Encyclopedia of Evolutionary Psychological Science

Living Edition
| Editors: Todd K. Shackelford, Viviana A. Weekes-Shackelford

Alloparenting and Female Same-Sex Behavior

  • Barry X. KuhleEmail author
  • Sara BrezinskiEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-16999-6_62-1


Heterosexual Woman Genital Arousal Paternal Investment Sexual Fluidity Sexual Responsiveness 
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The alloparenting hypothesis posits that female sexual fluidity – “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) – was selected because it facilitated the acquisition of and bonding to an alloparent (a non-biological caregiver for one’s offspring).


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; therefore, seemingly counterproductive behaviors such as same-sex sexual activity and romantic relationships pose an evolutionary puzzle. Although there have been several hypotheses put forward to attempt to explain same-sex sexual behavior in men [cite relevant Encyclopedia entries here], relatively little has been posed to explain such behavior in women. One exception is Kuhle and Radtke’s (2013; see also Kuhle 2013) alloparenting hypothesis positing that sexual fluidity in women evolved as an adaptation that increased ancestral women’s abilities to form pair-bonds with females who could help them rear children to reproductive age.


Alloparenting has been observed in an array of species and is particularly common in our primate cousins (Hrdy 1999). Among squirrel monkeys, relatives and non-kin engage in reciprocal alloparenting of infants (Roulin 2002; Williams et al. 1994). Among Japanese macaques, mothers allow other females to hold, watch, and protect infants (Bardi et al. 2001; Redmond 2008). Bonobo females form strong pair-bonds that last the duration of their lives (Furuichi 2011; Kano 1992). When a female reproduces, other females are significantly involved in the life of the young bonobo (De Lathouwers and Van Elsacker 2004; Furuichi 2011; Kano 1992). To cement pair-bonds within the troop, female bonobos engage in various forms of sex with troop members, especially with females who may serve as allomothers.

In humans, Kuhle and Radtke (2013) argue that alloparenting may have been a way for ancestral women to acquire a second form of parental investment for their children in the face of paternal desertion, death, or divestment of resources. Hrdy (1999, 2007, 2008) surmises that without cooperation from both kin and non-kin alloparents, humans may have been unable to flourish as a species because human infants are so altricial. Close kin are not always the dominant allo-caregiver; unrelated women often contribute substantial allomothering across cultures (Bentley and Mace 2009; Hrdy 1999; Meehan 2009).

Sexual Fluidity

Relative to men, women are more likely to report bisexual attractions than exclusive same-sex attractions (Baumeister 2000; Diamond 2006, 2007, 2008; Peplau 2001; Peplau and Garnets 2000). Additionally, US 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 et al. 2011). Women’s fluid sexuality is also evidenced physiologically (Chivers 2005, 2010). As opposed to men, women do not experience a significantly greater genital arousal from stimuli of their preferred versus non-preferred sex (Chivers and Bailey 2005; Chivers et al. 2004).

For the present purposes, a fluid sexuality is one that is potentially sexually responsive to both sexes (but not necessarily at the same point in time). It avoids a focus on changes in one’s sexual identity or sexual orientation.

Sexual Fluidity as a Conditional Female Mating Strategy

According to the alloparenting hypothesis, sexual fluidity increased ancestral women’s reproductive success by diminishing the costs of four adaptive problems that resulted in a deficiency of paternal investment and by promoting the acquisition of allomothering investment from unrelated women: (1) an absence of paternal investment due to rape, (2) reduced paternal investment due to paternal defection, (3) reduced paternal investment due to paternal death, (4) and reduced paternal investment due to a dilution of resources (Kuhle and Radtke 2013). Under this view, most heterosexual women are born with the capacity to form romantic bonds with both sexes. Female sexual fluidity is a conditional reproductive strategy where the pursuit of men is the default strategy, and same-sex sexual responsiveness is 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. The alloparenting hypothesis makes 14 testable predictions. Although there are several studies relevant to the first three predictions (see original article), no empirical evidence yet exists that bares upon predictions 4–14.
  1. 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. 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. 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.

