It is the oldest of questions. "What is love! It is a pretty thing, as sweet unto a shepherd as a king," wrote Robert Greene back in the 16th century. Four hundred years on, the poser is as tricky as ever. Ahead of Valentine's Day, theories still abound as to why two people are attracted to each other. To some, it is a romance-laden "bolt from the blue", a deftly timed arrow from Cupid. To the scientists, it is a mixture of factors: everything from status to age, similarity, and pheromones.

"Explaining pheromones is like trying to find the end of a piece of spaghetti in a big plate of spaghetti bolognese," says Dr Simon Moore, a psychology lecturer at London Metropolitan University. "There are so many areas you have to consider, despite the fact that we came down from the trees to the plains thousands of years ago. Psychologists still think we have these inherited cultural factors that underpin attraction, which are different for males and females." And women, it seems, tend to be more discriminating than men because they are fertile for a shorter part of their lives.

So what is the answer? "If you ask a load of men in a room what they find attractive, it is often eye contact. It produces a physiological response in the opposite sex," says Elizabeth Clark, the author for Flirting for Dummies (to be published in June). "If a lady is staring at you for 10 seconds, it's because she wants you. Women always get themselves into trouble by doing that. They think they're being friendly, but men interpret it differently."

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While most of us think the factors that attract us to others are under our control, there are many instinctive and conscious influences that affect our behaviour. Many of the reasons for attraction cited by behavioural scientists derive their principles from evolutionary pyschology – that is, the explanation of behaviour using evolutionary theory. Such theories are based on the assumption that, as animals, we select our partners on their suitability for transferring good genes to our offspring and not just, say, as an opportunity for casual sex. But some would say this clinical explanation takes the wonder out of dating, flirting and anything it might lead to.

So what determines who we think is hot or not? And can we do anything about it? The experts explain all.

Power and status

"Women find men who are successful and dominant attractive, and this is especially true in tribal societies," says Dr Mark Sergeant, a psychology lecturer at Nottingham Trent University. He says that, in Western culture, lots of money gives women the impression that men will be able to provide for them and for any offspring they may have together.

Equally, status indicates intelligence, another desirable characteristic for children. "Women look for things like good personality, leadership, resourcefulness and intellect," Sergeant adds. A powerful and rich man is also more likely to be able to provide for their partner during pregnancy. Studies show that older women aren't as influenced by status, and this may be linked to a decline in their fertility. "This evolutionary hold diminishes as women get older. Compatibility becomes more and more important."

Age

Women are fertile over a relatively short period of their lives – from puberty until the menopause – but some men can reproduce from puberty until late age. Traditionally, men are said to prefer blondes, and this may be because blonde hair can be interpreted as a sign of youthfulness, and thus of fertility (many children have blonde hair that darkens as they age).

Men also correlate youth with health and hence with an increased possibility that their spouse will have healthy children. This may be one reason why men generally link up with women who are younger than themselves, while women often look for older men who are established in their careers and are more mature, enjoy greater status, and have a greater ability to provide.

Gay men and women

Research shows that gay men generally look for similar characteristics in their partners as straight men do: namely, younger partners and short-term sexual liaisons. "Gay men can be more promiscuous than straight men as they have more chance to do it," Sergeant says.

Furthermore, studies in gender identity suggest that the higher gay women are on a "scale of masculinity", the more their preferences tend to resemble those of heterosexual men. Similarly, the higher gay men are on a "scale of femininity", the more similar their preferences are to those of heterosexual females. "Not everyone is going to act like this, but people probably won't realise how they are being unconsciously influenced," Moore adds.

Physical attractiveness

Scientists say attractiveness indicates the quality of someone's genes. We generally find people whose faces are the most symmetrical the best-looking. "The eye prefers symmetry or figures with some regularity," wrote Charles Darwin in The Descent of Man. Sergeant says that the reason for this is that humans are born with bilaterally symmetrical faces, and due to any inbreeding or mutations or malnutrition we or our family have experienced, we become less symmetrical over our lifespan. The more we preserve this symmetry, the better we are at dealing with genetic and environmental problems. Thus, those with the most symmetrical faces are the best catches.

Men also make inferences about women based on the colour of their hair. "Blondes are perceived to be fun and bubbly and possibly lower in intelligence. Some men might find that attractive. Such people are seen as being feminine and available," Sergeant says.

In African cultures, bigger women are perceived as more attractive because they are thought to be more resilient. In Britain, the preferred female body shape has changed greatly over the past 100 years, influenced by media pressures and what is seen to be attractive – thinner bodies are famously desirable. Some studies have showed that, in the most fertile period of their menstrual cycle, women show off more flesh and thus maximise the possibility of their best features being seen.

Familiarity

"From a social point of view, people will go for the same types of partners throughout their lives. For example, one woman might go for tall, brown-eyed men throughout her life," Sergeant says. Research shows that this preference stems from our parents. "This sounds Freudian, but it operates through an imprinting-like phenomenon. Men often look for qualities in a potential mate that they might have seen in their mothers," he adds.

There is an evolutionary reason for this. When we decide to have children, we want to do so with an individual whose genes we know have been tried and tested, and are therefore successful – hence the similarity factor. There is, however, an optimum level of similarity. Too similar is unattractive, as it suggests similar genes and inbreeding. We are after new genetic material, but equally material that we have seen has worked in the past.

Pheromones

Studies show that men's smell is more appealing to women around the time they are ovulating. This facilitates contact at the key moment for conception. Some other studies suggest that women most like the smell of men whose looks are symmetrical at the most fertile period of their menstrual cycle. They can also smell the difference between symmetrical and non-symmetrical faces.

"They are looking for the best genes," Sergeant says. "A few studies have also pointed out that men find women's odour most attractive when the female is most fertile." At around this time of ovulation, a woman's body odour and vaginal secretions are also more appealing to males. "Lots of secretions like body odour are dependent on hormones moving around the body. As people's hormones change, their smell changes."

People smell differently throughout their lifespan. This is also linked to fertility. The reliance on odours is something we have from birth. "Attraction to maternal odours has obvious survival benefits by keeping the offspring in close proximity to its mother," writes the American neuroscientist Gene Wallenstein in his book The Pleasure Instinct.

Similarity

Individuals tend to go for partners who are relatively similar to themselves – in lay terms, "in the same league". "If you go for someone roughly equally to you in attractiveness, it avoids two things," Sergeant says. "If they are much better-looking than you, you are worried about them going off and having affairs. If they are much less attractive, you are worried that you could do better."

Sergeant cites a study in which people were asked to rate themselves in attractiveness and then to rate their partners. This backed up the similarity theory that people want someone who is of the same overall "value" as they feel they are themselves. In terms of men, this might be linked to ambition, dominance and control over resources. In women, this "value" is focused on fertility and looks.