OK, so maybe not fully developed love, but a quick passionate response is possible. Your brain can release love-related chemicals in a split second after you spy someone you love. A 2010 study also showed that new love really is like a drug. It sparks the same euphoric feeling as cocaine -- in the same part of the brain.

Calvin Klein named a perfume for the early stage of love: Obsession. Italian scientists found a biological link between people with OCD -- a treatable type of obsessive thinking -- and those in love for less than six months. Both showed lower than normal levels of serotonin in their blood.

When you're love-struck, your brain is flooded with dopamine and norepinephrine, triggering euphoria. That’s why new lovers do things like talk all night, dote on little details about each other, or fly across the country for a weekend together.  Falling in love also triggers adrenaline, which may make your heart race and your knees shake when you’re near your lover.

You would probably never think of smelling a guy's sweaty T-shirt to see if he's the one! But researchers gave women this smell test -- and some were turned on by the sweat of men whose immune systems were different from their own.

If you're looking for a mate this makes sense: Children who get several different disease-fighting genes may be less likely to get sick. But it may also affect romance: Women whose immune genes are similar to their partners' may be more likely to be unfaithful.

Confirmed bachelors beware: In some studies, men preferred the odor of T-shirts worn by women while they were ovulating. And a study done in men's clubs showed that men tipped lap dancers who were ovulating almost twice what they tipped dancers who were menstruating.

No wonder George Clooney is such a heartthrob! There may be a biological reason why ovulating women prefer men with strong jaws and cheekbones: They are signs of testosterone, which signals good health. And a sense of humor may be a sign of intelligence, another plus when procreating. Research also shows that women prefer more feminine male faces at other times of the month. A softer face may suggest commitment and good parenting skills.

Testosterone -- in men and women -- is the only substance known to arouse sexual desire. Oysters have high levels of dopamine, but there's no proof that eating them boosts your libido. And chocolate has high levels of phenylethylamine (PEA) -- the “love molecule.” But there’s no evidence that eating chocolate releases the hormone the way a simple act like hands touching or eyes meeting can release it.

The same brain areas light up in people who are madly in love and those who use cocaine or opioids. Lovers also show the classic symptoms of addiction. As a romance builds, a lover craves more and more of the "drug" -- the other person. A breakup can cause withdrawal symptoms like poor sleep and depression. And scorned lovers can relapse: Hearing a favorite song can trigger the craving all over again.

At orgasm, the hormones vasopressin in men and oxytocin in women rise. These "cuddle hormones" can flood you with loving feelings for your partner. Scientists believe these chemicals help cement emotional bonds between lovers. The chemicals of romance may wane at the same time, so ecstasy gets replaced with devotion.

Women with a trim waist and womanly hips may get more manly attention than other women, at least if you're a line drawing!

Men shown drawings of women preferred those with a waist-to-hip ratio of 70% -- 25 1/2 inches by 36 inches, for example. And hundreds of art works from different cultures -- some dating back 32,000 years -- show this is not new. It may be a sign that a woman is a good mate.

Should you worry about your own shape? Probably not. These studies only showed a front view, so who knows how a curvaceous behind might change the laws of attraction?

In a study, men and women listened to recordings of people counting to 10, and then ranked their attractiveness. Voices rated most attractive came from men with broad shoulders and narrow hips. For women, the most fetching voices came from those with narrow waists and broad hips.

In two small studies, Swedish researchers asked people to sniff chemicals made from male and female hormones. The male hormone testosterone turned on a sexual part of the brain in gay men and straight women -- but not in straight men. Brain responses were more complicated when people sniffed a female hormone. But more lesbians and straight men said they liked the scent of the female hormone, than did straight women. 

Why do people with well-balanced facial features get more dates? It's a sign of greater ability to fight off disease. And research shows that to the symmetrical go the sexual spoils. Men with even features have sex earlier and with more partners, including after marriage. And women have more orgasms with them than with lopsided men.

Moths do it. Mice do it. And people may "give off" pheromones, too -- scents that turn on other people. But there isn't solid proof that human pheromones exist or create a sexual response in people, as they do in animals. Scientists are still researching it. Still, perfume companies sell pheromone-based sprays that claim to boost sexual attraction.

American men and women participating in a study ranked the qualities they want in a mate -- such as wealth, sexual loyalty, family devotion, and looks. Then they ranked those qualities in themselves. It turns out that most people were looking for someone like them. Other research shows that people want partners who share their sense of humor, values, and beliefs.