To figure out how we pick mates, scientists have measured every shape and angle of the human face, studied the symmetry of dancers, crafted formulas from the measurements of Playboy models, and had both men and women rank attractiveness based on smelling armpit sweat.

    After all this and more, the rules of attraction for the human species are still not clearly understood. How it all factors into true love is even more mysterious.

    But a short list of scientific rules for the game of love is emerging. Some are as clearly defined as the prominent, feminine eyes of a supermodel or the desirable hips of a well-built man. Other rules work at the subconscious level, motivating us to action for evolutionary reasons that are tucked inside clouds of infatuation.

    In the end, lasting love depends at least as much on behavior as biology. But the first moves are made before you're even born.

    Symmetry equals sex

    Starting at conception, the human body develops by neatly splitting cells. If every division were to go perfectly, the result would be a baby whose left and right sides are mirror images. But nature doesn't work that way. Genetic mutations and environmental pressures skew symmetry, and the results have lifelong implications.

    Good symmetry shows that an individual has the genetic goods to survive development, is healthy, and is a good and fertile choice for mating.

    "It makes sense to use symmetry variation in mate choice," said evolutionary biologist Randy Thornhill of the University of New Mexico. "If you choose a perfectly symmetrical partner and reproduce with them, your offspring will have a better chance of being symmetric and able to deal with perturbations."

    Thornhill has been studying symmetry for 15 years and scanned faces and bodies into computers to determine symmetry ratios. Both men and women rated symmetrical members of the opposite sex as more attractive and in better health than their less symmetrical counterparts. The differences can be just a few percent-perceivable though not necessarily noticeable.

    By questioning the study participants, Thornhill also found that men with higher degrees of symmetry enjoy more sexual partners than men of lower symmetry.

    "Women's sex-partner numbers are dependent on things other than attractiveness," Thornhill told LiveScience. "Because of the way that the sexual system in humans works, women are choosey. They are being sexually competed for. They have to be wooed and all that."

    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 （WHR）.

    Women with a WHR of 0.7-indicating a 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 the majority 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's 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.

    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.

    Face it

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

    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.

    Sniff this

    Research reported last month found women both smell and look more attractive to men at certain times of the month.

    And symmetrical men 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 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 to approximately 300 genes for photoreceptors in the eyes-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.

    Animal attraction

    Pheromones clearly act as sexual attractants in the animal world. Older male elephants, for example, exude sexual prowess with a mix of chemicals the younger bulls can't muster.

    Milos Novotny of the Institute of Pheromone Research at Indiana University has shown that special molecules produced by male mice can simultaneously attract females and repel, and even anger, rival males. Other studies have found similar responses throughout the animal kingdom.

    Yet many researchers are not sold on the idea that these odorless compounds play a role in human attraction. Count evolutionary biologist Jianzhi Zhang of the University of Michigan among the skeptical.

    In 2003, Zhang showed that a gene mutated 23 million years ago among primates in Africa and Asia that are considered to be human ancestors, allowing them to see color. This let the males notice that a female's bottom turned bright red when she was ready to mate.

    "With the development of a sexual color scheme, you don't need the pheromone sensitivity to sense whether a female monkey is ready to mate," Zhang said. "It's advantageous to use visual cues rather than pheromones because they can be seen from a distance."

    A study last year, however, suggested that human pheromones affect the sexual area of the brains of women and gay men in a similar manner.

    Sex goes visual

    Pheromones, like other scents, hitch a ride through the air on other particles, such as water droplets. They generally hover just 10 inches off the ground, however. So odds are slim they'll waft up to a human nose and fuel sudden passion at a nightclub.

    Watch any construction worker whistling at a passing woman from half a block away, and you can see how visual cues can be more powerful.

    And while they enter the nose like other scents, that's where the comparison stops. A pheromone's destination is a special organ called the volmeronasal organ, which humans now lack. From here the sexy scent travels along a neural pathway to the brain separate from other scents.

    Evolution played a role in this, too.

    After our ancestors began to see color, a gene important in the pheromone-signaling pathway suffered a deleterious mutation, making it impossible for the scent signals to reach the brain, Zhang said. Imagine a train, leaving from Los Angeles to New York, discovers that the tracks in St. Louis are destroyed.

    Although the classical pheromone pathway in both Old World primates and humans is dysfunctional, the mechanism for producing pheromones still works. Some scientists believe human pheromones might be influencing our decisions along the normal olfactory pathway.

