Research conducted in functional MRI machines have taught scientists a lot about sex in the brain, including how arousal works, and what happens when a person watches porn.

But an MRI study published in December 1999 answered some more… physical… questions about what happens in the body during sex. The method? You guessed it. Couples had sex inside an MRI machine while researchers looked on.

In the original study, eight couples and three individual women participated in 13 experiments. Prior to this study, no one had ever studied two people having sex in an MRI before.

One of the more memorable findings from the study was what happens during sex in the missionary position. The researchers observed that the penis assumes a boomerang shape — not straight, or S-shaped, as scientists had previously believed. The experiments also showed that the woman’s uterus does not get larger during arousal, disproving another old theory.

When the paper first came out, it (surprisingly) piqued the public’s interest. If nothing else, people were excited to see others photographed (well, imaged) while in the act, writes Tony Delamothe, a former editor at The BMJ, in a letter reflecting on the study. It remains one of the journal’s most downloaded papers, according to Delamothe.

Sex in an MRI — how they did it

The study’s actual contribution to science was negligible. But in the year after its publication, it received the IgNobel prize for medicine, a lighthearted award given to unusual research achievements “that first make people laugh then make them think.”

The anatomy of sexual intercourse, as imaged during the 1999 study. W. Weijmar Schultz et al, BMJ (1999)

“How much thinking this article has occasioned since publication is moot,” Delamothe writes. “But it is still making people smile (and laugh), much to the annoyance of one of the experimental subjects and author, Ida Sabelis.”

Sabelis, professor of organizational anthropology in the Netherlands, told VICE in September 2019 about what it’s really like to get down in an MRI.

“It’ll probably become my legacy,” she said. “But I’m lucky. You don’t get to choose your legacy, and some people don’t get one at all.”

During the study, Sabelis and her then-partner couldn’t make missionary style work in the machine. (The missionary position findings came from the other couples who participated.) But they didn’t give up: The couple assumed a butt-to-groin position instead.

Sabelis’s boyfriend at the time was naturally concerned he might have some performance issues under such scrutiny — but when the time came, they managed just fine, Sabelis told VICE. The experiment lasted about 45 minutes as researchers directed the couple to hold specific poses — and try not to laugh.

She was amazed by the resulting scans. “There was very clear features of both our insides, including the boundary between both our bellies. It showed so much detail it made me speechless.”

The ever-attractive science of sex

Later studies have also proven helpful in sussing out the science of sex. A July 2019 study of arousal revealed men and women experience similar changes in the brain when looking at images that spur arousal.

Those results might be more significant from researchers’ perspectives, but something about people getting it on in a big MRI machine still gets people interested today.

“Why that’s the case, 20 years after the article’s original publication,” Delamothe writes, “is worth a study of its own.”

Partial Abstract from 1999 Study:

Objective: To find out whether taking images of the male and female genitals during coitus is feasible and to find out whether former and current ideas about the anatomy during sexual intercourse and during female sexual arousal are based on assumptions or on facts.

Methods: Magnetic resonance imaging was used to study the female sexual response and the male and female genitals during coitus. Thirteen experiments were performed with eight couples and three single women.

Results: The images obtained showed that during intercourse in the “missionary position” the penis has the shape of a boomerang and 1/3 of its length consists of the root of the penis. During female sexual arousal without intercourse the uterus was raised and the anterior vaginal wall lengthened. The size of the uterus did not increase during sexual arousal.