has proposed several explanations for love. Monkey 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 an 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 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 are frequently cited in support of a cooperative past in humans.
Neurochemistry
The conventional view in biology is that there are three major drives in love - libido, attachment, and partner preference. The primary neurochemicals that govern these drives are testosterone, estrogen, dopamine, oxytocin, and vasopressin. Central dopamine pathways mediate partner preference behavior, while vasopressin in the ventral pallidum and oxytocin in the nucleus accumbens and paraventricular hypothalamic nucleus mediate partner preference and attachment behaviors. Sex drive is modulated primarily by activity in the mesolimbic dopamine pathway. Trace amines play a critical role in regulating neuronal activity in the dopaminergic pathways of the central nervous system. Testosterone and estrogen contribute to these drives by modulating activity within dopamine pathways. Adequate brain levels of testosterone seem important for both human male and female sexual behavior. Norepinephrine and serotonin have a less significant, contributing role through their neuromodulatory effects upon dopamine and oxytocin release in certain pathways. 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. Individuals who have recently fallen in love show higher levels of cortisol.
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
Brain scanning techniques such as functional magnetic resonance imaging have been used to investigate brain regions that seem to be involved in producing the human experience of love. 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. Ortigue et al. found that an unconscious prime of the name of a romantic partner activated similar brain regions as when subjects were consciously aware of seeing partners' faces. Subliminal priming with either a beloved's name or a favorite hobby activated emotion and motivational brain regions: caudate nucleus, insula, bilateral fusiform regions, parahippocampal gyrus, right angular gyrus, occipital cortex, and cerebellum. However, the love prime evoked more activation in bilateral angular gyri and bilateral fusiform regions than the hobby prime. These regions are associated with integrating abstract representations, and the angular gyrus in particular is involved with abstract representations of the self. The authors also found a correlation between activation of a region of the angular gyrus with a passionate-love scale measuring subjective feelings of love.
Love and motivation
Conscious thoughts about a romantic partner activate brain regions related to reward and motivation. Ortigue et al. investigated whether unconscious priming by a partner's name could also affect motivation. They found that priming by either a beloved or a favorite hobby improved reaction times in identifying whether a string of letters was a word or not compared against priming by a neutral friend. The authors suggest this effect happens because a beloved's name "may call for a goal-directed state" and produce "dopaminergic-driven facilitation effects."