    Lasting relationships

    The rules of attraction might drive our initial decisions, for better or worse. But lasting relationships are about much more than what we see and smell.

    Behavior plays a key role, with biology an intriguing contributing factor.

    One of the oldest theories about attraction is that like begets like. It explains that eerie perception that married couples sometimes look awfully similar.

    Last year, J. Philippe Rushton, a psychologist at the University of Western Ontario, looked into the relationships of people's genes. Based on a set of heritable personality traits, having similar genetics plays 34 percent of the role in friendship and mate selection, he found.

    "The main theory is that some genes work well in combination with each other," Rushton told LiveScience. "If these genes evolved to work in combination, then you don't want to break that up too much for your offspring. Finding a mate with similar genes will help you ensure this."

    If your spouse is genetically similar, you're more likely to have a happy marriage, for example. Child abuse rates are lower when similarity is high, and you'll also be more altruistic and willing to sacrifice more for someone who is more genetically like you, research shows.

    It probably comes as little surprise people are drawn to individuals with similar attitudes and values, as psychologist Eva Klohnen at the University of Iowa found in a 2005 study of newlywed couples. These characteristics are highly visible and accessible to others and can play a role in initial attraction.

    When it comes to sticking together for the long haul, researchers have shown that likeness of personality, which can take more time to realize, means more.

    Comedy can also help a relationship. But the importance of humor is different for men and women, says Eric Bressler of McMaster University.

    A woman is attracted to a man who makes her laugh, Bressler found in a 2005 study. A man likes a woman who laughs at his jokes.

    True love

    Somewhere amid attraction and sex, we all hope, are strong feelings of love. But which of all the motivations really drives us?

    Interestingly, brain scans in people who'd recently fallen in love reveal more activity related to love than sex. "Romantic love is one of the most powerful of all human experiences," says Helen Fisher, an anthropologist at Rutgers University. "It is definitely more powerful than the sex drive."

    The rules of attraction make up a pretty long list. No scientist knows the order of the list. But near the top is perhaps one of the toughest characteristics to gauge in advance in the search for the perfect partner.

    Despite all their differences, men and women place high value on one trait: fidelity.

    Cornell University's Stephen Emlen and colleagues asked nearly 1,000 people age 18 to 24 to rank several attributes, including physical attractiveness, health, social status, ambition, and faithfulness, on a desirability scale.

    People who rated themselves favorably as long-term partners were more particular about the attributes of potential mates. After fidelity, the most important attributes were physical appearance, family commitment, and wealth and status.

    "Good parenting, devotion, and sexual fidelity-that's what people say they're looking for in a long-term relationship," Emlen says.

    要知道我們怎么來挑伴侶，科學家們測量了人臉的所有形狀、每個角度，研究舞者的對稱性、從花花公子的模特身上建立標準身材的模型，甚至通過讓人聞腋下的汗液來判斷一個人吸引力。

    在進行了所有這些、以及更多的實驗之后，關于人類之間的吸引法則還是沒有完全弄清楚。而這些因素是如何變為"真愛"就更是個謎題了。

    不過還是有些科學的規律被總結了出來。有些界定得很明確，比如超級模特那種突出的、女人味十足的眼睛或者體格健壯的男性的翹屁股。有另外一些規律則而是一種潛意識的影響，在意亂情迷的表象之下隱藏的進化的原因、驅使我們采取行動。

    最后，雖然行為至少與生理因素對于持久的愛情同樣重要。但是最初的幾步在你出生之前就已經開始了。

    對稱=性感

    從受精那一刻開始，人的身體就是通過規則的細胞發展起來的。如果每次的都非常完美，那結果將是一個人身體的左邊和右邊就是彼此的鏡像。不過自然并不是這么運作的。基因突變和環境壓力破壞了這種對稱性，而結果將有終生的影響。

    很好的對稱性說明這個人有良好的基因來支持他的生存發展，說明健康，同時也說明是一個能生養的好配偶。

    "用對稱性這個因素來選擇伴侶很有道理。"新墨西哥大學的進化生物學家Randy Thornhill說，"如果你選了一個完美對稱的伴侶，并跟他生了孩子，你的后代對稱性也會不錯，并能夠更好的對付發育過程中的各種干擾。"

    Thornhill研究對稱性已經超過15年了，他將面孔和身體掃描進電腦來計算對稱比例。不論男性還是女性，他們認為，比起那些不那么對稱的異性，那些對稱的異性更吸引人、更健康。左右差異并不大，可能只有百分之幾--可以分辨，但并不明顯。

    通過詢問實驗被試，Thornhill還發現那些更對稱的男人擁有更多的性伴侶。

    "女性性伴侶的數量取決于除了吸引力之外的一些東西。"Thornhill說，"因為人體的生育機制，女性更為挑剔。她們是性競爭的裁判，你必須讓她們驚艷才成。"

    臀腰比例

    體型當然也很重要。科學家們已經用一些數字證實了這點。德克薩斯大學的心理學家Devendra Singh研究的就是腰-臀比例（WHR）.

