Some say they are attracted to a person's looks, others say it is more about personality - but everyone is subconsciously swayed by smell, according to new research.
Humans can smell whether a potential partner has compatible genes, an expert had concluded.
In his new book, The Compatibility Gene, Professor Daniel Davis explains that there are human genes that may influence how attractive a person is to a potential partner.
The University of Manchester professor believes that suitable mates can smell that they have found a partner with an optimal genetic code.
The idea is based on the so-called smelly T-shirt experiment in which Swiss scientists Claus Wedekind studied the DNA of a group of students to look at their histocompatibility genes, The Guardian reports.
The researchers asked the male students to wear a T-shirt for two nights while avoiding anything that might alter their natural smell.
At the end of the two days, the shirts were put in boxes and the female students were asked to smell them and then rank the smell by intensity, pleasantness and sexiness.
The results showed that the women tended to like the smell of men who have different compatibility genes.
This suggested that people are able to subconsciously choose a partner with whom they could have children with a genetic advantage.
Professor Davis said: "We each possess a similar set of around 25,000 human genes.
"Some of our genes vary from person to person, like those that give us a particular eye or hair colour.
"But my book is about the few genes - our compatibility genes - that vary the most between each of us.
"First and foremost these are immune system genes - they control how we combat disease.
"But recent research shows that they may be even more important than we once thought - there is evidence that they can influence how our brains are wired, how attractive we are, even how likely we are to reproduce."
It has already been shown that mice and stickleback fish choose their partners by their smell.
However, with humans it is not yet fully understood whether the same definitely applies.
- DAILY MAIL