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I have taught evolution and genetics at Williams College for about a decade. For most of that time, the only complaints I got from students were about grades. But that all changed after Donald Trump’s election as president. At that moment, political tensions were running high on our campus. And well-established scientific ideas that I’d been teaching for years suddenly met with stiff ideological resistance.

The trouble began when we discussed the notion of heritability as it applies to human intelligence. (Heritability is the degree to which offspring genetically resemble their parents; the concept can apply not only to physical traits, but also to behavioral ones.) In a classroom discussion, I noted that researchers have measured a large average difference in IQ between the inhabitants of the United States and those of my home country, Brazil. I challenged the supposed intelligence differential between Americans and Brazilians. I asked students to think about the limitations of the data, which do not control for environmental differences, and explained that the raw numbers say nothing about whether observed differences are indeed “inborn”—that is, genetic.

There is, of course, a long history of charlatans who have cited dubious “science” as proof that certain racial and ethnic groups are genetically superior to others. My approach has been to teach students how to see through those efforts, by explaining how scientists understand heritability today, and by discussing how to interpret intelligence data—and how not to.

In class, though, some students argued instead that it is impossible to measure IQ in the first place, that IQ tests were invented to ostracize minority groups, or that IQ is not heritable at all. None of these arguments is true. In fact, IQ can certainly be measured, and it has some predictive value. While the score may not reflect satisfaction in life, it does correlate with academic success. And while IQ is very highly influenced by environmental differences, it also has a substantial heritable component; about 50 percent of the variation in measured intelligence among individuals in a population is based on variation in their genes. Even so, some students, without any evidence, started to deny the existence of heritability as a biological phenomenon.

Similar biological denialism exists about nearly any observed difference between human groups, including those between males and females. Unfortunately, students push back against these phenomena not by using scientific arguments, but by employing an a priori moral commitment to equality, anti-racism, and anti-sexism. They resort to denialism to protect themselves from having to confront a worldview they reject—that certain differences between groups may be based partly on biology. This denialism manifests itself at times in classroom discussions and in emails in which students explain at length why I should not be teaching the topic.

To my surprise, some students even objected to other well-established biological concepts, such as “kin selection,” the idea that, when individuals take actions for the benefit of their offspring and siblings, they are indirectly perpetuating their own genes. Startled students, falling into what we call the “naturalistic fallacy”—the notion that what occurs in nature is good—thought I was actually endorsing Trump’s hiring of his family! Things have gone so far that, in my classes, I now feel compelled to issue a caveat: Just because a trait has evolved by natural selection does not mean that it is also morally desirable.

The duty of scientists is to study the world—including the human body and mind—as it is. Some of our students, however, are seeing only what they want to see and denying real-world phenomena that conflict with their ideology. Take, for example, the obvious biological differences between the sexes, not only in physical traits (men, on average, are clearly stronger and faster than women are), but also in aptitudes and preferences (boys generally prefer wheeled toys, and girls prefer plush toys, a preference that is also observed in baby monkeys!).

People expect an equal sex ratio across academic professions and sometimes ascribe the lack of such equality to bias. In the so-called STEM fields—science, technology, engineering, and mathematics—the relative paucity of women is frequently taken to reflect endemic sexism. While this is undoubtedly a factor, the effect of bias as opposed to other factors, such as differences in what male and female students prefer, requires detailed empirical study.

One set of data challenges the idea that bias is the only cause of sex-ratio differences in the STEM fields. The so-called gender-equality paradox involves the observation that, while women and men around the world perform equally well on standardized science tests, countries with the highest proportion of women in STEM are not the ones with the least discrimination or sexual harassment, but those with the greatest gender inequality. Where women are free to choose their own path and do not have to worry about pay, they gravitate toward the humanities. Countries such as Norway and Finland have relatively few women in STEM fields, while countries such as Algeria and Indonesia have an ample supply.

However, when one assumes that everyone is a blank slate, differences between what males and females do can be explained only by bias and harassment. The conclusion is obvious: All STEM fields are cesspools of sex discrimination. This is what happens when ideology replaces biology. It’s become taboo to even mention the possibility that men and women might have different preferences.

Sadly, students do not seem to realize that their good intentions may lead them to resist learning scientific facts, and can even harm their own goal of helping women and ethnic minorities. The existence of any genetic differences between males and females, or between different ethnic groups, does not imply that we should treat members of those groups differently. Denying reality and pretending that differences do not exist—as if this were the only possible path toward equality—is dangerous. If you believe that moral equality relies on biological equality, this makes your moral views susceptible to future research that might reveal biological inequalities. Instead, equality and equal opportunity for all should be the default position, regardless of potential biological differences.

When students at Williams or anywhere else try to protect their worldview by denying scientific evidence, it is bound to affect what professors teach and how they teach it. Campus norms proscribe any discourse that might offend women, minorities, or anyone perceived as a victim of patriarchal white societies. However, this rule, no matter how well intentioned, is harming the very people it aims to protect. The argument favoring a certain amount of self-censorship is that it is necessary to protect minority students from feeling unsafe when they hear what they see as “hate speech.” However, by not talking about science that some find unsettling, we deny students opportunities for learning and for intellectual empowerment. How well can they argue their positions effectively unless they are seeing the world as it really is?

This article is part of “The Speech Wars,” a project supported by the Charles Koch Foundation, the Reporters Committee for the Freedom of the Press, and the Fetzer Institute.

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