In Roughgarden’s view, biologists have categorized homosexuality and other non-reproductive sexual behaviors as “aberrations” or rare exceptions to the rule. She decided to study the diversity of animal sexual behaviors. She eventually compiled the material into a book titled Evolution’s Rainbow. Published in 2004 and written for the non-scientist, it catalogues hundreds of species with a plethora of scandalous behaviors, from ejaculatory anal sex in male bighorn sheep, to orgasmic genital rubbing in female bonobo chimps, to a sort of ménage-à-trois in the bluegill sunfish. According to Roughgarden, these behaviors, one after another, present the field of evolutionary biology with a challenge the sexual selection framework does not meet. 

In his 1871 treatise on sexual selection, Darwin wrote, “Males of almost all animals have stronger passions than females,” and “the female…with the rarest of exceptions is less eager than the male.” In the 20th century, biologists like Robert Trivers supported Darwin’s generalized narrative of passionate males and passive females using what is now called parental investment theory. In this model, females choose their partners carefully because of the time and effort of egg production, pregnancy, and/or childcare. Males, on the other hand, invest little energy in sperm production or rearing offspring, so their most effective strategy is to mate widely and often. In this way, the differential parental investment of the two sexes gives rise to the traditional “battle of the sexes,” which continues to surface in popular culture today—males chase quantity, while females choose quality. 

But in the 1970s and 1980s, the women’s movement questioned traditional sex roles and brought more women into the lab. Soon, sexual selection’s Victorian view was revised. Through the work of Sarah Hrdy, Patricia Gowaty, and many others, new data accumulated to challenge the stereotype of the “passive female.” Although most evolutionary biologists agree that the parental investment theory provided a useful framework that stimulated the acquiring of new data, many also agree that the data doesn’t support parental investment theory. The link between parental investment and reproductive behavior is not always predictable. In many species, males can be just as choosy as females, and females as sexually competitive as males. 

But in Roughgarden’s view, this challenge stops short. Rather than gain equal rights for females within sexual selection, Roughgarden seeks to do away with the theory that she believes nature’s sexual diversity has rendered obsolete. Beyond the categorizations of male and female, or even homo- and heterosexual, Roughgarden cites numerous examples of species that do not fit into what she calls the “gender binary.” Pointing to the prevalence of hermaphroditism, multiple genders, and species that change sex during the lifecycle, she believes that the real story of sexual behavior is being stifled by sexual selection theory. “The facts speak for themselves,” she says, “and the facts are that biologists have been covering up the extent of certain sexuality, homosexuality, and all kinds of gender and sexuality expressions.” She sees the theory of sexual selection as a rigid framework that highly complex behaviors have been needlessly shoehorned into. “I think it’s pretty clear that there are no known established cases where sexual selection is true,” she says. “There are lots of papers that invoke sexual selection after the fact, to rationalize the behavior, but they don’t test it and that’s in part because they don’t have an alternative.”