A new but small study suggests the amount of estrogen in a woman’s birth control pill may affect how she perceives other women.

Whereas past research has shown an estrogen effect from the pill on a woman’s mating preferences as well as mate-guarding behavior, the new findings suggest that higher doses of the hormone in birth control medication further focuses the female brain on sexual competition. For the study, published in the journal Personality and Individual Differences, researchers from the University of Trieste in Italy recruited 42 women mostly taking birth control medication, with several using a contraceptive vaginal ring. But although the study included no control group, lead researcher Valentina Piccoli said they were able to compare the effects of birth control pills with either ultra-low or high doses of estrogen. On a scale of 1 to 10, the women in the study ranked the importance of physical traits when shown photographs of men and women.

Notably, progesterone had no noticeable effect on how women ranked the photographic subjects on attractiveness and sexual appeal, as well as on other overall qualities such as health and perceived energy level. But higher doses of estrogen changed how women assessed the importance of physical attributes of other women. "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.

Yet, the researchers could tell nothing of how the hormones may have affected perceptions among women in the study, particularly without a control group of women not taking estrogen-laced birth control. However, Piccoli said the findings do not demonstrate a simple causal relationship between doses of estrogen and changes in perception some researchers refer to as the “objectification of women.” The limited study only examined the effects of a couple of different types of estrogen-driven birth control medication, she added.

A 2009 study from the University of Sheffield in the United Kingdom found that estrogen from the pill affects the hormonal balance, shifting a woman’s body to a hormonal state consistent with pregnancy, and therefore affecting mating behavior and choices.

"Although mate choice studies in humans have routinely recorded pill use during the last decade to control for its confounding effects, little effort has been invested in understanding the consequences of such effects of the pill," said Alexandra Alvergne, of the University of Sheffield, in a press statement.

Piccoli’s team said more study should be done to better understand the subtle and not-so-subtle effects of birth control pills containing estrogen.

Source: Piccoli V, Cobey KD, Carnaghi A. Hormonal contraceptive use and the objectification of women and men. Personality and Individual Differences. 2014.