The scientific rules for the game of love

Those hips
Body shape is of course important, too. And scientists have some numbers to prove it. Psychologist Devendra Singh of the University of Texas studied people's waist-to-hip ratio, or WHR.

Women with a WHR of 0.7 — indicating waist significantly narrower than the hips — are most desirable to men.

And an analysis of hourglass figures of Playboy models and Miss America contestants showed that most of these women boast a WHR of 0.7 or lower.

In general, a range of 0.67 to 1.18 in females is attractive to men, Singh concluded in a 2004 study, while a 0.8 to 1.0 WHR in men is attractive to women, although having broad shoulders is more of a turn-on.

What exactly is encoded in the hip ratio? A big fat clue to whether the person will have enough energy to care for offspring.

Where fat is deposited on the body is determined by sex hormones; testosterone in men, and estrogen in women. If a woman produces the proper amount and mixture of estrogen, then her WHR will naturally fall into the desired range. The same goes for a male testosterone.

People in the ideal hip-ratio range, regardless of weight, are less susceptible to disease such as cardiovascular disorders, cancer and diabetes, studies have shown. Women in this range also have less difficulty conceiving.

"The idea is that beauty is conveying information about health and fertility, and we admire that," Singh said in a telephone interview.

Face it
The structure of a person's face also gives insight to fertility.

NBC VIDEO Dissecting a dance Dec. 21: The University of Washington’s Keith Grochow explains how the movements of Jamaican dancers were digitized to study how spectators link body symmetry with artistic ability. University of Washington

Estrogen caps bone growth in a woman's lower face and chin, making them relatively small and short, as well as the brow, allowing for her eyes to appear prominent, Thornhill explained. Men's faces are shaped by testosterone, which helps develop a larger lower face and jaw and a prominent brow.

Men and women possessing these traits are seen as attractive, Thornhill said, because they advertise reproductive health.

Thornhill also points to the booming nip-'n'-tuck business — which is very much about improving a person's symmetry — as evidence that people find the quality attractive.

Another recent study revealed that symmetrical dancers are seen as more attractive. And research reported last month found women both smell and look more attractive to men at certain times of the month.

Sniff this
Symmetrical men also smell better.

Borrowing sweaty undershirts from a variety of men, Thornhill offered the shirts to the noses of women, asking for their impressions of the scents. Hands down, the women found the scent of a symmetrical man to be more attractive and desirable, especially if the woman was menstruating.

By now you might be wondering how much of this we're consciously aware of. The rules of attraction, it turns out, seem sometimes to play out in our subconscious.

In some cases, women in Thornhill's study reported not smelling anything on a shirt, yet still said they were attracted to it.

"We think the detection of these types of scent is way outside consciousness," Thornhill said.

A 2002 study found that women prefer the scent of men with genes somewhat similar to their own over the scent of nearly genetically identical or totally dissimilar men.

These subconscious scents might be related to pheromones, chemical signals produced by the body to communicate reproductive quality. The human genome contains more than 1,000 olfactory genes — compared with approximately 300 genes for photoreceptors — so pheromones have received a lot of attention from basic research scientists as well as perfume manufacturers.

But the role of pheromones in the human realm remains controversial.

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