Biological models of sex tend to view love as a mammalian drive, much like hunger or thirst. Helen Fisher, a leading expert in the topic of love, divides the experience of love into three partly-overlapping stages: lust, attraction, and attachment. Lust exposes people to others, romantic attraction encourages people to focus their energy on mating, and attachment involves tolerating the spouse long enough to rear a child into infancy.
Lust is the initial passionate sexual desire that promotes mating, and involves the increased release of chemicals such as testosterone and estrogen. These effects rarely last more than a few weeks or months. Attraction is the more individualized and romantic desire for a specific candidate for mating, which develops out of lust as commitment to an individual mate forms. Recent studies in neuroscience have indicated that as people fall in love, the brain consistently releases a certain set of chemicals, including pheromones, dopamine, norepinephrine, and serotonin, which act similar to amphetamines, stimulating the brain's pleasure center and leading to side-effects such as an increased heart rate, loss of appetite and sleep, and an intense feeling of excitement. Research has indicated that this stage generally lasts from one and a half to three years.
Since the lust and attraction stages are both considered temporary, a third stage is needed to account for long-term relationships. Attachment is the bonding which promotes relationships that last for many years, and even decades. Attachment is generally based on commitments such as marriage and children, or on mutual friendship based on things like shared interests. It has been linked to higher levels of the chemicals oxytocin and vasopressin than short-term relationships have.
In 2005, Italian scientists at Pavia University found that a protein molecule known as the nerve growth factor (NGF) has high levels when people first fall in love, but these levels return to as they were after one year. Specifically, four neurotrophin levels, i.e. NGF, BDNF, NT-3, and NT-4, of 58 subjects who had recently fallen in love were compared with levels in a control group who were either single or already engaged in a long-term relationship. The results showed that NGF levels were significantly higher in the subjects in love than as compared to either of the control groups.