text: The benefits for the altruist may be increased, and the costs reduced by being more altruistic towards certain groups.Research has found that people are more altruistic to kin than to no-kin, to friends than strangers, to those attractive than to those unattractive, to non-competitors than competitors, and to members in-groups than to members of out-groups.The study of altruism was the initial impetus behind George R.Price's development of the Price equation, a mathematical equation used to study genetic evolution.An interesting example of altruism is found in the cellular slime moulds, such as Dictyostelium mucoroides.These protists live as individual amoebae until starved, at which point they aggregate and form a multicellular fruiting body in which some cells sacrifice themselves to promote the survival of other cells in the fruiting body.Selective investment theory proposes that close social bonds, and associated emotional, cognitive, and neurohormonal mechanisms, evolved to facilitate long-term, high-cost altruism between those closely depending on one another for survival and reproductive success.Such cooperative behaviors have sometimes been seen as arguments for left-wing politics, for example, by the Russian zoologist and anarchist Peter Kropotkin in his 1902 book Mutual Aid: A Factor of Evolution and Moral Philosopher Peter Singer in his book A Darwinian Left.Neurobiology Jorge Moll and Jordan Grafman, neuroscientists at the National Institutes of Health and LABS-D'Or Hospital Network, provided the first evidence for the neural bases of altruistic giving in normal healthy volunteers, using functional magnetic resonance imaging.In their research, they showed that both pure monetary rewards and charitable donations activated the mesolimbic reward pathway, a primitive part of the brain that usually responds to food and sex.