    WHR為0.7的女性，也就是腰明顯比臀細的女性最受男性歡迎。

    另一項對花花公子模特和美國小姐參賽者們沙漏般的身材進行的分析現實，她們中大多數人的WHR都是0.7或者更低。

    Singlh在2004年的研究中總結，一般來說，那些0.67到1.18之間的女性對男性有吸引力。而對女性來說WHR在0.8-1的男性對女性有吸引力，雖然寬闊的肩膀更讓女人心動。

    在腰臀比例中蘊含著什么信息呢？他可是能告訴我們一個人有沒有足夠的氣力去養育后代。

    脂肪在身體何處堆積取決于性荷爾蒙，男性的雄性激素和女性的雌性激素。如果女人體內的荷爾蒙的成分與含量適當，那么她的WHR很自然就會在理想的區間內。男性也是如此。

    研究表明，那些在理想腰-臀比之內的人，不論體重如何，罹患諸如心血管疾病、癌癥、糖尿病等疾病的可能性較低。在這個區間內的女性受孕也較容易。

    Thornhill還指出，正在蓬勃發展的nip-'n'-tuck 服務--主要是幫助人們提高對稱性，正說明了人們覺得這個特征吸引人。

    另一項研究則發現，對稱的舞者看上去更有吸引力。

    看得到

    臉的結構也可以提供生育能力的一些信號。

    雌性激素會抑制女性臉部下方和臉頰的骨骼生長，使得這兩個部位相對較小、較短，額頭部位也是如此，讓她的眼睛更深邃。而男性的面孔則受到雄性激素的作用，幫助臉部下方和下巴的骨骼發育，并長出更突出的額頭。

    有這樣特征的男女看起來更吸引人，Thornhill說，因為他們宣告著生殖健康。

    聞得到

    最近的研究指出，女性在一個月中的特定時間聞起來、看起來更吸引人。

    對稱的男人圍起來更好。

    Thornhill收集了很多男人汗淋淋的襯衫，并讓女人用嗅覺感覺判斷對哪個男人印象更深。和前面的一樣，那些對稱男性的期為聞起來更有吸引力、更誘人。尤其如果女人正處于月經期。

    你現在可能會想，我們能夠意識到的有多少。吸引力的法則往往只在我們的下意識中起作用。

    在Thornhill 的研究中，女人都說并不能聞出t-shirt上有什么期為，不過他們還是說被這些吸引。

    "我們覺得意識到這些氣味的區別，可能在我們的意識之外。"Thornhill說。

    一項2002年的研究發現，比起基因相差甚遠或者完全相同的男性，女性對那些基因和自己相似的男性的氣味更感興趣。

    這些潛意識的的嗅覺可能與信息素有關，這是一種身體產生的化學物質，作用是溝通生育質量。人類的基因組上攜帶了超過1000個嗅覺基因，而人眼的視覺接收器的相關基因只有不到300個，因此不論從事基礎研究的科學家還是香水的制造商，都對信息素有極大的關注。

    不過信息素在人體的作用還存在爭議。

    動物間的吸引

    信息素在動物世界里是種致命吸引物。比如，年長的公象就會分泌一種混合了多種化學物質的汗液，這是年輕的公象可望不可即的。

    印第安那大學信息素研究所的Milos Novotny發現，雄性老鼠分泌的一種特殊的物質可以馬上吸引雌性，同事擊退、甚至激怒其他的雄性。其他的研究也發現，在整個動物界中都存在這類似的現象。

    不過，很多研究對于這種沒有氣味的物質在人類吸引力中的作用并不買賬。其中密歇根大學的遺傳生物學家Jianzhi Zhang就是持懷疑態度的人之一。

    2003年，zhang發現，在2千3百萬年前生活非洲和亞洲的靈長類人類祖先們，他們身上有一種基因，這讓他們可以看到顏色。這讓雄性可以注意到雌性的屁股變紅，這是他們準備好交配的信號。

