Science tells us that humans have evolved over the years to make better decisions about whom to choose as a spouse, but a growing body of research suggests that women could undo all that evolution with a simple pill many are already taking.
Birth control pills, according to a review of the literature published today in the journal Trends in Ecology and Evolution, may alter the menstrual cycle in such a way that both women and men are affected in their appetites for the opposite sex.
But researchers caution it is too early to draw conclusions about the effects of the pill, and some even doubt if science could ever answer how much hormones dictate human attraction.
"While the general trend certainly points to the direction that the pill might indeed affect mate preferences in both sexes in a very intriguing way, we badly need further studies experimentally testing these effects," said Virpi Lummaa, a Royal Society University research fellow at the University of Sheffield in the United Kingdom and one of the review authors. "So, at this point, I wouldn't like to raise any concern among the pill users, just awareness that these type of effects might also be possible."
Over time, research has helped scientists hypothesize on how women might be attracted to different types of men at different points in their fertility cycle.
Around the time of ovulation, women would be more attracted to men who could provide good genetic material to their children. However, during periods when they were less fertile, they would be attracted primarily to men who would make good father figures and provide time and funding for offspring.
Men, meanwhile, have been found to be more attracted to women during the time of the month when they are most fertile. One study of exotic dancers published last year suggested that those who were not on the pill would see a rise in their tips around the time they reached peak fertility.
Research has suggested this to be the case, in part because when women were on the pill, the fluctuations in attraction do not swing as wildly, and exotic dancers who had been using birth control did not see significant variation in their tips as the month progressed.
But even if this hypothesis becomes a well-evidenced theory, it still may not mean women should go off the pill.
"We have to weigh the benefits as well as the costs," said Dustin Penn, director and senior scientist for the Konrad Lorenz Institute for Ethology in Vienna. "If it turns out to be true that the pill alters [women's] mating preferences. … They may be willing to accept those consequences given the benefits that they enjoy from the pill."
It is similar, he said, to the case of a medication that might reduce symptoms but prolong the illness itself slightly.
"We might choose to be sick another day because we don't want to feel bad," said Penn.
Dr. Karen Boyle, a fertility expert at the Greater Baltimore Medical Center, said doctors are pretty clear about what the pill can do to a woman biologically.
"Some of the newer birth control pills, the ones that have different hormone combinations that make menstrual cycles disappear, have been known to change the moodiness with the cycle," said Boyle. "The pill is also protective against ovarian and endometrial cancer."
But Boyle was a little intrigued by the sudden increased interest in the pill by sociologists.
"The interest that science and the community at large has had with female sexual health has changed over the last couple of years," she said.
But while Boyle would love to know whether the pill could influence who we settle down and marry, she doubted whether any studies could prove the connection.
"You could extrapolate from the pill, but the women who use birth control have other biases -- their education levels, their communication skills," said Boyle. "It's really interesting … but because it's such a difficult thing to study, it hasn't been something that we would discuss with patients who are going to start the pill."
One of the most controversial, and most pressing arguments for studying the pill's effects on mate selection was the impact on future generations.
A few small studies in the published review found a genetic difference between the men women find attractive during a natural cycle, and the men women found attractive while on the pill. Women on the pill found men who were more genetically similar to themselves more attractive, while women who were off the pill found men who were genetically dissimilar more attractive.
Animal studies have shown that genetically similar mates have a harder time producing offspring, and the offspring they do produce are less fertile in turn.
But Dr. William Hurd, a fertility expert at the University Hospitals Case Medical Center, was suspicious of the jump from the animal kingdom to modern problems with infertility.
"In the human world, infertility from genetic similarity has not been found to be true, even though people have suspected it," said Hurd. "Doctors tried for years -- they thought that recurrent miscarriages were related to genetic similarity, but they couldn't find an example."
Hurd said doctors used to test infertile couples to see if they were HLA compatible -- a genetic similarity on blood cells that can determine if a person is a match in organ transplants. However, the HLA testing proved unhelpful to treating fertility.
Hurd was also suspicious about the leap from small studies asking which person you find attractive in a picture, and asking which person you would choose for a life partner.
"This study really didn't even touch that. This study didn't even talk about selection of a mate, they talk about preferences," said Hurd.
"I would venture to guess that the changes within our interpersonal relationships would influence future mating patterns -- think about how online dating has changed who we pick to date," said Hurd. "You're picking a person online."
As Penn noted, research may be done for scientific reasons, but that doesn't mean its potential use would end there.
"The article did a very good job of putting together the evidence and making the case that something might be going on that we should be concerned about," he said.
However, he continued, "The pill's effects on mate choice should not be exploited by those who want to see the pill go away for political or religious reasons. There's really a lot of benefits of the pill to be considered."
Despite those concerns he said -- that research on the pill might be used in an attempt to take it away -- he said research needed to be done to give women a better way of evaluating their decision to use contraception.
"This story could be abused," he said. "[But] certainly we need to do the science to find out exactly how the pill can affect people's behavior -- mating preferences and otherwise."
In the end, researchers recommended that contraceptive use be a personal decision. As one of the authors noted, women shouldn't take studies of contraceptives and mate preference to mean that they should drop birth control as a knee-jerk reaction.
"It's up to everyone themselves to evaluate any costs and benefits of the pill in their personal circumstances, and as we make clear, the pill has a whole range of many positive effects too that cannot be overseen," said Lummaa. "I met my current boyfriend on the pill, and I am not too worried."