For all the mystique that surrounds love, coupling is in fact a complex mix of primal instincts, evolution, social conditioning and a cocktail of hormones.

Falling truly, madly, deeply in love has been the subject of infinite historical legends, fairy tales, literature, theatrical and film productions, and has helped to create industries worth countless billions in modern times.

But as unromantic as it sounds, ‘falling in love’ is simply sexual attraction by another name. The process of attraction is hard-wired into our genes, and is the culmination of an evolutionary process.

According to scientists, psychologists and sociological pundits, sexual attraction comes down to a primal human quest: to reproduce and nurture healthy offspring. Levels of attractiveness, and what both men and women consider attractive in each other, appear to be a function of both evolution and psychology.

Of course, every man and woman has their unique taste, but an overwhelming number of studies show that our biological need to procreate leads most of us to pick mates who have certain physical traits over those who don’t. These physical characteristics stimulate the brain’s hypothalamus, leading to elevated heart rate, perspiration and feelings of sexual arousal.

It is also a chemical state, with six or seven hormones at play. Perhaps the most obvious physical trait that arouses attraction is a youthful appearance, which is linked to reproductive capacity. In humans, mate ‘value’ therefore declines with age, beginning in early adulthood. It follows, then, that we are naturally inclined to enhance our appearance.

Throughout history, men and women have sought to add to their attractiveness and appear more youthful through dress, makeup and hairstyles. From the late 20th century, surgical and non-surgical breakthroughs have allowed people to ‘turn back the clock’; enjoy smoother, more radiant skin, tauter faces and necks, more symmetrical noses, plumper and shapelier breasts and, through liposuction and body-contouring procedures, to emulate the ‘ideal’ waist-to-hip ratio for male attention of 0.7.

But what attracts a man to a woman fundamentally differs from what women seek in a male partner. ‘In virtually all of the human populations sampled, males rated physical attractiveness (usually associated with youth) significantly more important in mate choice than females did,’ says Australian evolutionary anthropologist Sean McBride.

‘Females, by contrast, mostly rated earning potential and ambition-cum-industriousness as more important factors when choosing a mate.’ In other words, the attractiveness of the man is directly linked to his skills and prowess rather than his looks. ‘An evolutionary perspective on human behaviour suggests that human beings, like members of other animal species, are likely to have adaptations for assessing the “value” of potential sexual partners, using visual and other cues, and that our standards of physical attractiveness arise from these adaptations,’ McBride continues.

What men are attracted to in women

Regardless of culture or country of origin, men find similar female traits attractive. Their preferences are biologically and evolutionarily programmed to find signs of youth and health attractive, so as to seek females that are best suited to carry on their genes. Studies have found that female figures with slender bodies, a low waist-to-hip ratio (WHR) and large breasts are rated as the most attractive, healthy, feminine-looking and desirable for both casual and long-term romantic relationships, according to McBride.

‘Large breasts consistently enhance the attractiveness rating of both slender and heavy figures, so long as they have a low WHR,’ he says.

Polish research has shown that women with large breasts and a small waist had larger amounts of female hormones than women with a broad waist and large hips; narrow waist and small breasts; or broad waist and small breasts.

The significance of the WHR is directly linked to fertility, as a low WHR is believed to correspond to the optimal fat distribution for high fertility. If a woman is seen to be more fertile, she is also seen to be more attractive.

‘Women with a low WHR may also be healthier, given that a high WHR resulting from a bloated abdomen can be a sign of parasite infection. A high WHR in a female can also predict menstrual irregularity, hirsutism (excess hairiness), elevated plasma triglycerides, diabetes, hypertension, cardiovascular disease, gall bladder disease and cancer of the endometrium, ovaries and breast,’ McBride explains.

But large breasts and a small waist aren’t the only physical assets men are drawn to. Other characteristics said to be prized by our male ancestors in their potential mates were full red lips, clear and smooth skin, clear eyes, lustrous hair and good muscle tone; all indications of a healthy, youthful woman.

