For those who try to examine how the pressures of natural selection have affected human behavior (an area of thinking known as "evolutionary psychology," "Darwinian psychology," "selection thinking," and probably several other terms), there have been some significant gains in understanding, but also some persistant problems.
Evolutionary psychologists try to examine human behavior from the standpoint of the theory of evolution -- to explain a human behavior by examining how that behavior contributes to the reproductive success of the individual exhibiting that behavior, and therefore to the proliferation of the genetic material which, presumably, at some level is responsible for causing that behavior.
For these theorists, the problem of adoption is a thorny one -- at least when the adoption occurs outside of genetic relatives. Adoption requires a person to devote resources and energy to the upbringing of a child that carries unrelated genes.
While it has been shown that non-biological (i.e. step- or adoptive-) children are statistically treated in an inferior way by their parents than biological children are, the question remains in the case of adoption -- why are they treated well at all? Why are adoptive parents willing to put out so much time, energy and resources to help the reproductive success of a stranger?
A standard theory is that adoption, pursued mainly by otherwise childless couples, is a pathological hijacking of a parental instinct that would normally go towards encouraging the parents to raise biologically related offspring. If the parents are infertile, the psychological desires to raise a child remain and are transferred to an adopted child.
I believe that meme theory can take the pathology out of adoption, and eliminate the above awkward theory for one which uses more straightforward natural selection thinking.
The theory of evolution is based on the following facts:
In evolutionary psychology, the restriction of natural selection thinking to the genetic level is a drawback. Memes, which can be thought of as "ideas," or "idea systems," or "systems of processing ideas," or indeed all three, also follow similar rules.
Memes are passed epigenetically, and can be passed to people other than biological offspring. Memes can be passed from person to person in conversation, or below the level of communication "by example." Memes can even be propagated by the dead; the authors of the gospels continue to have their very successful memes read.
One way memes spread is from primary caretakers to children. The science of psychology tells us that a great deal of personality and thought is fixed in humans within the first five years of life. The primary caretakers of a child have a great opportunity to pass durable memes into their children. These memes can range from a religion or a language which is explicitly taught, to the location of the silverware drawer, which is absorbed less directly.
If the trait which encourages adoption is a genetic trait, we have the following situation (greatly simplified, of course, for illustration):
MOTHER FATHER AA BB <- Genes (BIOLOGICAL CHILD) (BIOLOGICAL CHILD) (ADOPTED CHILD) AB AB MN <- Genes
If the gene that encourages adoption is, for instance, a "B" gene (again, forgive the simplification) -- that gene has been passed to two biological children in the above example. Clearly, from that gene's point of view, it would be more likely to succeed if the resources allocated to the adopted child not carrying the gene were instead allocated to the two children carrying the gene, or to the production of another child carrying the gene.
If we imagine, instead, that adoption is a memetic trait, and one which can be passed to children in the care of an adult with the trait (through religous or moral teaching, for instance), then we have the following situation:
MOTHER FATHER AA BB <- Genes cc dd <- Memes (BIOLOGICAL CHILD) (BIOLOGICAL CHILD) (ADOPTED CHILD) AB AB MN <- Genes cd cd cd <- Meme
So in this case, if we assume that the "c" meme was the one that encouraged the trait of adoption, that meme has not been hurt in the slightest by adoption -- in fact it has been helped. And the adoption of a child already born may be a more reliable way to get a subject for a meme than waiting for a pregnancy and the attendant possibilities of miscarriage, infant death or defect, etc.
The meme that encourages the trait of adoption, if that meme also encouages passing itself on to the possessor's children, is a meme that has a built in method of propagation to a wider and wider segment of the "meme pool." It doesn't require the posessor of the meme even to be biologically capable of having children.*
One would expect, if this meme theory of the adoption trait is true, that adoptive parents would be more likely to prefer to adopt children which A) will be more likely to absorb the adoption meme themselves, and B) will be likely to pass that meme on to other children.
As a child passes infancy and early childhood, it becomes harder to implant memes on the child. This theory would suggest, then, that adoptive parents would prefer to adopt younger children.
If a child is unlikely to reach adulthood, or if when it reaches adulthood it is unable to care for children, it will be unable to pass on memes to its children. This theory would suggest, then, that adoptive parents would prefer physically and mentally healthy children.
* Note that a possible complication of this meme becoming prominent the "meme pool" is that it would encourage a genetic adaptation to it -- i.e. a gene which would encourage the possessor to have children and place them up for adoption, without adopting any children of the possessor's own. A gene which encouraged the possessor to put all of his/her energy into producing children which are adopted by other families would be at an advantage in such a world and would tend to counteract the adoption meme.
In fact a gene which had the effect of immunizing the bearer from the adoption meme would be a successful gene, because the possessor of that gene would be encouraged to put its resources into raising only its genetic children. On the other hand, a meme which was able to overcome this immunization would also be successful, because it would pass itself to more children (if, for example, the possessor of that gene was unable to have biological children, the meme would survive while the gene would not)
Genes and memes are likely to be at odds like this often.