On altruism and love:

Earlier I said that natural selection selects selfish replicators. If organisms were replicators, all organisms should be selfish. But organisms do not replicate. Your parents did not replicate when they had you, because you are not identical to either of them. The blueprint that made you—your set of genes—is not the same as the blueprint that made them. Their genes were shuffled, randomly sampled to make sperm and eggs, and combined with each other’s during fertilization to create a new combination of genes and a new organism unlike them. The only thing that actually replicated were the genes and fragments of genes whose copies made it into you […]

DNA, of course, has no feelings; “selfish” means “acting in ways that make one’s own replication more likely.” The way for a gene to do that in an animal with a brain is to wire the brain so that the animal’s pleasures and pains cause it to act in ways that lead to more copies of the gene. Often that means causing an animal to enjoy the states that make it survive and reproduce. A full belly is satisfying because full bellies keep animals alive and moving and reproducing, leading to more copies of the genes that build brains that make full bellies feel satisfying.

By building a brain that makes eating fun, a gene helps to spread copies of itself lying in the animal’s gonads. […] But here is an important twist. The genes in an animal’s gonads are not the only extant copies of the brain-building genes; they are merely the most convenient ones for the brain-building genes to help replicate. Any copy capable of replicating, anywhere in the world, is a legitimate target, if it can be identified and if steps can be taken to help it replicate. A gene that worked to replicate copies of itself inside some some other animal’s gonads could do as well as a gene that worked to replicate copies of itself inside its own animal’s gonads. As far as the gene is concerned, a copy is a copy; which animal houses it is irrelevant. […] If a gene could build a brain that could tell when copies of itself were sitting in another animal’s gonads, it would make the brain enjoy the other animal’s wellbeing, and make it act in ways that increased that other animal’s wellbeing.

When does a copy of a gene in one animal also sit inside another? When the animals are related. In most animals, there is a one-in-two chance that any gene in a parent will have a copy lying inside its offspring […]. There is also a one-in-two chance that a copy is lying inside a full sibling […]. There is a one-in-eight chance that a copy is lying inside a first cousin, and so on. A gene that built a brain that made its owner help its relatives would indirectly help to replicate itself. [… I]f the benefit to the relative, multiplied by the probability that a gene is shared, exceeds the cost to the animal, that gene would spread in the population. […]

When an animal behaves to benefit another animal at a cost to itself, biologists call it altruism. When altruism evolves because the altruist is related to the beneficiary so the altruism-causing gene benefits itself, they call it kin selection. But when we look into the psychology o the animal doing the behaving, we can give the phenomenon another name: love.

The essence of love is feeling pleasure in another’s well-being and pain in its harm. These feelings motivate acts that benefit the loved one, like nurturing, feeding, and protecting. We now understand why many animals, including humans, love their children, parents, grandparents, grandchildren, siblings, […]: people helping relatives equals genes helping themselves. […]

Many people think that the theory of the selfish gene says that “animals try to spread their genes.” That misstates the facts and it misstates the theory. Animals, including most people, know nothing about genetics and care even less. People love their children not because they want to spread their genes (consciously or unconsciously) but because they can’t help it. […] Genes “try” to spread themselves by wiring animals’ brains so the animal loves their kin […].