Now is the time to read Cordelia Fine’s Testosterone Rex. In the context of sexual harassment scandals rocking the nation from Hollywood to the US capitol, clarity about where sexism does and does not come from is crucial. Excuses such as “locker room talk” and “boys will be boys” serve to naturalize sexism. Although it is not always articulated clearly, the underlying premise is that men are biologically hardwired to behave the way they do, and this is why sexual harassment and wage inequality exist. Differences between men’s and women’s brains and hormones allegedly cause the Mars and Venus behaviors we see. Men are supposedly more competitive, more physically imposing, and more sexually aggressive.

That these ideas continue to hold sway is evidenced  this statement by University of Glasgow psychology professor Gijsbert Stoet, who explains the continued gender gap in science fields in this way:

People are often guided by their unconscious desires. In the Stone Age, it was useful for men to be hunters and women to look after babies, and nature has helped by encoding some of those skills in the hardware of our brain. That still influences how we think today.

These are not just casual opinions, but beliefs that some scientists claim are supported by studies. Fine cites, for example, a study conducted on fruit flies with identifiable mutations. The reproductive success of male and female flies was evaluated. The data suggested that if male flies mated with ever-larger numbers of females, their reproductive success increased accordingly. The authors did not observe this for the female flies. They concluded that, because of the time and energy the females put toward gestation, there was no real point in “sleeping around.” But for the male flies, evolution favored the promiscuous. So, sperm is cheap and gestation expensive, therefore males should spread sperm everywhere and females should be choosy and careful.

However convenient the narrative, there are some problems with the study—much less the transposition of its conclusions to humans under modern capitalism. First, some of the mutations actually decreased the viability of the offspring, skewing the counts. Also, the author only focused on a subset of the data he obtained in order to draw the larger conclusions about sex-specific differences in the evolutionary benefit of promiscuity for all species. But when species other than fruit flies are examined for the effect of promiscuity, the results vary widely. For example, female foam-nesting tree frogs and yellow-pine chipmunks (among many others) do much better when they mate with as many males as possible. It’s quite easy to find examples of species where the males mate as little as possible, suggesting they don’t consider their sperm so “cheap.” These observations fly in the face of a narrative that suggests evolution always favors male “promiscuity.”

It is undeniable that it takes more time for a human woman to carry a baby to term than for a man to ejaculate. Because of this, psychologist David Schmitt once suggested that there was a selective pressure for “men’s mating strategies to favor at least some desire for sexual variety.” (Subtext: monogamy is unnatural for men). And it is true that studies have documented that young men are more likely than young women to agree to have casual sex when propositioned by a stranger. But this constitutes not proof of any natural sexual predispositions on the part of men or women, but rather social and cultural factors. Fine, who unpacks some of the differences in social sanction for promiscuity in men and women, puts it eloquently when she writes, “What this study is actually primarily showing is women’s lack of interest in being murdered, raped, robbed, or inflaming the interests of a potential stalker.”  Drawing on the work of historian Hera Cook, she relates how sexually repressive norms were established during Victorian England and persist in modified forms to this day.

Fine also explores the biological basis of sex, with an appropriate mention of the estimated 2 percent of people who have more complex anatomical, hormonal, and genetic characteristics than a sex binary typically acknowledges. She challenges the historical view of the magic Y chromosome being the sole determinant of sex. The influence of this chromosome, in addition to the sex-associated hormone levels, supposedly determines highly gendered brains. But the reality is much more complex: young rats that experience profound stress have altered brain development that is sex specific. The observed differences observed between brains of men and women result from a variety of environmental, genetic, hormonal, and epigenetic (heritable non-DNA changes that affect which genes are turned on and when) factors. And unlike average genital anatomy, sex influences brain development and function in unpredictable and complex ways. Finally, sex is not the sole determinant of any feature of the brain studied thus far.

Critiques of gender norms are becoming much more widespread, especially among young people. More and more people are examining the gender role they are told to assume and saying, “That’s just not me.” Some of the recent data on behaviors that are more common in one sex than another help validate that sentiment. A meta-analysis was conducted of studies on twenty-five behaviors where a sex difference had been shown. If you track it down to an individual level, less than 1 percent of the people consistently behaved as “the right” gender. Between 55 and 70 percent displayed a more mosaic pattern of gendered behaviors. So, when we consciously reject gender roles, it seems we are only articulating what already happens in our day-to-day lives.

But Fine’s analysis would be incomplete if she didn’t address the elephant in the room: testosterone. This magic elixir is blamed for phenomena as diverse as the collapse of the housing bubble in 2008 to roughhousing by five-year-old children. She tells the story of one of the more direct examples of the physiology of testosterone, involving African cichlid fish. Some of the males of Haplochromis burtoni are territorial—they spend their days fighting less dominant males and copulating with females. They have higher levels of testosterone, larger testes, and bright red and orange markings compared to the non-territorial males. These fish would appear to affirm the narrative of the power of testosterone. However, it turns out that the physiological changes associated with dominance are the effect, not the cause, of dominant behaviors. These fish’s perception of their relative social power can cause them to either gain or lose these features.

I particularly liked the cichlid fish story because it sums up something important: sex differences exist, and chromosomes, brains, and hormones vary, but an attempt to claim that something as complex as human behavior can be reduced to the two-hundred-odd genes on a Y chromosome is ludicrous. This is not some abstract scientific debate, either. If biology explains the very real differences in the lived experience of men and women, then the fight for sex equality is basically already lost. Watching the news over the last six months, it appears that many people of all genders agree with Cordelia Fine’s thesis: nothing about sexism in our society is “natural.”