Limerence is an involuntary state of mind which results from a romantic attraction to another person combined with an overwhelming, obsessive need to have one's feelings reciprocated.
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In biology, Bateman's principle is the theory that females almost always invest more energy into producing offspring than males invest, and therefore in most species females are a limiting resource over which the other sex will compete. It is named for English geneticist Angus John Bateman (1919–1996).
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Homogamy is marriage between individuals who are, in some culturally important way, similar to each other. Homogamy may be based on socio-economic status, class, gender, ethnicity, or religion.[1] It is a form of assortative mating.

The Scarcity Principle

The Ben Franklin effect is a psychological finding: A person who has done someone a favor is more likely to do that person another favor than they would be if they had received a favor from that person. Similarly, one who harms another is more willing to harm them again than the victim is to retaliate.
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"all the girls get prettier at closing time" phenomenon
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The Science of the Friend Zone
-- youtube