‘These in-built preferences seem to be aimed at ensuring males find suitably fertile females who are healthy enough to reproduce and, in turn, produce healthy children,’ says McBride.

‘These mechanisms are instinctual and generally subconscious. Men don’t think to themselves: “I must find a woman with a WHR of 0.7 and smooth skin”. We just find these characteristics appealing and they are almost universally sought after,’ he adds.

‘When men see beautiful women – those who have these characteristics, for instance – the same section of the male brain associated with gambling and thrill-seeking behaviour is stimulated.’

According to McBride, facial neotony, or the continuation of juvenile traits into adulthood, is also very appealing to males, because it signifies youth. A woman’s higher voice, doe-like eyes, small nose and chin and lack of body hair are all traits shared with children. Men have evolved to protect their children from harm, and therefore these somewhat ‘childlike’ features make women more attractive to men.

Breasts figure prominently in the attraction equation. Humans are the only primates to mate face-to-face. In other primate species the males approach females from the rear and use her swollen red buttocks as a signal that she’s ready for mating.

Human females are the only mammals with permanently enlarged breasts. In his ground-breaking 1967 book, The Naked Ape, British zoologist Desmond Morris proposed that the breasts we know today are an evolutionary side- effect of bipedal locomotion (the ability to walk on two feet). He asserted that their more rounded shape means they are mainly a sexual signalling device rather than simply for providing milk for infants.

According to Morris’ hypothesis, as humans began to walk upright, enlarged female breasts evolved as a mimic of the rear end – a relic from the days when humans walked on all fours and buttocks were the primary sexual signal.

Bipedal locomotion gave our ancestors an edge by freeing them up to carry items, use tools and have a higher vantage point when they walked on the ground. However, it meant that the shape of the pelvis changed, and thus the typical arrangement during intercourse.

Although research confirms that modern-day men appear to adore breasts of any shape or size, cleavage has been shown to be the most stimulating aspect of the breasts. In fact, researchers at Purdue University in the US state of Indiana found that female hitchhikers can double the number of lifts she is offered by increasing her bust size by adding 5cm of padding to enhance the cleavage.

Round, peach-shaped buttocks are another sexual turn- on for similar reasons. ‘Humans have evolved from animals that walked on all fours, when the bottom was a big turn-on to males, who would mate from behind,’ writes Morris in another book, The Naked Woman: A Study of the Female Body. Further, such curves in women have also evolved as attractive because women needed a layer of fat to keep themselves and their babies alive during times of famine.

This all points to the popularity of wearing designer jeans that highlight the buttocks and give them a firm, rounded look. Similarly, wearing high heels, which make the wearer arch her back, push out her bottom and make her wiggle when she walks, invariably draws male attention. Indeed, Marilyn Monroe reputedly chopped 2cm off the heel of her left shoe to emphasise her wiggle.

High heel shoes also accentuate long legs, or give the illusions of legs being longer than they actually are – another sexual attractant for men. When a girl reaches puberty, her legs undergo rapid lengthening as hormones flood her body and change her into a woman. Long legs become a powerful non-verbal signal telling males a woman is sexually mature and capable of childbearing.

Most men also prefer a woman with shapely, thicker legs than those with thinner ones. Additional fat in the legs highlights the sex difference between males and females and has even been said to be an indicator of better lactation.

As women age, their WHR tends to increase slowly in step with declining levels of fertility. Cartilaginous tissues continues to grow throughout life, so ears get bigger and noses longer, wider and more conspicuous. Also the vermillion – or red zone – of the lips gets thinner with the loss of connective tissue. Males subconsciously associate all these characteristics with a lack of fertility.

If men were attracted to only menopausal women, there would not be much chance of passing on their genes to future generations.

