The theory of a chemical basis for love has been explored by such biological sciences as evolutionary psychology, evolutionary biology, anthropology and neuroscience. Specific chemical substances such as oxytocin are studied in the context of their roles in producing human experiences and behaviors that are associated with love.
From the perspective of evolutionary psychology the experiences and behaviors associated with love can be investigated in terms of how they have been shaped by human evolution. For example, it has been suggested that human language has been selected during evolution as a type of "mating signal" that allows potential mates to judge reproductive fitness. Miller described evolutionary psychology as a starting place for further research: "Cognitive neuroscience could try to localize courtship adaptations in the brain. Most importantly, we need much better observations concerning real-life human courtship, including the measurable aspects of courtship that influence mate choice, the reproductive (or at least sexual) consequences of individual variation in those aspects, and the social-cognitive and emotional mechanisms of falling in love." Since Darwin's time there have been similar speculations about the evolution of human interest in music also as a potential signaling system for attracting and judging the fitness of potential mates. It has been suggested that the human capacity to experience love has been evolved as a signal to potential mates that the partner will be a good parent and be likely to help pass genes to future generations.
Evolutionary theory also suggests that love keeps two people together, and this would help raise a child. For example, a man and a woman who love each other would be together and work together to raise a child. Back in the tribal days—when much of human evolution took place—it would probably require two people to successfully raise an offspring, and a mother with a supporting partner would probably have more surviving offspring than a mother who does not have such a partner. Thus, people with the ability to form love would produce more offspring than those without that ability. And these offspring would have the genes for love. Thus, the genes for love would become common, and that is why most people today have the ability to love.
Studies in neuroscience have involved chemicals that are present in the brain and might be involved when people experience love. These chemicals include: nerve growth factor, testosterone, estrogen, dopamine, norepinephrine, serotonin, oxytocin, and vasopressin. Adequate brain levels of testosterone seem important for both human male and female sexual behavior. Dopamine, norepinephrine, and serotonin are more commonly found during the attraction phase of a relationship. Oxytocin and vasopressin seemed to be more closely linked to long term bonding and relationships characterized by strong attachments.
The conventional view in biology is that there are two major drives in love — sexual attraction and attachment. Attachment between adults is presumed to work on the same principles that lead an infant to become attached to his or her mother or father– or both.
The chemicals triggered that are responsible for passionate love and long-term attachment love seem to be more particular to the activities in which both persons participate rather than to the nature of the specific people involved.
Anthropologist Helen Fisher also adds lust to the experience of love. Lust exposes people to others, and 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.
Chemically, the serotonin effects of being infatuated have a similar chemical appearance to obsessive-compulsive disorder; which could explain why people experiencing infatuation cannot think of anyone else. For this reason some[who?] assert that taking SSRIs and other antidepressants impede one's ability to fall in love. In one particular case anthropologist Helen Fisher noted:
I know of one couple on the edge of divorce. The wife was on an antidepressant. Then she went off it, started having orgasms once more, felt the renewal of sexual attraction for her husband, and they're now in love all over again.
Simplified overview of the chemical basis of love.
The long-term attachment felt after the initial "in love" passionate phase of the relationship ends is related to oxytocin, a chemical released after orgasm. Moreover, novelty triggers attraction. Even exercising for several minutes can make one more attracted to other people on account of increased heart rate and other physiological responses.
Nerve growth factor
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 return to previous levels after one year. Specifically, four neurotrophin levels (NGF, BDNF, NT-3, and NT-4) of 58 subjects who had recently fallen in love were compared with levels in two control groups 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.
Role of the limbic system
In A General Theory of Love, three professors of psychiatry from UCSF provide an overview of the scientific theories and findings relating to the role of the limbic system in love, attachment and social bonding. They advance the hypothesis that our nervous systems are not self-contained, but rather demonstrably attuned to those around us and those with whom we are most close. This empathy, which they call limbic resonance, is a capacity which we share, along with the anatomical characteristics of the limbic areas of the brain, with all other mammals. Their work builds on previous studies of the importance of physical contact and affection in social and cognitive development, such as the experiments conducted by Harry Harlow on rhesus monkeys, which first established the biological consequences of isolation.
Brain scanning techniques such as magnetic resonance imaging have been used to investigate brain regions that seem to be involved in producing the human experience of love.
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