As if the game of love isn’t difficult enough to understand, the rules of attraction aren’t much clearer. We think we know why we’re physically attracted to one person over another, citing preferences for blue eyes over brown, or muscular body types over lanky ones. But there are many factors that propel us toward certain people and repel us from others. Our noses, as well as our eyes and ears, pick up on much more than we realize when we’re attracted to someone else. You may think it’s a hot body or a symmetrical face that draws you in, but these underlying elements could play a role as well.

The Color Red
Colors evoke different reactions in people. Blue is associated with calm, yellow makes people happy and social, and red ignites confidence and passion. In fact, a couple of studies have shown that wearing the color red automatically makes you more attractive to the opposite sex. A 2008 study published in the Journal of Personality and Social Psychology showed that men find women who are wearing red or surrounded by a red border in photographs to be more attractive, even when compared to pictures of the same women in different colors. What’s more, the men said they’d spend more on a date with the women wearing red. Women also like a rosier hue: research published in a 2010 edition of the Journal of Experimental Psychology found that women thought men wearing red shirts or framed by the color red in a picture were more desirable and held a higher status in society.

Voice Pitch
The question of whether we’re altos or baritones is related to hormones, specifically estrogen, progesterone, and testosterone. The first two make voices higher and more feminine, while the latter produces deep, manly pitches. Because hormones have such a startling effect on whom we’re attracted to (attraction fluctuates significantly throughout a woman’s monthly cycle), it’s no surprise that hormone-level indicators like voice pitch have an effect, too.

Research has shown that women prefer men with low-pitched voices, especially as women near ovulation. One 2010 study at Aberdeen University found that, when they’re ovulating, women with the highest-pitched voices go for guys with the most masculine ones. And generally, men like ladies with high-pitched voices as well; a 2008 study published in the journal Perception showed that men rated higher-pitched voices as more attractive, and when female participants’ voices were made higher digitally, the men preferred those pitches to the women’s original voices.

These preferences have a biological basis: men and women instinctively look for the most advantageous mates, the ones most likely to produce successful offspring. And fertility is positively correlated with high amounts of sex-specific—and voice-influencing—hormones like estrogen in women and testosterone in men.

Birth Control Pills
Many animals use scent to determine the viability of a potential mate. Female mice, for instance, smell the urine of male mice to make sure that they’re genetically different—or more scientifically speaking, they unconsciously sniff out indicators of their major histocompatibility complex (MHC), a DNA segment that helps the immune system detect disease. They want to choose mates with MHC genes that are most different than their own so that any offspring will have superior immune systems. For years, scientists struggled to find a way that humans do this, too, since we obviously don’t smell each other’s pee. Then a recent study at the University of Bern in Switzerland found that we use body odor instead. Women who were asked to smell men’s sweaty T-shirts most preferred the ones belonging to men with vastly different MHC genes than their own.

However, the Pill does a number on this mate selection trait. A 2008 study at the University of Liverpool found that women on the Pill are actually more likely to go for men with similar MHC genes, possibly due to the Pill’s mimicry of beginning pregnancy stages and our inherent need to be around comforting, familiar presences during that time. That tendency could affect fertility if the genes are too close for successful compatibility, not to mention what it could do to the relationship if the woman goes off the Pill and isn’t nearly as attracted to her mate.

From the male perspective, there’s some evidence that women taking birth control aren’t as attractive as their non-hormone-regulating counterparts. A 2008 study out of the University of New Mexico analyzed the varying amounts exotic dancers made in tips during their shifts. Those off the Pill made about $70 per hour at their most fertile and made $53 on average for the entire cycle. Women on the Pill made only $37 on average, without any significant differences throughout the study.

Ovulation
Birth control alters the natural progression of a woman’s hormonal cycle, which affects how possible mates see her and how she sees them. But when the Pill isn’t at play and ovulation occurs normally, men are more attracted to women when they’re fertile. A 2006 study out of Prague’s Charles University asked men to judge the smells of cotton pads worn under women’s underarms at different cycle stages. Men responded best to the smells associated with the follicular phase, which happens right before ovulation. A 2004 study showed that men and women rated pictures of women’s faces taken during that same phase to be most attractive. Women also unconsciously change their outward appearance and mannerisms during that time—studies suggest they make their voices higher and dress more fashionably.

It’s crazy to think about how many factors must line up for one person to be attractive to another, and even more mind-boggling to think of how many are beyond our control. Here I thought a winning personality and smile were all it takes, but there’s clearly a lot more going on behind the scenes—or under the skin. I guess the nose, as well as the eyes and ears, really does know.