Birth control pill blister packs
The pills did not impair study subjects’ memory, UCI graduate researcher Shawn Nielsen emphasizes: “It’s a change in the type of information they remember, not a deficit.” Hoang Xuan Pham / University Communications

A carefully controlled experiment involving women using birth control pills and those not on them has yielded surprising results.

The volunteers, females aged 18 to 35, were each shown the same series of slides by UC Irvine researchers: a boy and his mother, a car on a sidewalk, and various hospital scenes.

The audio narrative, however, differed. Some were told the car had hit a curb, while others were told it had hit the boy and critically injured him.

One week later, all were given surprise tests about what they recalled. The results were striking: Women taking hormonal contraceptives such as the pill for as little as one month remembered more clearly the main points of the disturbing story line – that there had been an accident, that the boy had been rushed to the hospital, that doctors had worked to save his life and had successfully reattached his feet, for instance. Conversely, women with natural hormonal cycles retained more details, such as a fire hydrant next to the car.

The findings were published this month in the journal Neurobiology of Learning & Memory and widely reported in newspapers and at online news sites.

“What’s most exciting about this study is that it shows the use of hormonal contraception alters memory,” says UCI graduate researcher Shawn Nielsen. “There are only a handful of studies examining the cognitive effects of the pill, and more than 100 million women use it worldwide.”

She stresses that the medications did not damage memory. “It’s a change in the type of information they remember,” she notes, “not a deficit.”

This difference makes sense, says Nielsen, who works with neurobiologist Larry Cahill, because oral contraceptives prevent pregnancy by suppressing sex hormones such as estrogen and progesterone. And these hormones were previously linked to women’s strong “left brain” memory by Cahill’s research group.

Nielsen and colleague Nicole Ertman say the findings could help shed light on why women experience post-traumatic stress syndrome more often than men, as well as other disorders.

“We’ll be interested to see what other cognitive effects the pill and similar contraceptives have,” Ertman says. “We’re hopeful that this research will lead to better health outcomes, especially in mental health conditions that disproportionately affect women.”

The work could also further understanding of how men remember differently than women. Males typically rely more on right-hemisphere brain activity to encode memory, and they retain the gist of things better than details. Women on the pill, who have lower levels of hormones associated with female reproduction, may recall emotional events similar to the way men do.

“This new finding may be surprising to some,” Cahill says, “but it’s a natural outgrowth of the research we’ve been doing on sex differences for 10 years.”

Neurobiologist Pauline Maki of the University of Illinois at Chicago agrees. “Larry Cahill is already well known for his phenomenal work linking sex to memory,” says the professor of psychiatry and psychology, who specializes in memory and brain functioning. “The fact that women on oral contraceptives remembered different elements of a story tells us that estrogen has an influence on how women remember emotional events.”

The study was funded by the National Institute of Mental Health. Additional authors include Cahill and UCI undergraduate research assistant Yasmeen Lakhani.