  4. 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 from being diluted among other women.

  5. 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.

  6. 6.

    Women whose husbands have died are more likely to have subsequently engaged in same-sex sexual behavior (especially if they have children) relative to women whose husbands are alive and investing in them.

  7. 7.

    In the absence of paternal defection, desertion, and death, wives of husbands whose investment has diminished are more likely to have subsequently engaged in same-sex sexual behavior (especially if they have children) relative to women whose husbands’ investment is sufficient.

  8. 8.

    A woman’s mate value (MV) will be an important moderating variable on her likelihood of engaging in same-sex sexual behavior. All things being equal, lower MV women will be more likely than higher MV women to engage in same-sex sexual behavior in the face of male abuse, rape, divestment, desertion, and death because they are less able to acquire sufficient paternal investment from other men.

  9. 9.

    Women who have formed deeper, emotional friendships with women who exhibit alloparenting potential are more likely to have engaged in same-sex sexual behavior than women with fewer such friendships.

  10. 10.

    Women who experience extreme stress associated with childrearing are more likely to report having engaged in same-sex sexual behavior than women without such stress.

  11. 11.

    Women with an unrestricted sociosexuality (Jackson and Kirkpatrick, 2007; Simpson and Gangestad, 1991) will be more likely to engage in same-sex sexual behavior than women with a restricted sociosexuality. The more willing and comfortable a woman is in engaging in casual sex without love, commitment, or closeness, the more likely she is to experience a dearth of paternal investment postpartum and hence need allomothering investment.

  12. 12.

    Women with few kin available to alloparent will be more likely to engage in same-sex sexual behavior than women with abundant alloparenting help from kin.

  13. 13.

    Women will be more likely to engage in same-sex sexual behavior during non-fertile versus fertile phases of their menstrual cycles. In the context of a plural marriage, same-sex sexual behavior during ovulation comes with opportunity costs that detract from reproduction. However, such behavior during non-fertile phases could promote the forming and grooming of alloparenting relationships among women.

  14. 14.

    If sexual fluidity serves to promote female-female bonds, heterosexual women who evidence high levels of fluidity (e.g., the most nonspecific patterns of genital arousal) should have a larger number of close female friends compared to heterosexual women with lower levels of fluidity.


But Why the Sex?

Sex is an effective means of forming, increasing, and sustaining pair-bonds between people (Brigman and Knox 1992; Hazan and Diamond 2000; Leigh 1989; Meston and Buss 2009). Sexual behavior with male mates promotes women’s feelings of commitment to these partners (Meston and Buss 2009). A similar process of sexual behavior-induced commitment is likely to occur between female partners. As female same-sex behavior in bonobos appears to increase both the survival of the mother and her offspring (Furuichi 1989; Hohmann and Fruth 2000; Parish 1994, 1996), it is likely that future research will reveal that sexual relations between female bonobos increase pair-bonding and help ensure that a mother’s offspring are cared for by alloparents (Radtke 2012). The alloparenting hypothesis suggests that psychological mechanisms underlying a similar process of same-sex sexual behavior in the service of alloparenting evolved in human females and is particularly likely to be triggered among women who encounter an absence of paternal investment, or the availability of allomothering investment.


The alloparenting hypothesis entwines several diverse phenomena including (a) female sexual fluidity in human and nonhuman 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 nonhuman primates. The alloparenting hypothesis also outlines 14 testable predictions, 12 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.

Since the engine of evolution is reproduction, same-sex sexual behavior appears to be an enigma. However, it is resolved if, instead of hindering reproduction, the trait actually facilitates it. In light of the alloparenting hypothesis, a trait that formerly appeared maladaptive – sexual behavior between women – is recast as an adaptive outcome. 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.



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© Springer International Publishing Switzerland 2016

Authors and Affiliations

  1. 1.University of ScrantonScrantonUSA