According to research published in the journal Trends in Ecology and Evolution, over half a century of widespread use of the oral contraceptive pill may well have changed the preferences of young women away from masculine-looking men to those with more feminine features.

Need an illustration of this?  Consider that baby face of the wildly popular Canadian singer Justin Bieber. 

Earlier studies suggested that in the days before ovulation, women start to become more attracted to men who have deeper voices and more chiseled, masculine and symmetrical faces. This may be because men who look like the lusciously rugged Javier Bardem are more likely to have dominant social roles, better genes and stronger immune systems.

Dr. Marina Adshade, in her Big Think column called Dollars and Sex, cites a number of previous studies suggesting that women actually change their mate preference according to where they are in their menstrual cycle. For example:

“Participants in one study preferred, at significantly higher rates, poor yet creative men for short-term sexual relationships over financially successful and not creative men when they were ovulating (93% versus 58%).  Another study found that women are more likely to seek extra-marital relationships when they are ovulating as well.

“This suggests that at peak fertility, women seek men for their reproductive benefits rather than other important characteristics such as their abilities as caretakers.”

But Dr. Adshade, who is an assistant professor at Dalhousie University where she teaches a popular undergraduate course called The Economics of Sex and Love, points to the more recent study published in Trends in Ecology & Evolution.

This research shows that when taking The Pill, women lose their variations in preference over their cycle and, in particular, they lose the approximately six days in which they have a strong preference for a man who is masculine in appearance. She writes:

“It might very well be the case that in societies where large numbers of women are taking oral contraceptives, the societal ideal of an ‘attractive’ mate is moving away from a man who looks like he will provide good genes towards a mate who looks like he might be a caretaker.”

The study authors took male face images and altered them to be either more feminine or more masculine. They then randomly showed the images to the women in the study and asked them to record their preferences for short- and long-term relationships.

“They found that non-pill users preferred less masculine faces in long-term relationships but masculine faces for short-term relationships. The women in the study who were using the pill preferred feminine features in men regardless of the context of the relationship. What is the interesting question here is how pill use effects divorce rates if a woman’s choice of partner changes when she stops taking the pill after she has already committed.”

A study done by Dr. Benedict Jones out of the University of Aberdeen, published in the journal Behavioral Ecology, found that the masculine trait of a low-pitched voice tends to be preferred by women who themselves have deeper voices. Might that preference help to explain why the relatively higher-pitched (feminized) singing voice of the young Justin Bieber is most appealing to his younger female fan base who have higher-pitched voices as well?

But a research team led by Dr. Greg Bryant at UCLA has also measured a subtle but detectable shift upwards in the pitch of an average woman’s voice when she’s approaching ovulation. In fact, her voice reaches its highest pitch on the day before and the day of the egg’s release. So does that mean that even pre-menopausal women may start humming Bieber’s hit tune “Baby” around their ovulation days?

Another possible explanation for preferring less-masculine faces may well lie in the state of our general health. For example, another University of Aberdeen research study, this one from Dr. Jones’ colleague Dr. Lisa DeBruine, reported in The Royal Society last February that women’s masculinity preferences will be stronger in cultures where poor health is particularly harmful to survival. Her team investigated the relationship between women’s preferences for male facial masculinity and their health factors like mortality rates, life expectancies and the impact of communicable disease. Across 30 countries, masculinity preference increased as health decreased, findings that were independent of cross-cultural differences in wealth or women’s mating strategies. In other words, women in relatively good health can afford to prefer baby-faced men. Women who take The Pill, then, can arguably afford to be taking it both financially and physically.

Read Dr. Adshade’s Big Think column called Justin Bieber and Birth Control, or Why Young Women Prefer Girly-Faced Men.