We all think we know instinctively what we find attractive in other people. Off the top of our heads, we will probably mention attributes such as facial appearance, physical build, mannerisms and behaviours. But how do we define physical appeal and attraction? What, precisely, makes an attractive woman or man?

The idea of applying scientific analytical methods to such questions may seem about as appropriate as analysing Shakespeare's love sonnets through the mathematics of rhythm and the structure of language and vocabulary to better understand their seductive effects. Nevertheless, breaking down the aspects of attractiveness into their component parts and then subjecting them to rigorous scientific testing has provided answers to many of the basic questions about the judgements we make in the first few moments of meeting a potential partner.

The psychological mechanisms underlying these judgements of attractiveness in humans have evolved with the primary purpose of finding a high quality mate. Animals display traits and receive multiple signals related to some basic physical quality or attribute and science shows we are not very far removed from animals in these respects. It might be interesting to take a moment and consider the things you would look for if you were, for example, about to embark on a speed dating mission to find the partner of your dreams. In a BBC internet survey of the top three most desirable traits in a potential partner, after breaking down the results according to gender, men ranked good looks and facial attractiveness higher than women, while women preferred honesty, humour, kindness and dependability in their men. However, the latest research suggests men and women are completely unaware at a conscious level of what truly attracts them to another person. When a group of young people were asked about their preferences before a speed dating session, the usual gender difference was found, in that men said they would prefer good looking women while women would seek men with good earning potential and nurturing capabilities. However, as these people made their choices during the speed dating session, it became apparent that the gender divide had disappeared, and there was no difference in the number of men and women who were attracted to a partner through looks. Women were also much more choosey about the type of man they were attracted to, while men were far less discriminating. This is in line with the Darwinian theory of mate selection, with choosey females and competitive males. It seems that people are closer to animals than they might care to admit, and they are also intrinsically unaware of what they actually find attractive in a partner.

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WHAT MEN FIND ATTRACTIVE ABOUT WOMEN

A pretty face

What makes a pretty face? Studies have found that average, symmetrical faces are attractive and it is thought they honestly signal good traits such as healthiness, including how well a person has adapted to the stresses of genetic and environmental development. Hence facial symmetry suggests "good genes". In a recent survey, women with symmetrical faces were considered to have more feminine facial proportions and such feminine features are considered to be more attractive.

This attractiveness is not just about looks though, because a study has found that feminine features are empirically linked to higher levels of oestrogen in women.

However, familiar or typical faces are also viewed as more attractive, while more unusual and distinctive faces are rated less attractive. Furthermore, seeing faces more often increases their attractiveness rating. Hence facial appearance is a cue to hormone levels in women but presumably only at the first meeting and providing the woman is not too distant, ethnically or genetically.

While facial symmetry is regarded as an attractive quality, most people don't actually realise they are looking for symmetry. Once again, unconscious mechanisms come into play in determining face preferences. This may help explain why the reasons behind attraction are often so difficult to describe.

The question of make-up

The link between attractiveness and hormone levels is lost when women wear make-up. However, both men and women judge full facial make-up to be more attractive than wearing no facial makeup. Men prefer women with full eye make-up and foundation, but lipstick is not necessarily considered an enhancement to beauty. Men find a greater contrast between the darkness of the eyes and lips and the lightness of the surrounding skin to be most beautiful.

Voice pitch

Men prefer women with higher pitched voices, even when artificially manipulated. A more recent report, however, indicates that men perceive raised pitch more attractive only if the women are demonstrating an interest in them. Breathiness when speaking is also considered to be a feminine characteristic women might therefore wish to consider cultivating a "Nicole Kidman" approach to speech.

Body shape

The question of body shape is a contentious issue. Studies have broken down the analysis of what makes a perfectly shaped body into body mass index (BMI), waist:hip ratio (WHR) (the circumference of the waist compared to the circumference of the hip), waist:bust ratio (WBR) and body weight. Curvaceousness, the hourglass shape, has also been found to be a factor.

