Read: Are male and female brains biologically different?

The perception of a hard-and-fast separation between the sexes started to disintegrate during the second wave of feminism in the 1970s and ’80s. In the decades that followed, we learned that about 1.7 percent of babies are born with intersex traits; that behavior, body shape, and size overlap significantly between the sexes, and both men and women have the same circulating hormones; and that there is nothing inherently female about the X chromosome. Biological realities are complicated. People living their lives as women can be found, even late in life, to be XXY or XY.

Skeletal studies, the field that I work in as a doctoral student in anthropology, and the history of this field show how assumptions about sex can lead to profound mistakes, and how acknowledging that things are not really as binary as they may seem can help resolve those errors. Trump and his advisers should take note.

If you’ve ever watched the TV series Bones, you’ve heard Temperance “Bones” Brennan, the show’s protagonist and star forensic anthropologist, call out to her colleagues whether the skeleton she’s analyzing is male or female. That’s because sex distinctions are very helpful to know for missing persons and archaeological sites alike. But just how easy is it to make this determination?

In the early 1900s, the U.S.-based anthropologist Aleš Hrdlička helped found the modern study of human bones. He served as the first curator of physical anthropology at the U.S. National Museum (now the Smithsonian Institution). The skeletons Hrdlička studied were categorized as either male or female, seemingly without exception. He was not the only one who thought sex fell into two distinct categories that did not overlap. The scientists Fred P. Thieme and William J. Schull of the University of Michigan wrote about sexing a skeleton in 1957: “Sex, unlike most phenotypic features in which man varies, is not continuously variable but is expressed in a clear bimodal distribution.” Identifying the sex of a skeleton relies most heavily on the pelvis (for example, females more often have a distinctive bony groove), but it also depends on the general assumption that larger or more marked traits are male, including larger skulls and sizable rough places where muscle attaches to bone. This idea of a distinct binary system for skeletal sex pervaded—and warped—the historical records for decades.

Read: Race is not biology.

In 1972, Kenneth Weiss, now a professor emeritus of anthropology and genetics at Pennsylvania State University, noticed that there were about 12 percent more male skeletons than female ones reported at archaeological sites. This seemed odd, since the proportion of men to women should have been about half-and-half. The reason for the bias, Weiss concluded, was an “irresistible temptation in many cases to call doubtful specimens male.” For example, a particularly tall, narrow-hipped woman might be mistakenly cataloged as a man. After Weiss published about this male bias, research practices began to change. In 1993, 21 years later, the aptly named Karen Bone, then a master’s student at the University of Tennessee at Knoxville, examined a more recent data set and found that the bias had declined: The ratio of male to female skeletons had balanced out. In part that might be because of better, more accurate ways of sexing skeletons. But also, when I went back through the papers Bone cited, I noticed there were more individuals categorized as “indeterminate” after 1972 and basically none prior.