What is desire? How would one describe and study it? How does it change over one’s lifetime?

“And what are its units — centimeters per second?” joked professor Bill Summers, who has been teaching “The Biology of Gender and Sexuality,” more commonly known as “Porn in the Morn,” for four years. “I mean, it sounds silly because we don’t know how to talk about it.”

This debate about desire embodies the ambiguities at the intersection of biology, sexuality and gender — an area of study still plagued by “unanswered questions,” he said.

Summers started the class as a way of introducing sexuality — from a scientific standpoint and as a broader process encompassing reproduction — as a course of study within the Women’s, Gender & Sexuality Studies major. A professor in the History of Science, History of Medicine Department, Summers said he took on the role as a way to explore the interaction of humanities and science and to make the latter relevant to non-science-oriented students. This field of interaction, which bridges several disciplines, seems appropriate to Summers’ similarly eclectic range of interests, from the biology of viruses to Chinese culture and history.

What Summers didn’t expect from the course was the overwhelming response from Yale students — admittedly, some political science majors searching for ways to fill distributional requirements, but others legitimately drawn by the intrigue of the subject matter.

But with entire lectures dedicated to differentiating between the G-spot and clitoral female orgasm, pinpointing sexual arousal and its visual symptoms, and understanding the anatomy of the penis and vagina, perhaps its popularity should not come as a surprise.

“I’d never really had a sex ed class before Yale — I thought it’d be a useful sex ed course, but it went even beyond that,” said Matthew Adams ’10, who explained he took the class on a whim and ended up finding the subject matter interesting and thought provoking. “The class opens your eyes to a lot of real issues,” he said.

The class looks at issues of sexuality and gender from a variety of standpoints, including anatomy, physiology, neuroscience, genetics and the social sciences, Summers said.

He said the study of the field necessitates this multidisciplinary approach. The method is a substitute that helps us grapple with questions we currently don’t have the tools to solve, he said.

“It’s really about the human mind, which we don’t have a good grasp about,” he said. “Whatever you think is a product of the brain — so psychology is biology.”

Most people believe gender and sexuality are very natural, universal processes, but that idea is challenged by views and expressions of the two concepts that vary from individual to individual, he said.

The study of how this comes about draws upon the social science framework more than its does the biological framework, said professor Maria Trumpler, director of undergraduate studies for the Women’s, Gender & Sexuality department, who is teaching a freshman seminar called “The History of Sexuality” this semester.

“Almost nothing is biology in my own professional framework,” she said. “The issue of gender expression — the degree to which you choose to express femininity or masculinity — is 90 percent cultural.”

Summers said conceptions of gender, distinct from the concept of biological sex, vary from culture to culture, complicating the relationship between the two.

For example, skirts are associated with femininity in the United States, but in Scotland, where kilts are the national male dress, they are associated with masculinity. A good way to think about the distinction, he said, is to use the terms femininity and masculinity in reference to gender, but female and male in reference to biological sex.

“Gender is much more culturally embedded,” he said. “It has to do with how society interprets what you’re doing.”

But the scientific study of sexuality has historically had many pitfalls, Trumpler said.

She said the dominant course of study on gender and sexuality has been about “trying to label and box and typologize” — an approach too narrow for the multiplicity of sexual and gender identities individuals prescribe to in the real world.

These labels, Trumpler added, have immense political import. For instance, contemporary political debates have tied individuals’ eligibility for civil rights to their sexual orientations or gender identities — a debate that has been traditionally fraught with the message that, if such identities are personal choices, not inherent biological characteristics, the individual should not be eligible. So understanding some of these gender processes, and the distinction between their biological and sociological components has broad ramifications for all of society, she sad.

But Trumpler said the department “readily admits” that the field as it stands today asks more questions than provides answers.

“Why can we talk about how drugs work on nerve endings, but not why you’re turned on by one sort of person and not another?” Summers asked. “We don’t know whether its going to be the biochemists who are going to give us the answers or the cognitive scientists or the psychologists.”