There are many aspects involved in the perception and judgement of body shape that clearly cannot be measured in isolation. Motion and three-dimensional presentation affect the attractiveness of shape and weight, and provoke basic social perceptions of biological gender and health, and of fitness for particular environments. Hence measures of a woman's attractiveness vary depending on whether the she is in motion or is posing. Using the frequencies with which female celebrities star in film or in magazines as a measure for attractiveness, women with low BMI are more likely to be seen in film (and, therefore, in motion) but women with low WHR or WBR are more likely to be seen posing in magazines. A study examined the changes in idealised female body images in the media (such as Playboy magazine Playmates of the Year, Miss America Pageant winners, and fashion models) and young women in general over a period of eight decades. In the early and latter parts of the 20th century the ideal was for less curvaceous women, while during the middle decades the ideal was for very curvaceous women. Over the period studied, models tended to have smaller bosoms and hips, but Playmates had larger bosoms and smaller waists, indicating a difference between the media's ideal woman and that of men in the real world.

Bosom

Bigger bosoms are more attractive to men. We are probably all aware of that, but just to prove it, in one study a female member of a research team, wearing a bra that permitted her to vary her bust size, sat in a nightclub and on the pavement area of a bar for an hour at each location. She was approached by men more often while exhibiting the bigger bust. A similarly equipped female researcher was offered more hitch-hiking lifts from thumbing when she had a larger bosom, which may reinforce the idea that men do not offer lifts just to be kind and sociable.

WHAT WOMEN FIND ATTRACTIVE ABOUT MEN

As most men would agree, and most women would deny, hormones play a major role in female attraction to males. However, the degree to which hormones (and pheromones) play a role in attraction is much greater than women would perhaps care to discover.

Nesting instincts and hormones

Facial attractiveness in men signals better genetic stock, greater genetic variability and higher testosterone levels. However, men who are genetically good stock make poorer partners and parents than men of genetically lower quality. Very masculine facial characteristics larger jawbones and more prominent cheekbones suggest to women negative attributes relevant to relationships and paternal investment. The more masculine a face, the more the perceived dominance and negative behaviour aspects (such as coldness or dishonesty). Therefore women often prefer men with slightly more feminine faces. Men who possess the childlike features of large eyes, the mature features of prominent cheekbones and a large chin, the expressive feature of a big smile, and high-status clothing are seen as the most highly attractive.

So women in stable relationships are therefore with men of poorer genetic stock. Hence a woman might theoretically invest in a stable relationship but obtain high quality genetic stock by straying outside the partnership, most logically at her most fertile period. Most women (and men) would be shocked at such an inference, but the science points in that direction. In a study of partnered women, most found single men were most attractive only when they were briefly in their fertile phase; otherwise they were not attracted to them.

Women's preference for men who display more masculine traits varies with the menstrual cycle. Women prefer the odour of dominant men, and prefer men who act in a dominant fashion and who have more masculine faces, at the peak fertility time of their menstrual cycle, particularly at the follicular phase. It may be shocking, but women fancy men with the most masculine traits when they are most likely to conceive. One study found that this is linked to oestradiol levels, which track with a woman's preference for testosterone levels in men over the menstrual cycle. A group of genes called the major histocompatibility complex (MHC) influence individual odours in men. If women prefer the odour of MHC-dissimilar males it is perhaps to increase genetic diversity in their offspring or to reduce inbreeding. Women prefer the odour of men who are more dissimilar in MHC when they are approaching the most fertile part of their menstrual cycle. Women on the pill, however, do not demonstrate this preference, suggesting that the contraceptive pill might affect the choices made by women and hence fertility of the human species as a whole except of course that women come off the pill in order to conceive.

All this suggests that women may be driven to seek less attractive partners in order to provide a stable and nurturing environment for their children, but may secretly improve the genetic quality of their offspring through extra-pair matings with the most masculine and attractive men while at their most fertile. Either way, as with partner selection, and with how we define facial beauty, it appears that powerful forces are at work behind the scenes in our reproductive strategies, of which we are oblivious.

VIVE LA DIFFERENCE

So what can we do to make ourselves more attractive to the opposite sex? The simplest answer for women is to reinforce the gender difference. Everything men are attracted to in women points towards an emphasis on femininity. Therefore a woman should dress herself as femininely as possible, emphasise her feminine features and wear make up. In this day and age, that sounds almost sexist! Does it just come down to clothes? Science also tells us that smiling and eye contact make people significantly more attractive, to both sexes. As for men, if looking for a stable relationship, the opposite advice applies: reduce the gender difference both in looks and behaviour, and emphasise the loving and caring partner aspects. Both sexes should try to remember, though: it's the men who compete and the women who choose. If it's something else, you're doing it wrong.