A team from the University of Pisa in Italy found the bodily chemistry which makes people sexually attractive to new partners lasts, at most, two years.
The Italian researchers tested the levels of the hormones called neutrophins in the blood of volunteers who were rated on a passionate love scale.
Levels of these chemical messengers were much higher in those who were in the early stages of romance.
Testosterone was also found to increase in love-struck women, but to reduce in men when they are in love.
But in people who had been with their partners for between one and two years these so-called “love molecules” had gone, even though the relationship had survived.
The scientists found that the lust molecule was replaced by the so-called “cuddle hormone” – oxytocin – in couples who had been together for several years.
Oxytocin, is a chemical that induces labour and milk-production in new and pregnant mothers.
Donatella Marazziti, who led the research team, said: “If lovers swear their feelings to be ever-lasting, the hormones tell a different story.”
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