I'll give an example of willful gullibility. In the 1970s, there was a popular idea that men and women are genetically identical, with the exception of a few key organs. So, if boys and girls behave differently, it's because society teaches them to behave differently. As this article in the Economist recalls, "there was a fad for giving dolls to baby boys and fire-engines to baby girls. The idea was that differences in behaviour between the sexes were solely the result of upbringing: culture turned women into ironers, knitters and chatterboxes, and men into hammerers, drillers and silent types. Switching toys would put an end to sexual sorting."
It didn't work. From birth, boys and girls are different in rather key ways, and the little girls wanted to calmly socialize, care for dolls and so forth, while the little boys wanted to run around and beat the hell out of things. It's pretty likely that the exaggerated sex stereotypes that reign in pop culture today are as stupid as they seem. But, it's also unlikely that a boy is just a girl with a penis.
In fact, a notorious misstep in that direction was taken by the controversial doctor John Money, when he decided that a baby boy whose penis was damaged in a botched circumcision should be given a vagina and raised as a girl. "The experiment didn't work, and the consequences were tragic." The 'girl' was miserable, and eventually got another sex change as an adult, battled depression, and finally committed suicide.
People still fiercely debate nature vs. nurture, and I think it's very mistaken to come down on one side or the other. There is an interplay with the natural world and culture that shapes us. To give an example, nearsightedness is associated with a genetic disposition, and yet people with this disposition who live in the Amazon jungle don't need glasses. Why? Well, they have the gene, but they don't strain their eyes by reading print, so the gene isn't activated. Simply put, nature and nurture make us who we are.
But, hormone research has made leaps and bounds in the last decade or so. "That boys and girls—and men and women—are programmed by evolution to behave differently from one another is now widely accepted." At least, it is in the sciences. But, the Economist doesn't point out how widely denied it is in the humanities. I constantly encounter academics who use the term 'essentialist' as a slur for those who believe that any behavior is encouraged by biology. And this is fascinating to me. When I point them to recent studies on hormones, particuarly those on testosterone, they often become derisive about science itself, accusing it of attempting to control and oppress us. "Remember the Nazi doctors? They were scientists!" (Not really). They take the Nietzschean line (via Foucault) about science as another way of knowing that expresses the will to power of those who are scientists. A line that the intelligent design people also borrowed directly from Foucault, oddly enough.
And so, I often encounter people who believe something in the face of all evidence to the contrary. It scares me to think that certain corners of academia are populated by flat-earthers. You saw a bit of this in the dust-up at Harvard. I've said that Larry Summers was a much lousier president than his defenders are willing to admit, and that's true. But, it's bizarre to demonize someone for suggesting that biological diferences might be researched. And it demonstrates how certainty becomes the death of learning. People who are certain aren't curious about things they don't know about.