IRVINE, California, September 15, 2011 ( – Women on the birth control pill are less likely to retain factual details in memories of emotional events, according to new University of California, Irvine (UCI) research.

“What’s most exciting about this study is that it shows the use of hormonal contraception alters memory,” UCI graduate researcher Shawn Nielsen said.

In the study, which appears in the September issue of the journal Neurobiology of Learning and Memory, groups of women either on the pill or experiencing natural hormonal cycles were shown photographs of a mother, her son, and a car accident. Some in each group were told the car had hit a curb, while others were told the car had hit the boy and critically injured him.

One week later, all were given surprise tests about what they recalled. Women not using hormonal contraceptives remembered more details, such as a fire hydrant next to the car, than women on the pill for as little as one month. Women on the pill remembered more clearly the “gist” of the event, that there had been an accident, that the boy had been rushed to the hospital, and that doctors worked to save his life and successfully reattached both his feet.

The change makes sense, said Nielsen, who co-authored the publication with UCI neurobiologist Larry Cahill, because contraceptives suppress sex hormones such as estrogen and progesterone to prevent pregnancy. Those hormones have previously been linked to women’s strong “left brain” memory.

Nielsen said that only a handful of studies currently examine the cognitive effects of the pill, which is used by an estimated 100 million women around the world.

“It’s a change in the type of information they remember, not a deficit,” she said.

Pauline Maki, professor of psychiatry and psychology at the University of Illinois at Chicago, and a specialist in memory and brain functioning, noted that Cahill is known as an expert for researching “linking sex to memory.”

“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,” she said.

Nielsen said that the findings, which were made possible by National Institute of Mental Health funding, could help lead to answers about why women experience post traumatic stress syndrome more frequently than men, and how men remember differently than women.

According to the UCI press release, men typically rely more on right-hemisphere brain activity to encode memory. They retain the gist of things better than details. Women on the pill, who have lower levels of hormones associated with female reproduction, may remember emotional events similarly to men.

Other side effects pointed out by critics against the popular drug are less benign: the pill has been classed a Group 1 carcinogen, or a “definite” cause of cancer, by the World Health Organization for its effect on breast cancer risk.


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