Zita chews her pencil in frustration. No matter what she does, she cannot get the equation to work. She has tried subtracting x and y, and dividing by the square root of every single figure. Is there anything worse than love homework? At this rate she will never find the ideal partner!

Considering the number of couples in human history, the reason why one person chooses another has been extraordinarily hard to pin down. A popular saying is that opposites attract - but researchers in the US¹ have concluded that this is not the case.

The team studied 133 traits in almost 80,000 couples.² They also took the results of 199 previous studies which had focused on 22 traits.

A scale was devised with a score of 1 given for a trait that was always shared and 0 for one that never was. The conclusion was that for over 80% of the traits, partners were likely to be similar.

The most common similarities were in year of birth (0.87), political beliefs (0.58), religious attitudes, intelligence and level of education. Other traits looked at ranged from hair colour to wearing glasses. 

Only a few traits were likely not to be shared, such as hearing difficulties. "Our findings demonstrate that birds of a feather are more likely to flock together," says Tanya Horwitz, who led the research.

"A lot of models in geneticsA branch of science that studies genes. Genetics can include looking at how some diseases are passed down the generations, like cancer or heart problems. assume that human mating is random," adds her colleague Matt Keller. "This study shows this assumption is probably wrong."

Journalist Signi Livingstone-Peters has no doubt that love is a science. Romantic love, she writes,³ divides into lust, attraction and attachment. Each relates to hormonesChemicals created inside living creatures. They are used to send messages from one part of the body to another. released from the brain.

Lust releases testosteroneThe hormone that causes people to develop male sexual characteristics. Women also produce some testosterone. or oestrogenFemale sex hormones. After the menopause, these can increase the risk of cancer.. Attraction produces serotoninA chemical messenger believed to boost people's mood. , dopamineA hormone and neurotransmitter that plays several important roles in the brain and body, providing us with positive sensations, rewarding us for evolutionarily good behaviours like eating food. and norepinephrineA hormone that plays a role in the body's "fight or flight" response. , which give us warm feelings. For attachment, oxytocin produces positive emotions while vasopressin prepares you to protect a partner.

"As unromantic as it may sound," she concludes, "there is a formula for love."

The mathematician Bobby Seagull set out to calculate how many women there were that he might marry. Since most of his time was spent in London and Cambridge, he started by taking their female populations. Then he narrowed these down to those who were his age or up to ten years younger and had been to university.

Reckoning that he found one in 20 attractive, he was left with 29,639 possible girlfriends. But when he factored in those who were single and likely to find him attractive, the number fell to 73.

An equation which is sometimes found as a tattoo is (i∂+m) Φ = 0. Devised by the British physicist Paul Dirac to describe the electronA negatively charged subatomic particle. , it combines the principles of relativityA theory stating that space and time are relative to each other. and quantum mechanicsA fundamental theory in physics describing the properties of nature on an atomic scale. Unlike general relativity, which works when applied to large objects, quantum mechanics helps describe the world on a tiny level - at a scale where things can be in several places at once, and measurement can affect reality..

But it is also known as "the formula for love." Writing in El Pais, Montero Glez explains why:  "The equation implies that if two particles are connected for a time and then separate, what happens to one will continue to affect the other despite the separation."

Yes: As Signi Livingstone-Peters says, it all comes down to a biological process - "Beneath the flushed cheeks, there is a series of complex chemical reactions taking place between the brain and the body."

No: Some things cannot be explained by science. Even if you found two people with exactly the same traits, there is no guarantee that they would get on with each other, let alone fall in love.

Or... It is just as well that there is no equation. The search for a perfect match is one of the most exciting things in life, and the more random it is the better: even dating apps detract from it.