Attraction, like romantic love, works in mysterious ways.

While we’d like to think that we know why a particular person catches our eye, there are a number of invisible forces at work that determine which members of the opposite sex we become interested in — and which ones we don’t.

Of course, there are a number of factors that go into who we choose to be with, including personality traits, interests and values and physical appearance. But when it comes to immediate, knee-jerk physical attraction, we often can’t pinpoint why exactly we’re drawn to someone. Even as scientific research has shed more light on the factors that contribute to our selection of a sexual mate, the biology of attraction is complex and not yet fully understood — and it doesn’t help that attraction is particularly difficult to replicate in a lab.

So what really is happening when the sight of a hot guy or girl makes us instantly swoon? Human biology and evolutionary psychology has some answers.

Here are some of the subtle but powerful factors that may help determine who we’re attracted to.

We fall in love at first “smell.”

“Smell” is the woefully inadequate way we describe sensing someone’s pheromones — a type of scent-bearing chemical secreted in sweat and other bodily fluids. Pheromones are known to be involved in sexual attraction in animals, and research suggests that they may also play a role for people. A type of pheromone called a “releaser” — which includes the compounds androstenone, androstadienone and androstenol — may be involved in sexual attraction, according to a Reactions: Everyday Chemistry video.

“We’ve just started to understand that there is communication below the level of consciousness,” psychologist Bettina Pause, who studies pheromones, told Scientific American. “My guess is that a lot of our communication is influenced by chemosignals.”

In one study, female participants were tasked with the unpleasant directive to smell men’s sweaty undershirts. The researchers found that women could smell how symmetrical a man was, and using that information, judged his attractiveness. (In both men and women, symmetry is known to be an important factor in attractiveness.)

Men can detect a fertile woman.

Men can actually sense fertility on a woman, perhaps due in part to her pheromones. During the most fertile time in her menstrual cycle, a woman gives off a different scent which may make her more attractive to potential male suitors. Research from the University of Texas at Austin investigated this phenomenon by asking a group of women to wear T-shirts to sleep during both fertile and infertile points in their cycles, and then asked men to smell the T-shirts and assess which ones they found most pleasing. Overwhelmingly, they judged the shirts worn by the fertile women to be more “pleasant” and “sexy.”

A woman’s face may also appear more attractive to men during the most fertile point in her cycle. A British study conducted in 2004 asked a group of 125 men to look at two pictures of the same woman, at times of high and low fertility in her cycle, and to assess which photo was more attractive. Nearly 60 percent of the men rated the photos of the women’s faces at peak fertility (eight to 14 days after her last period) to be more attractive.

The sound of a woman’s voice also plays into a man’s judgements of a woman’s attractiveness. A recent study found that a woman’s voice sounds most seductive at the most fertile point in her menstrual cycle — and that hearing a woman’s peak-fertility voice can literally make a man’s skin tingle.

Women quickly assess markers of masculinity.

A large body of evolutionary psychology research has shown that, in general, women tend to prefer more masculine-looking men — perhaps because masculine features like broad shoulders or a strong jawline are indicators of virility and good health. But today, this doesn’t always hold true.

Women may have evolved to seek out virility, but that doesn’t mean that their preference in a modern context is always for “manly” men (and ditto for men’s attraction to “fertile looking” women). Not all — or even the majority — of women prefer more masculine men. One study found that context matters: Women living in poorer environments may have a greater preference for masculine men, but women in more developed areas prefer more feminine-looking men, according to a study from the Face Research Laboratory.

“From an evolutionary perspective, masculinity is basically man’s way of advertising good genes, dominance and likelihood to father healthier kids,” the Wall Street Journal explained. “When disease is a real threat, as it had been—and arguably still is—heritable health is invaluable.”

One time this preference may hold true is when a woman is at the most fertile point in her cycle. One study found that women whose partners had less masculine facial features reported attraction to more masculine-looking men when they were ovulating. However, women whose partners had more masculine features did not report the same eye-wandering. However, these findings only applied to women in short-term relationships — not serious, committed partnerships.

The Pill might change a woman’s preference in men.

Is she really attracted to you — or is it just her birth control? A number of studies have suggested that hormonal contraception may have some effect on women’s preferences for sexual partners.

A man’s smell provides a woman with information about his major histocompatibility complex (MHC) genes, which play an important role in immune system function. As the thinking goes, women prefer men whose MHC genes differ from their own because children with more varied MHC profiles are more likely to have healthy immune systems — which makes a whole lot of sense from an evolutionary perspective. However, research has shown that women on the pill actually display a preference for men with more similar MHC genes to their own. Scientists aren’t entirely sure why this happens, but one hypothesis is that the hormonal changes involved in pregnancy (which the Pill mimics) might draw women more to “nurturing relatives.”

Even within long-term committed relationships, changes in hormonal contraception use might affect a woman’s sexual satisfaction with her male partner. “Women who had met their partner while taking the pill and were still currently taking it — as well as those who had never used the pill at any point — reported greater sexual satisfaction than those women who had begun or stopped using the pill during the course of the relationship,” lead researcher Dr. Craig Roberts said in a statement.