section_path: Interpersonal/Biological basis
section_name: Biological basis
paragraph_text: Biological basis
  
  Biological models of sex tend to view love as a mammalian drive, much like hunger or thirst. Helen Fisher, an anthropologist and human behavior researcher, divides the experience of love into three partly overlapping stages: lust, attraction, and attachment. Lust is the feeling of sexual desire; romantic attraction determines what partners find attractive and pursue, conserving time and energy by choosing; and attachment involves sharing a home, parental duties, mutual defense, and in humans involves feelings of safety and security. Three distinct neural circuitries, including neurotransmitters, and three behavioral patterns, are associated with these three romantic styles.
  
  Lust is the initial passionate sexual desire that promotes mating, and involves the increased release of hormones 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 form. Recent studies in neuroscience have indicated that as people fall in love, the brain consistently releases a certain set of chemicals, including the neurotransmitter hormones dopamine, norepinephrine, and serotonin, the same compounds released by amphetamine, stimulating the brain's pleasure center and leading to side effects such as increased heart rate, reduced appetite and sleep, and an intense feeling of excitement. Research indicates 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 that promotes relationships lasting for many years and even decades. Attachment is generally based on commitments such as marriage and children, or mutual friendship based on things like shared interests. It has been linked to higher levels of the chemicals oxytocin and vasopressin, to a greater degree than what is found in short-term relationships. Enzo Emanuele and coworkers reported the protein molecule known as the nerve growth factor (NGF) has high levels when people first fall in love, but these return to previous levels after one year.