A behaviour that increases the direct fitness of the actor is mutually beneficial if the recipient also benefits, and selfish if the recipient suffers a loss. A behaviour that reduces the fitness of the actor is altruistic if the recipient benefits, and spiteful if the recipient suffers a loss. - from WikipediaThe levels of genetic relatedness between individuals influence the levels of mutually beneficial, selfish, altruistic and spiteful behaviors that occur between those individuals.
A few years ago, Stanley Kurtz wrote a couple of very insightful articles on why many Muslim countries demand that their women cover and/or conceal themselves (hijab; purdah). The reasons are connected to both resource (wealth) control and cousin marriage, i.e. preventing assets from being sieved away from the (extended) family by marrying relatives to relatives. From "Veil of Fears":
If a husband's tie to his wife should become more important than his solidarity with his brothers, the couple might take their share of the property and leave the larger group, thus weakening the strength of the lineage.*****
There is a solution to this problem, however — a solution that marks out the kinship system of the Muslim Middle East as unique in the world. In the Middle East, the preferred form of marriage is between a man and his cousin (his father's brother's daughter). Cousin marriage solves the problem of lineage solidarity. If, instead of marrying a woman from a strange lineage, a man marries his cousin, then his wife will not be an alien, but a trusted member of his own kin group. Not only will this reduce a man's likelihood of being pulled away from his brothers by his wife, a woman of the lineage is less likely to be divorced by her husband, and more likely to be protected by her own extended kin in case of a rupture in the marriage. Somewhere around a third of all marriages in the Muslim Middle East are between members of the same lineage, and in some places the figure can reach as high as 80 percent. It is this system of "patrilateral parallel cousin marriage" that explains the persistence of veiling, even in the face of modernity.
Randall Parker and Steve Sailer took this idea of cousin marriage (consanguinity) in the Middle East affecting social behaviors and applied it to the political situation in that area of the world -- specifically to Iraq. From Sailer:
Are Muslims, especially Arabs, so much more loyal to their families than to their nations because, due to countless generations of cousin marriages, they are so much more genealogically related to their families than Westerners are related to theirs? Frank Salter, a political scientist at the Max Planck Institute in Germany whose new book "Risky Transactions: Trust, Kinship, and Ethnicity" takes a sociobiological look at the reason why Mafia families are indeed families, told me, "That's my hunch; at least it's bound to be a factor."*****
One of the basic laws of modern evolutionary science, quantified by the great Oxford biologist William D. Hamilton in 1964 under the name "kin selection," is that the more close the genetic relationship between two people, the more likely they are to feel loyalty and altruism toward each other. Natural selection has molded us not just to try to propagate our own genes, but to help our relatives, who possess copies of some of our specific genes, to propagate their own.
Nepotism is thus biologically inspired. Hamilton explained that the level of nepotistic feeling generally depends upon degree of genetic similarity. You share half your personally variable genes with your children and siblings, but one quarter with your nephews/nieces and grandchildren, so your nepotistic urges will tend to be somewhat less toward them. You share one eighth of your genes with your first cousins, and one thirty-second with your second cousin, so your feelings of family loyalty tend to fall off quickly.
But not as quickly if you and your relatives are inbred. Then, you'll be genealogically and related to your kin via multiple pathways. You will all be genetically more similar, so your normal family feelings will be multiplied.
Natural selection has, however, not only "molded us ... to help our relatives, who possess copies of some of our specific genes, to propagate their own." Natural selection has also molded us to attempt to control -- to wield power (if possible) over -- those relatives who possess copies of some of our specific genes.
For example, parents in the West frequently try to exert pressure on their children when it comes to their choice of a spouse. In the (fairly recent) past, they may have even reserved a right of approval or rejection. Since each parent shares half their genes with each of their children -- and given that 'purpose' of life is to pass one's genes on to successive generations -- it naturally follows that they would want to exert some amount of control in ensuring those genes are well 'invested' for the future.
When, as in the case of cousin marriage, parents share MORE than half of their genes with their children, it follows that they will want to exert a GREATER amount of control over where those genes are invested. I propose that this is precisely what we see in populations where cousin or other consanguineous marriage practices are the norm -- greater controls on reproduction than in areas with low levels of consanguineous marriages.
Women are often the target of such control efforts simply because their reproductive capabilities are a quite limited resource. Men can, if given the opportunity, father hundreds (thousands?) of children -- women are limited to twenty-something (births) at best. Thus, in the West -- a population with low levels of consanguinity -- we have "8 Simple Rules for Dating My Teenager Daughter", but none for the teenage son. In populations with greater levels of consanguinity, control efforts are much more stringent and we see such behaviors as the covering/concealment of women, female genital mutilation (female circumcision), even 'honor killings' (and, perhaps, foot binding?) -- all behaviors which are best accounted for by the drive to control genetic flow.
The attempts of families to control their members' reproduction are, certainly, related to resource control (keeping wealth in families) as Kurtz and Sailer have indicated. But, ultimately, the objective is the successful reproduction of genes and the control of that reproduction.
Appended below is some suggestive evidence (clearly much further research -- ANY, in fact -- needs to be done to either support or disprove these assertions). Compare the second two maps with the first one:
1) % of consanguineous marriages by country;
2) map of various hijab types (*India=Muslim minorities only);
3) frequency of FGM in Africa: