The New Atlantis has published an extensive report about homosexuality, transgenderism, and their causes. The authors—Lawrence S. Mayer and Paul R. McHugh, both affiliated with Johns Hopkins University—take aim at the idea that gays and the transgendered are “born that way.” Perhaps this sentence, from their section focusing on homosexuality, best captures their overall attitude: “Research suggests that while genetic or innate factors may influence the emergence of same-sex attractions, these biological factors cannot provide a complete explanation, and environmental and experiential factors may also play an important role.”

Mayer and McHugh are absolutely correct that the reality of human sexuality is complicated and messy—no explanation would be complete unless it addressed both the “genetic or innate” and the “environmental and experiential.” What’s more, the authors have a praiseworthy willingness to confront the taboo, most evident in their discussion of whether childhood sexual abuse might increase the odds of homosexuality in adulthood.

But Mayer and McHugh don’t strike quite the right balance between the innate and the environmental—in fact, they gloss over some of the most compelling evidence on the innate side of the ledger. The problem is easily seen if one, upon finishing their New Atlantis report, proceeds immediately to “Sexual Orientation, Controversy, and Science,” a paper from earlier this year by J. Michael Bailey, one of the nation’s leading sex researchers, and several prominent colleagues.

Before we begin in earnest, though, I should confess I’m biased: Bailey is the reason I’ve been following this research casually for more than a decade. During my undergraduate years at Northwestern, where he teaches, a nasty controversy swirled about his book The Man Who Would Be Queen: The Science of Gender-Bending and Transsexualism (free PDF). Among much else, the book advanced the theory that some male-to-female transsexuals are driven by “autogynephilia,” or sexual arousal at the thought of oneself as a woman, which some activists find offensive.

I never took any of Bailey’s classes, but I did interview him once for the Northwestern Chronicle, the conservative student newspaper—and as editor of the Chronicle in 2005, I published Bailey’s own account of the controversy. We haven’t kept in touch, but I have kept paying attention to the subjects he studies.

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As Mayer and McHugh note, the exact boundaries of a sexual orientation are hard to nail down. If we define orientation by one’s behavior, for example, we have to address the fact that such behavior can be shaped by circumstances and culture—sometimes people resort to same-sex behavior just because they lack access to opposite-sex partners; many cultures strongly discourage homosexual behavior while others accept or even ritualize it. People may also lie about their behavior when asked, whether out of shame or just as a joke. Similar problems bedevil people’s self-identifications as straight or gay or something else.

Despite being fully aware of these issues, the authors give a lot of emphasis to self-report evidence—citing, for example, surveys in which adolescents change the sexual identities and attractions they report over time. They refer to other measures, such as “tests that include observing the way pupils dilate in response to sexually explicit imagery,” only in passing, while discussing the difficulty of defining sexual orientation. But tests like these actually teach us quite a bit—at least when it comes to men.

Unfortunately, the most well-established test involves measuring blood flow to a man’s most sensitive organ while he views stimulating material. (Women are a trickier bunch, both because their sexuality seems to be more “fluid” and because female genitals work in confusing ways.) But despite its limitations, this research has produced some key insights. For one thing, men do have distinctive arousal patterns. In general, they respond strongly to one sex but hardly at all to the other, though men with bisexual arousal patterns also exist. For another, men’s arousal patterns usually match their self-identifications.

This suggests that while there’s plenty of gray in human sexuality, there’s also some black-and-white. On a very physical level, men react to some stimuli but not others, with self-identified gay men reacting to sexy males the same way that straight men react to sexy females.

Of course, this says nothing about why we observe these arousal patterns. Is it genes? Something else biological? Social conditioning?

Mayer and McHugh do a good job of summarizing a few of these lines of inquiry. For example, they concede there’s evidence that genes and hormones play some role. They also note studies showing differences between the brains of gays and straights, which are certainly consistent with a biological explanation even if they don’t prove it (as life experiences can shape people’s brains). But they miss some things.

One important piece of evidence is that adult homosexuality correlates strongly with gender nonconformity in early childhood—boys wearing dresses, etc. This has been shown with both retrospective studies (where adults are asked how masculine or feminine they were as kids) and prospective ones (where gender-nonconforming kids are followed to see how they turn out), and across cultures. This is consistent with homosexuality being both innate and a stable trait, though the distinction between “is consistent with” and “proves” again comes to mind.

Another key piece of evidence is the “fraternal birth order effect”: the more older brothers a man has, the more likely he is to be gay. This phenomenon is well-established. From a 2011 review (citations removed):

Although there have been a few failures to replicate the FBO effect, the original finding … has been confirmed many times, including replications by independent investigators. The FBO effect has also been demonstrated in diverse samples, including very different clinical samples of men with same-sex attractions, including homosexual transsexual men [i.e., male-to-female transsexuals who are attracted to men], gay men from different historical eras, men with same-sex attractions from different cultures, in male GID (gender identity disorder) children who will very likely … be gay in adulthood, and in gay men from convenience and representative, national probability samples. … In summary, FBO is likely the most reliable epidemiological finding in almost a century of research on sexual orientation.

Whatever drives the correlation, it seems to happen before birth, as adoptive older brothers don’t have the same effect. One would expect the Mayer/McHugh report to discuss this in depth. Instead there’s only one offhand reference to it, commenting on one of the studies that failed to replicate the effect: “This study also found no correlation between experiencing same-sex attraction and having multiple older brothers, which had been reported in some earlier studies.” (Later studies, too, actually.)

An advantage the New Atlantis report has over similar research reviews, though—including the paper by Bailey et al.—is its willingness to dive deep into the controversial notion that sexual abuse in childhood could increase one’s chance of becoming gay. To be absolutely clear, this is a very different claim from the one holding that gays themselves are pedophiles, which is not true. (Pedophiles are overwhelmingly men, and the aforementioned sexual-arousal studies show that for men, age and gender seem to be separate components of sexual orientation. Many pedophiles target kids of one gender or the other, but standard-issue homosexuals, like standard-issue heterosexuals, are not attracted to children of either gender.)

The correlation is there, as Bailey et al. do discuss:

In a recent, carefully sampled, and very large (N = 33,902) study, T. Sweet and Welles (2012) found that nonheterosexual adults of both sexes were much more likely than heterosexual adults to have experienced childhood sexual abuse, defined as “sexual experiences with an adult or any other person younger than 18 years when the individual did not want the sexual experience or was too young to know what was happening.” … 38.1% for lesbians, 43.5% for bisexual women, and 14.2% for heterosexual women; 18.6% for gay men, 19% for bisexual men, and 4.6% for heterosexual men.

But Bailey et al. downplay (as did the authors of the study they cite) the possibility that sexual abuse might play some role in causing homosexuality. Skepticism is justified, and Mayer and McHugh share it; for example, gender-nonconforming kids could be targeted for sexual abuse, and of course not all gays are abuse victims. But where the Bailey paper leaves it at that and walks away, Mayer and McHugh look a bit closer, for instance noting a study from three Harvard researchers that employed a complicated statistical technique that—at least if certain strict assumptions are met—can distinguish between correlation and causation. That study suggested there is in fact a causal element, though Bailey and a coauthor criticized it when it was released. The possibility should at least be left on the table for now.

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Mayer and McHugh cover transgenderism as well as homosexuality. I was glad to see that they address head-on what I find to be the most striking results in this area:

In a 2004 paper, William G. Reiner, a pediatric urologist and child and adolescent psychiatrist, and John P. Gearhart, a professor of pediatric urology, followed up on the sexual identities of 16 genetic males affected by cloacal exstrophy—a condition involving a badly deformed bladder and genitals. Of the 16 subjects, 14 were assigned female sex at birth, receiving surgical interventions to construct female genitalia, and were raised as girls by their parents; 6 of these 14 later chose to identify as males, while 5 continued to identify as females and 2 declared themselves males at a young age but continued to be raised as females because their parents rejected the children’s declarations. The remaining subject, who had been told at age 12 that he was born male, refused to discuss sexual identity. So the assignment of female sex persisted in only 5 of the 13 cases with known results. … It is important to note that the ages of these individuals at last follow-up ranged from 9 to 19, so it is possible that some of them may have subsequently changed their gender identities.

Reiner and Gearhart’s research indicates that gender is not arbitrary; it suggests that a biological male (or female) will probably not come to identify as the opposite gender after having been altered physically and immersed into the corresponding gender-typical environment. The plasticity of gender appears to have a limit.

I interpret these results much more aggressively. They don’t just show that “The plasticity of gender appears to have a limit”; they show that even extreme measures—cutting female genitalia into a male infant and raising him as a girl—are unlikely to make a boy think he’s a girl. And yet some biological boys do think they’re girls, and vice versa, from a young age, and without such measures’ being taken. In fact they often want such measures to be taken, to make their bodies conform to their self-concepts. To me, this indicates that something fundamental is going on here. It doesn’t seem like your run-of-the-mill mental illness that could be addressed with medication or therapy.

But we don’t really know. Transgenderism is rarer than homosexuality and has been studied less. Mayer and McHugh have little trouble making short work of the evidence we do have, though for an alternative view I highly recommend this piece in The Public Discourse by the biologist Jennifer Gruenke, who urges conservatives to rethink their approach to transgenderism. Especially interesting is some work on twins suggesting that, as with homosexuality, genes may play some role here.

On a far more practical level, Mayer and McHugh take up the question of what to do when a patient comes to a doctor seeking a sex-change operation. The research is somewhat mixed on whether such surgery improves mental health, though the American Medical Association is fine with it, and as someone with a libertarian streak I’d let people do it regardless. McHugh, who helped end the surgery during his time as chief psychiatrist at Johns Hopkins Hospital, has long been a critic of it. Hopefully future research will flesh out when, if ever, it’s a good idea.

Especially pressing is the issue of how to treat children who show signs of transgenderism. Many activists would like to reinforce these behaviors and medically change the course of puberty. But an astonishingly high proportion—about 80 percent—of these kids “desist” once puberty is naturally underway, meaning they end up being fine with their birth gender. (Importantly, though, they usually end up gay or bisexual, not straight.) We’ll put it this way: I would absolutely not do this to my own child, especially given that, as Rod Dreher has been extensively documenting on his blog, transgenderism seems to be something of a fad in schools today.

Mayer and McHugh are to be commended for an extensive and thought-provoking report. But rather than treating it as definitive, readers should seek out the other side of the story before making up their minds. The case for “born this way” is stronger than they let on.

Robert VerBruggen is managing editor of The American Conservative