Chemical basisBiological 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 (or indeed the child) long enough to rear a child into infancy.
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 on 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 short-term relationships have. 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.