What women are attracted to in men

Women are ‘judicious, prudent, and discerning about the men they consent to mate because they have so many valuable reproductive resources to offer,’ according to University of California research scientists Elizabeth Pillsworth and Martie Haselton in their study, Ovulatory Shifts in Female Sexual Desire.

‘While males produce sperm by the thousands, females only produce about 400 eggs in their entire lifetime. In addition, the process of pregnancy and child-rearing is long and risky, so preferences are based on the potential security and longevity of the relationship.’

Historically, women have looked for men with strong square jaws and other deeply masculine features such as large muscles, all of which imply high testosterone levels. This is because the more dominant men are, the more likely they will provide resources and status for the woman and her offspring. In the modern world, men who are wealthy and/or powerful in society are attractive to women for similar reasons.

Physical strength is an indication that the male is a good hunter and provider for a family. However, material success and generosity are both seen as desirable. Women are attracted to more generous men as this is an indication of future treatment – of whether a male will share or withhold his resources from a female and their offspring.

Citing previous researchers, Pillsworth and Haselton also claim that women tend to choose sexually attractive men for short-term relationships but rich men for longer term relationships.

When a man isn’t wealthy, good looking or both, self- proclaimed American dating guru David DeAngelo, author of Double Your Dating: What Every Man Should Know About How To Be Successful With Women, asserts that to attract more women, a man has to show his value through a change in his behaviour by making them feel special. For instance, by being attentive, interested, entertaining and sexually playful and avoiding behaviours that are ‘guaranteed’ to bore females.

While women are biologically programmed to seek out so-called ‘Alpha males’, in reality they are so scarce that women will likely choose Alpha men in a more limited context. That is, a man they can call their own rather than chase a geographically or socially unattainable Brad Pitt or Barack Obama. This limited context may be at work or within someone’s own suburb or town.

A man of modest means can support a family and still has the potential to create wealth. He doesn’t have to be a billionaire to be sexually attractive to many women.

Studies have also indicated that less-attractive females seek less-masculine males who have slightly feminised faces for relationships. This may be more of a psychological function than that of physiology, indicating that if a man or woman perceive themselves as less attractive, they may be drawn to someone of the opposite sex who tends to mirror a lower self-image.

Beautiful people usually receive preferential treatment in a variety of situations, from work to romance to criminal cases, according to McBride. ‘There are evolutionary reasons as to why this is so,’ he says. ‘It isn’t fair and it isn’t particularly nice, but that is the reality.’

However, McBride cites a few positive examples. ‘The writer Henry James met the English novelist George Eliot (pseudonym of Mary Ann Evans) when she was 49. He wrote to his father: “She is magnificently ugly. She has a low forehead, a dull grey eye, a vast pendulous nose, a huge mouth, full of uneven teeth. Now in this vast ugliness resides a most powerful beauty which, in a very few minutes, steals forth and charms the mind, so that you end as I ended, in falling in love with her.’ Importantly, McBride believes that beauty alone does not infer happiness.

‘Beautiful people are not necessarily happier than the rest of us,’ he says. ‘They are usually happier in their ability to attract mates, but one only has to look to Hollywood to realise that attracting mates and living happily ever after don’t always go together.’

The scent of attraction

Scientists have also discovered that scent plays an important role in the sexual attraction of males to females. At certain points during the menstrual cycle, women produce more or less oestrogen and, accordingly, their scent becomes more or less appealing to men. Research indicates that oestrogen triggers blood flow to the hypothalamus in the male, but not the female, brain.

In fact, research into our sense of smell has determined that a simple kiss has evolved in the Western world from the universal human greeting of smelling one another’s hands or faces. Although such smells are not blatant, and may not register in the conscious mind, such smells influence mood and sexual mating preferences.

Unlike pheromones, which are long-distance chemical messengers in the Animal Kingdom, these are subtle protein secretions detected at close quarters. They enable humans to determine whether they are genetically similar or different, proving that opposites do attract.