Donatella Marazziti, a psychiatrist at the University of Pisa, made a name for herself in 1999 by trying to identify the physiological origins of love. Her original research involved measuring serotonin levels in the blood of people suffering from obsessive compulsive disorder. Then, perhaps thinking "people in love are kind of obsessive," she expanded her study.
Her team recruited students who had recently fallen in love, and who had obsessed about their new love for at least four hours every day but who had not yet consumated the relationship with sex. Seventeen women and three men volunteered. A separate group of people with OCD was studied at the same time, as was a control group.
Members of all three groups were tested for a serotonin transporter protein in their blood platelets. The control group was normal. The obsessive-compulsives were low by 40% (as Marazziti knew to expect from earlier studies). And the people in love? Also low by 40%.
Blood serotonin is not a perfect proxy for serotonin levels in the brain, but it's an intriguing suggestion that, biochemically at least, falling in love provokes madness.
They retested some of the students in love a year later and found that their serotonin levels were back to normal. How quickly love fades!
Marazziti won an Ig Nobel Prize for this work in 2000. You can find her study written up in Psychological Medicine (vol 29, p 741, 1999). I have included the abstract below.
The evolutionary consequences of love are so important that there must be some long-established biological process regulating it. Recent findings suggest that the serotonin (5-HT) transporter might be linked to both neuroticism and sexual behaviour as well as to obsessive-compulsive disorder (OCD). The similarities between an overvalued idea, such as that typical of subjects in the early phase of a love relationship, and obsession, prompted us to explore the possibility that the two conditions might share alterations at the level of the 5-HT transporter. METHODS: Twenty subjects who had recently (within the previous 6 months) fallen in love, 20 unmedicated OCD patients and 20 normal controls, were included in the study. The 5-HT transporter was evaluated with the specific binding of 3H-paroxetine (3H-Par) to platelet membranes. RESULTS: The results showed that the density of 3H-Par binding sites was significantly lower in subjects who had recently fallen in love and in OCD patients than in controls. DISCUSSION: The main finding of the present study is that subjects who were in the early romantic phase of a love relationship were not different from OCD patients in terms of the density of the platelet 5-HT transporter, which proved to be significantly lower than in the normal controls. This would suggest common neurochemical changes involving the 5-HT system, linked to psychological dimensions shared by the two conditions, perhaps at an ideational level.