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The amount of estrogen in women's birth control pills may affect how they perceive other women, a small new study suggests.
Among the women in the study, the higher the dose of estrogen in the birth control they were using, the more likely they were to rank other women's appearance as very important to them, the researchers found.
The study included 42 women who were using birth control that contained both estrogen and progesterone. Most of the women were taking birth control pills, but four of them were using a vaginal ring such as NuvaRing. Half of the women were taking ultra-low dose contraceptives (that contain between 0.015 mg and 0.020 mg of estrogen), and the other half were taking low-dose contraceptives (containing 0.030 mg to 0.035 mg of estrogen).
The women in the study also looked at pictures of men and women and rated several attributes related to their appearance, such as their attractiveness and sex appeal, and other attributes not related to their appearance, such as their health and energy level. The women in the study ranked the importance of all these attributes, from the most to the least important, on a scale from 1 to 10.
The results of the new study as well as previous research suggest that the way women rank other women's traits is likely related to the hormones in the contraceptives they are taking, said study researcher Valentina Piccoli, of the University of Trieste in Italy. However, without measuring the women's hormone levels in the blood, the researchers could not pinpoint exactly how the hormones may have affected the participants' rankings of other women, she said. [7 Surprising Facts about The Pill]
"This mechanism may be a direct result of the hormones ingested via contraceptive pill use," or it could result from changes in the body's natural hormone levels that occur in women using the contraceptive, Piccoli told Live Science.
She noted the findings are preliminary and should be treated with caution. The study did not have a control group in which women were taking a placebo instead of birth control pills, she said.
The investigators found no link between the level of progesterone in the pills and the women's perceptions of other women, according to the study, published in the August issue of the journal Personality and Individual Differences.
Previous research has shown a relationship between the levels of estrogen in combined hormonal contraceptives and increased jealousy and so-called "mate-guarding behavior," in which a person tries to ensure that his or her mate is not interested in pursuing other men or women. The new findings show that estrogen levels may be also be related to women's paying more attention to potential female competitors, the researchers said.
Piccoli stressed that, in the new study, the researchers did not administer different doses of estrogen but only assessed the doses of the hormone that were in the contraceptives the women took. Therefore, the study does not show a cause-and-effect relationship between the different levels of estrogen and what the researchers have called "objectification" of other women, she said.
The researchers will next look at a larger sample of participants to better understand the results, she said.