OK, so maybe not fully developed love, but a passionate love response is possible. The brain can release love-related chemicals and hormones within one-fifth of a second of first sight, researchers say. A 2010 study also showed that new love really is like a drug -- it sparks the same euphoric feeling (and brain area) as cocaine.

Calvin Klein named a perfume for the early stage of love: Obsession. In a small study, Italian researchers found that people who had been in love for less than six months share more than just obsessive thinking with people who have OCD. Both also had 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.

Women asked by researchers to smell men's sweaty T-shirts were turned on by those of men whose immune system genes were different from theirs. From a mating perspective, this makes sense: Children who get different immune genes from each parent can better fight off disease. But it may also affect romance: Women whose immune genes are similar to their partners' are 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.

Brain scans show that the same regions of the brain are turned on in people who are madly in love as those who have injected 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 sleeplessness 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. This can cause feelings of closeness and attachment, which is why they're known as "cuddle hormones." Scientists believe they also help cement bonds between lovers and parents and children. The chemicals of romance may wane at the same time, so ecstasy gets replaced with devotion.

In experiments done around the world, men who were shown line drawings of women preferred a waist-to-hip ratio of 70% -- 25 1/2 inches to 36 inches, for example. A study of women portrayed in 330 works of art from different cultures -- some dating back 32,000 years -- shows that this is not new. It may be that a 70% ratio is a cue to a woman's value as a mate. However, another study disputes those findings because they are based on frontal views of women that don’t include the buttocks.

In a study, men and women ranked the attractiveness of people’s voices based only on a recording of them counting to 10. It turned out that the voices rated most attractive came from men with broad shoulders and narrow hips and women with narrow waists and broad hips.

In two small studies, Swedish researchers asked people to sniff chemicals made from male and female hormones that some scientists believe trigger a sexual response. When sniffing a chemical from the male hormone testosterone, the sexual part of the brain turned on in gay men and straight women. It did not turn on in straight men. Lesbians and straight men processed the female scent in different parts of the brain but, unlike straight women, they liked the female hormone more than the male one.

Why do people get more romantic interest if their features are even and well balanced? 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 that turn on other people too. 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.