Why the Sex? 

Sex is an effective means of forming, increasing, and sustaining pair bonds between people (Brigman & Knox, 1992; Hazan & Diamond, 2000; Leigh, 1989; Meston & Buss, 2009).  Sexual behavior with male mates promotes women’s feelings of commitment to these partners (Meston & Buss, 2009).  There may be something unique about sex that serves to promote bonding and trust between women, two things that may have been necessary before being willing to hand one’s kids over to a non-relative for care.  Sex also comes with costs, particularly for women: If a woman is willing to bear those costs (e.g., the possibility of contracting an STD from you), it may suggest that she is committed to you (à la Zahavi, 1977).  Sex also comes with emotions, particularly for women: the act's associated hormonal changes may be particularly useful in bonding women together.


In one coherent framework, the alloparenting hypothesis weaves together several diverse phenomena including (a) female sexual fluidity in human and non-human primates, (b) heterosexual women’s potent genital arousal to both sexes, (c) the rates of rape, physical abuse, and sexual abuse as a function of sexual orientation, and (d) the ubiquity of alloparenting among human and non-human primates.  The alloparenting hypothesis also outlines fourteen testable predictions, twelve of which specify variables that will shunt some women into forming same-sex romantic bonds that facilitate alloparenting.  No other hypothesis for sexual fluidity is as wide-ranging or as falsifiable. 

As the engine of evolution is reproduction, same-sex sexual behavior poses a paradox.  This paradox is resolved if, far from impeding reproduction, the trait in question actually facilitates it.  In light of the alloparenting hypothesis, a trait that formerly appeared maladaptive—sexual behavior between women—is recast as an adaptive outgrowth of sexual fluidity.  This hypothesized contingent adaptation may have increased ancestral women’s ability to form pair bonds with women who helped them rear children to reproductive age in the face of male rape, death, desertion, and divestment of resources, as well as during stressful childrearing times or simply when a suitable allomother presented herself.  Being born with the ability to go both ways may have been beneficial to ancestral women. 

[“Surmise” “may,” “should,” “might.”  Here’s hoping DATA will shed more definitive light on the origin and development of sexual fluidity in the future. If you’re looking to test some of these predictions (or others!) feel free to drop me a line!]


See references list in original article.

Follow me on Facebook and Twitter  

Copyright © 2013 Barry X. Kuhle. All rights reserved.

Disclaimer: The views expressed in this blog do not necessarily reflect the views of Psychology Today and the University of Scranton, or my friends, family, probation officer, gut bacteria, darkest thoughts, and personal mohel.

Be sure to read the following responses to this post by our bloggers:

More Posts