    "隨著性色彩系統的發育，他們不需要特別對信息素敏感才可以注意到雌性是非準備好交配。"Zhang說，"用視覺比信息素的優勢在于，從很遠的地方就可以看到。"

    不過，在2005年的一項研究發現人類信息素對于女性和同性戀男性大腦區域的作用十分相似。

    視覺吸引

    信息素就和其他味道一樣，需要通過通過空氣中的其他微粒進行傳播，比如水蒸氣。不過它們只在距地面10英寸左右的地方存在。所以但夜總會里，這些氣味能夠進入鼻子里、并馬上點著激情的機會并不大。

    相反的，如果你見過建筑工人們在女人從老遠走過來就大吹口哨的話，你就明白視覺的信息是多么強大。

    另外，當信息素和其他氣味一樣進入我們的鼻子，比較就停止了。信息素的最終接收器是一種叫做梨鼻骨的特殊器官，而這種器官是現代人類所缺乏的。從那里，這些性信息素會與其他氣味分別隨著神經傳導到大腦中。

    在這個過程中，進化選擇也起到了一定作用。

    在我們的祖先能看到顏色之后，一種在信息素分辨中起到重要基因收到了不良的突變，使得氣味信號不能進入大腦。我們來想象一列火車，要從洛杉磯出發去紐約，但鐵路在圣路易斯就被破壞了。

    雖然在古代靈長類和人類身上，這種典型的信息素途徑失去了作用，制造信息素的功能還繼續存在。有些科學家相信人類的信息素會通過正常的氣味傳導而影響我們的決定。

    長久的關系

    人與人之間的吸引規律可能最促使我們做出最初的決定，不論決定是好還是壞。但是維持一段關系就遠不是光看或者聞聞就夠了。

    行為起到了重要的作用，這其中生理原因發生了復雜的影響。

    有一條很古老的吸引規律是，已婚的夫妻看起來出去的相似。在2005年，西安大略州大學的心理學家Philippe Rushtopn研究了人類基因之間的關系，根據一系列有遺傳性的人格特征，他發現，擁有相似的基因在朋友關系或選擇配偶中起到了34%的作用。

    "主要的觀點是有些基因，當結合在一起的時候將發揮更好的作用。"Rushton說，"如果這些基因在一起工作是進化的結果，那為了后代著想，你最好不要拆開他們。找到一個和你基因相似的伴侶能幫你確保這一點。"

    如果你和伴侶基因相似，你的婚姻可能會更幸福。研究發現，如果相似度高，虐待子女的比例就比較低，而且你也會更愿意幫助那些和你基因相似的人、為他們奉獻自己。

    說到我們會被態度和價值觀相似的人吸引，人們可能不會吃驚，而這正式愛荷華大學的心理學家Eva Klohnen2005年對新婚夫妻做的研究中所發現的。這些特點非常明顯、而且也會很容易被別人發現，這在最初的吸引中發揮了作用。

    而對那些老夫老妻，研究人員發現，那些需要更長時間才能發現的人格特征上相似性，更有影響。

    喜劇也對經營一段關系有幫助。不過幽默感的重要性對男人和女人不太一樣，麥克馬斯特大學的Eric Bressler說。2005年，他的一項研究，發現女人會被那些能讓讓她笑的男人吸引。而男人則喜歡那些會被他的笑話逗笑的女人。

    真愛？

    不論是吸引還是性愛，我們都希望是源于濃濃愛意。不過到底是那個動機最終促使我們行動呢？

    有趣的是，對那些才墜入愛河的人進行的腦部掃描發現，和愛有關的活動比和性有關的更多。"浪漫的愛情是在人類所有的體驗中最強烈的一種。"羅特格斯大學的人類學家Helen Fisher說，"它絕對比性沖動要強烈得多。"

    我們可以列出一長串的吸引法則。沒有人知道哪個更為重要。不過名列前茅的應該是那些尋找完美伴侶時所提出的要求。

    不論區別多大，男人和女人都對一項品質很重視：忠誠。

    康奈爾大學的Stephen Emlen和同事們采訪了近1000名18-24歲的年輕人，讓他們對一些特征的吸引程度打分，這些特征包括了身體的吸引力，健康，社會地位，工作野心，忠誠度等。

    人們對長期伴侶的要求比暫時的伴侶要求更具體，僅次于忠誠的特征包括身體吸引、對家庭的承諾、財產和地位。

    "能當個好父母、奉獻精神、性方面的忠誠，這些是人們在尋覓長期伴侶時看重的東西"