Biological basis of love
The theory of a biological basis of 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.
Evolutionary psychology 
Evolutionary psychology has proposed several explanations for love. Human infants and children are for a very long time dependent on parental help. Love has therefore been seen as a mechanism to promote mutual parental support of children for an extended time period. Another is that sexually transmitted diseases may cause, among other effects, permanently reduced fertility, injury to the fetus, and increase risks during childbirth. This would favor exclusive long-term relationships reducing the risk of contracting a STD.
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. Biologist Jeremy Griffith defines love as 'unconditional selflessness', suggesting utterly cooperative instincts developed in modern humans' ancestor, Australopithecus. Studies of bonobos (a great ape previously referred to as a pygmy chimpanzee) are frequently cited in support of a cooperative past in humans.
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.
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, such as anthropologist Helen Fisher, assert that taking SSRIs and other antidepressants impede one's ability to fall in love. In one particular case 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.
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 imaging 
In 2000, a study led by Semir Zeki and Andreas Bartels of University College London concluded that at least two areas of the brain become more active when in love. These were foci in the media insula, which the brain associates with instinct, and part of the anterior cingulate cortex, which is associated with feelings of euphoria.
See also 
- The Handbook of Evolutionary Psychology, edited by David M. Buss, John Wiley & Sons, Inc., 2005. Chapter 14, Commitment, Love, and Mate Retention by Lorne Campbell and Bruce J. Ellis.
- "Evolutionary psychology: the emperor's new paradigm" by D. J. Buller in Trends Cogn. Sci. (2005) Volume 9 pages 277-283.
- The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature by Geoffrey F. Miller in Psycoloquy (2001) 12,#8.[page needed]
- Evolution of human music through sexual selection by G. F. Miller in N. L. Wallin, B. Merker, & S. Brown (Eds.), The origins of music, MIT Press, (2000). pp. 329-360.
- Sexual selection and mate choice in evolutionary psychology (PDF) by C. Haufe in Biology and Philosophy doi:10.1007/s10539-007-9071-0
- Griffith J. 2011. What is Love?. In The Book of Real Answers to Everything ISBN 9781741290073. http://www.worldtransformation.com/what-is-love/
- Sussman, Robert W. (2004). The Origins and Nature of Sociality. Transaction Publishers. p. 432. ISBN 978-0-202-30731-2.
- Emanuele E, Politi P, Bianchi M, Minoretti P, Bertona M, Geroldi D (April 2006). "Raised plasma nerve growth factor levels associated with early-stage romantic love". Psychoneuroendocrinology 31 (3): 288–94. doi:10.1016/j.psyneuen.2005.09.002. PMID 16289361.
- "The neurobiology of love" by S. Zeki in FEBS Lett. (2007) Volume 581 pages 2575-2579. PMID 17531984
- Bancroft J (September 2005). "The endocrinology of sexual arousal". The Journal of Endocrinology 186 (3): 411–27. doi:10.1677/joe.1.06233. PMID 16135662.
- Slater, Lauren (February 2006). "Love". National Geographic. Retrieved 19 November 2009.
- Sandroni P (October 2001). "Aphrodisiacs past and present: a historical review". Clinical Autonomic Research 11 (5): 303–7. doi:10.1007/BF02332975. PMID 11758796.
- True Love, L. Slater, in National Geographic Magazine 2006 Retrieved 22 May 2009
- Carmichael MS, Humbert R, Dixen J, Palmisano G, Greenleaf W, Davidson JM (January 1987). "Plasma oxytocin increases in the human sexual response". The Journal of Clinical Endocrinology and Metabolism 64 (1): 27–31. doi:10.1210/jcem-64-1-27. PMID 3782434.
- Emanuele, E.; Polliti, P.; Bianchi, M.; Minoretti, P.; Bertona, M.; & Geroldi, D. (2005). “Raised plasma nerve growth factor levels associated with early-stage romantic love.” Abstract. Psychoneuroendocrinology, Sept. 05.
- Lewis, Thomas; Lannon, Richard; Amini, Fari (2000). A General Theory of Love. Vintage Books USA. ISBN 978-0-307-42434-1.
- Reward, Motivation, and Emotion Systems Associated With Early-Stage Intense Romantic Love by Arthur Aron1, Helen Fisher, Debra J. Mashek, Greg Strong, Haifang Li and Lucy L. Brown in Journal of Neurophysiology (2005) Volume 94, pages 327-337.
- Bartels, Andreas; Zeki, Semir (November 27, 2000). "The Neural Basis of Romantic Love". NeuroReport (Lippincott Williams & Wilkins) 11 (17): 3829–3834. Retrieved 27 March 2012. Lay summary – BBC News (5 July 2000).
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