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.

    要知道我们怎么来挑伴侣，科学家们测量了人脸的所有形状、每个角度，研究舞者的对称性、从花花公子的模特身上建立标准身材的模型，甚至通过让人闻腋下的汗液来判断一个人吸引力。

    在进行了所有这些、以及更多的实验之后，关于人类之间的吸引法则还是没有完全弄清楚。而这些因素是如何变为"真爱"就更是个谜题了。

    不过还是有些科学的规律被总结了出来。有些界定得很明确，比如超级模特那种突出的、女人味十足的眼睛或者体格健壮的男性的翘屁股。有另外一些规律则而是一种潜意识的影响，在意乱情迷的表象之下隐藏的进化的原因、驱使我们采取行动。

    最后，虽然行为至少与生理因素对于持久的爱情同样重要。但是最初的几步在你出生之前就已经开始了。

    对称=性感

    从受精那一刻开始，人的身体就是通过规则的细胞发展起来的。如果每次的都非常完美，那结果将是一个人身体的左边和右边就是彼此的镜像。不过自然并不是这么运作的。基因突变和环境压力破坏了这种对称性，而结果将有终生的影响。

    很好的对称性说明这个人有良好的基因来支持他的生存发展，说明健康，同时也说明是一个能生养的好配偶。

    "用对称性这个因素来选择伴侣很有道理。"新墨西哥大学的进化生物学家Randy Thornhill说，"如果你选了一个完美对称的伴侣，并跟他生了孩子，你的后代对称性也会不错，并能够更好的对付发育过程中的各种干扰。"

    Thornhill研究对称性已经超过15年了，他将面孔和身体扫描进电脑来计算对称比例。不论男性还是女性，他们认为，比起那些不那么对称的异性，那些对称的异性更吸引人、更健康。左右差异并不大，可能只有百分之几--可以分辨，但并不明显。

    通过询问实验被试，Thornhill还发现那些更对称的男人拥有更多的性伴侣。

    "女性性伴侣的数量取决于除了吸引力之外的一些东西。"Thornhill说，"因为人体的生育机制，女性更为挑剔。她们是性竞争的裁判，你必须让她们惊艳才成。"

    臀腰比例

    体型当然也很重要。科学家们已经用一些数字证实了这点。德克萨斯大学的心理学家Devendra Singh研究的就是腰-臀比例（WHR）.

    WHR为0.7的女性，也就是腰明显比臀细的女性最受男性欢迎。

    另一项对花花公子模特和美国小姐参赛者们沙漏般的身材进行的分析现实，她们中大多数人的WHR都是0.7或者更低。

    Singlh在2004年的研究中总结，一般来说，那些0.67到1.18之间的女性对男性有吸引力。而对女性来说WHR在0.8-1的男性对女性有吸引力，虽然宽阔的肩膀更让女人心动。

    在腰臀比例中蕴含着什么信息呢？他可是能告诉我们一个人有没有足够的气力去养育后代。

    脂肪在身体何处堆积取决于性荷尔蒙，男性的雄性激素和女性的雌性激素。如果女人体内的荷尔蒙的成分与含量适当，那么她的WHR很自然就会在理想的区间内。男性也是如此。

    研究表明，那些在理想腰-臀比之内的人，不论体重如何，罹患诸如心血管疾病、癌症、糖尿病等疾病的可能性较低。在这个区间内的女性受孕也较容易。

    Thornhill还指出，正在蓬勃发展的nip-'n'-tuck 服务--主要是帮助人们提高对称性，正说明了人们觉得这个特征吸引人。

    另一项研究则发现，对称的舞者看上去更有吸引力。

    看得到

    脸的结构也可以提供生育能力的一些信号。

    雌性激素会抑制女性脸部下方和脸颊的骨骼生长，使得这两个部位相对较小、较短，额头部位也是如此，让她的眼睛更深邃。而男性的面孔则受到雄性激素的作用，帮助脸部下方和下巴的骨骼发育，并长出更突出的额头。

    有这样特征的男女看起来更吸引人，Thornhill说，因为他们宣告着生殖健康。

    闻得到

    最近的研究指出，女性在一个月中的特定时间闻起来、看起来更吸引人。

    对称的男人围起来更好。

    Thornhill收集了很多男人汗淋淋的衬衫，并让女人用嗅觉感觉判断对哪个男人印象更深。和前面的一样，那些对称男性的期为闻起来更有吸引力、更诱人。尤其如果女人正处于月经期。

    你现在可能会想，我们能够意识到的有多少。吸引力的法则往往只在我们的下意识中起作用。

    在Thornhill 的研究中，女人都说并不能闻出t-shirt上有什么期为，不过他们还是说被这些吸引。

    "我们觉得意识到这些气味的区别，可能在我们的意识之外。"Thornhill说。

    一项2002年的研究发现，比起基因相差甚远或者完全相同的男性，女性对那些基因和自己相似的男性的气味更感兴趣。

    这些潜意识的的嗅觉可能与信息素有关，这是一种身体产生的化学物质，作用是沟通生育质量。人类的基因组上携带了超过1000个嗅觉基因，而人眼的视觉接收器的相关基因只有不到300个，因此不论从事基础研究的科学家还是香水的制造商，都对信息素有极大的关注。

