The very source of love has been found. And is it that smouldering look exchanged across a crowded room? Those limpid eyes into which you feel you could gaze for ever? No. It's NGF, say unromantic spoilsport scientists who have made the discovery, - that's short for nerve growth factor.
And now, the really deflating news: its potent, life-enhancing, brain-scrambling effect doesn't last. It subsides within the year of first falling in love - presumably within the same period it takes lovers to notice that the object of their affections can't get the lid on the toothpaste.
"We have demonstrated for the first time that circulating levels of NGF are elevated among subjects in love, suggesting an important role for this molecule in the social chemistry of human beings," says Enzo Emanuele of the University of Pavia in Italy.
Dr Emanueleand his researchers compared 58 men and women, aged 18 to 31, who had recently fallen in love with people in established relationships and those who were single.
"Potential participants required to be truly, deeply and madly in love," said the researchers. Only people whose relationships had begun within six months were studied. The "in love" had to be spending at least four hours a day thinking about their partner.
When the levels of blood chemicals were measured, it was found that both men and women who had recently fallen in love showed very high levels of NGF - 227 units compared with 123 units recorded in those in long-lasting relationships. The study also found that those who reported the most intense feelings had the highest NGF levels.
However, when researchers revisited people from the "in love" group who were still in the same relationship more than a year later, the levels of NGF had declined to the same levels as the established relationship and singles groups.
Love is a neglected area of research and little work has been done on its mechanisms. Dr Emanuele and his team believe they have conducted the first investigation into the peripheral levels of neurotrophins in people in love.
While the role of NGF in falling in love remains unclear, the researchers suggest that some behavioural or psychological features associated with falling in love could be related to the higher chemical levels.
"The raised NGF levels when falling in love could be related to specific emotions typically associated with intense early-stage romantic love, such as emotional dependency and euphoria," the researchers say.
"The specificity of NGF increase during early-stage love seems to suggest that it could be involved in the formation of novel bonds, whereas it does not appear to play a major role in their maintenance.''
Rocketing NGF, however, could be a necessary step on the way to an enduring love because NGF is thought to play an important part in the release of another chemical which plays a pivotal role in social bonding.
In a report about to be published in the journal Psychoneuroendocrinology, the research team ends with a justification for more love research that seemsquintessentially Italian: "Given the complexity of the sentiment of romantic love, and its capacity to exhilarate, arouse, disturb, and influence so profoundly our behaviour, further investigations on the neurochemistry and neuroendocrinology of this unique emotional state are warranted."