    不过信息素在人体的作用还存在争议。

    动物间的吸引

    信息素在动物世界里是种致命吸引物。比如，年长的公象就会分泌一种混合了多种化学物质的汗液，这是年轻的公象可望不可即的。

    印第安那大学信息素研究所的Milos Novotny发现，雄性老鼠分泌的一种特殊的物质可以马上吸引雌性，同事击退、甚至激怒其他的雄性。其他的研究也发现，在整个动物界中都存在这类似的现象。

    不过，很多研究对于这种没有气味的物质在人类吸引力中的作用并不买账。其中密歇根大学的遗传生物学家Jianzhi Zhang就是持怀疑态度的人之一。

    2003年，zhang发现，在2千3百万年前生活非洲和亚洲的灵长类人类祖先们，他们身上有一种基因，这让他们可以看到颜色。这让雄性可以注意到雌性的屁股变红，这是他们准备好交配的信号。

    "随着性色彩系统的发育，他们不需要特别对信息素敏感才可以注意到雌性是非准备好交配。"Zhang说，"用视觉比信息素的优势在于，从很远的地方就可以看到。"

    不过，在2005年的一项研究发现人类信息素对于女性和同性恋男性大脑区域的作用十分相似。

    视觉吸引

    信息素就和其他味道一样，需要通过通过空气中的其他微粒进行传播，比如水蒸气。不过它们只在距地面10英寸左右的地方存在。所以但夜总会里，这些气味能够进入鼻子里、并马上点着激情的机会并不大。

    相反的，如果你见过建筑工人们在女人从老远走过来就大吹口哨的话，你就明白视觉的信息是多么强大。

    另外，当信息素和其他气味一样进入我们的鼻子，比较就停止了。信息素的最终接收器是一种叫做梨鼻骨的特殊器官，而这种器官是现代人类所缺乏的。从那里，这些性信息素会与其他气味分别随着神经传导到大脑中。

    在这个过程中，进化选择也起到了一定作用。

    在我们的祖先能看到颜色之后，一种在信息素分辨中起到重要基因收到了不良的突变，使得气味信号不能进入大脑。我们来想象一列火车，要从洛杉矶出发去纽约，但铁路在圣路易斯就被破坏了。

    虽然在古代灵长类和人类身上，这种典型的信息素途径失去了作用，制造信息素的功能还继续存在。有些科学家相信人类的信息素会通过正常的气味传导而影响我们的决定。

    长久的关系

    人与人之间的吸引规律可能最促使我们做出最初的决定，不论决定是好还是坏。但是维持一段关系就远不是光看或者闻闻就够了。

    行为起到了重要的作用，这其中生理原因发生了复杂的影响。

    有一条很古老的吸引规律是，已婚的夫妻看起来出去的相似。在2005年，西安大略州大学的心理学家Philippe Rushtopn研究了人类基因之间的关系，根据一系列有遗传性的人格特征，他发现，拥有相似的基因在朋友关系或选择配偶中起到了34%的作用。

    "主要的观点是有些基因，当结合在一起的时候将发挥更好的作用。"Rushton说，"如果这些基因在一起工作是进化的结果，那为了后代着想，你最好不要拆开他们。找到一个和你基因相似的伴侣能帮你确保这一点。"

    如果你和伴侣基因相似，你的婚姻可能会更幸福。研究发现，如果相似度高，虐待子女的比例就比较低，而且你也会更愿意帮助那些和你基因相似的人、为他们奉献自己。

    说到我们会被态度和价值观相似的人吸引，人们可能不会吃惊，而这正式爱荷华大学的心理学家Eva Klohnen2005年对新婚夫妻做的研究中所发现的。这些特点非常明显、而且也会很容易被别人发现，这在最初的吸引中发挥了作用。

    而对那些老夫老妻，研究人员发现，那些需要更长时间才能发现的人格特征上相似性，更有影响。

    喜剧也对经营一段关系有帮助。不过幽默感的重要性对男人和女人不太一样，麦克马斯特大学的Eric Bressler说。2005年，他的一项研究，发现女人会被那些能让让她笑的男人吸引。而男人则喜欢那些会被他的笑话逗笑的女人。

    真爱？

    不论是吸引还是性爱，我们都希望是源于浓浓爱意。不过到底是那个动机最终促使我们行动呢？

    有趣的是，对那些才坠入爱河的人进行的脑部扫描发现，和爱有关的活动比和性有关的更多。"浪漫的爱情是在人类所有的体验中最强烈的一种。"罗特格斯大学的人类学家Helen Fisher说，"它绝对比性冲动要强烈得多。"

    我们可以列出一长串的吸引法则。没有人知道哪个更为重要。不过名列前茅的应该是那些寻找完美伴侣时所提出的要求。

    不论区别多大，男人和女人都对一项品质很重视：忠诚。

    康奈尔大学的Stephen Emlen和同事们采访了近1000名18-24岁的年轻人，让他们对一些特征的吸引程度打分，这些特征包括了身体的吸引力，健康，社会地位，工作野心，忠诚度等。

    人们对长期伴侣的要求比暂时的伴侣要求更具体，仅次于忠诚的特征包括身体吸引、对家庭的承诺、财产和地位。

    "能当个好父母、奉献精神、性方面的忠诚，这些是人们在寻觅长期伴侣时看重的东西"