There was a decline in consanguineous marriage rates in Lebanon between 1950 and 1970, from an overall rate of ca. 30% down to 20%.[1]

There has been an increase in consanguineous marriage rates since 1983-84 when the overall rate was 25%[2] to the 2000s (2008?) when the overall rate was 35.5%[3].

Reportedly, there has also been an increase in the wearing of headscarves in this period, i.e. since the 1980s:

The wearing of headscarves has risen since the 1980s, even though Lebanon is generally more liberal than other Mideast countries and also has larger Christian and secular communities. Women who support Hezbollah, the Shiite militant group, wear headscarves generally and some wear the Iranian-style chador, which covers the hair and body but not the face or hands.[4]

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[1] Parental consanguinity and congenital heart malformations in a developing country

[2] Consanguineous Marriage and Reproduction in Beirut, Lebanon

[3] Consanguinity in Lebanon: prevalence, distribution and determinants

[4] A Look At The Wearing Of Veils Across The Muslim World, International Herald Tribune, Oct 31, 2006.

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Iran is a nation composed of multiple ethnic groups and religions. Islam is by far the most important religion — something in the order of 98% of the population claim to be Muslims — and Shia Islam is the most important branch. The peoples of Iran include Persians, Azeris, Kurds, Lurs, Arabs, Baluchi, Turkmens, Armenians, Assyrians, Jews, and the Qashqai amongst others. Many of these groups now live in urban centers, but many are still rural-based agriculturalists while some remain nomadic herders.

The overall consanguinity rates reported for Iran are as follows: in 1977, 40%[1]; in 2001, 38.6%[2]; and in 2003, 58.2%[3]:

The overall consanguinity rates for Tehran were 25.1% in 1966[4] and 31.59% in 1991[5]; in rural areas the rates were 32.8% in 1966[4] and 46.86% in 1991[5]:

Given and Hirschman noted that in 1977, consanguineous marriage was higher among the younger cohort of women than amongst older women.[6] And in 2005-06, Akrami, et. al., found that the number of consanguineous marriages had increased across three generations of Tehranis[7] (my own chart based on their data):

It appears, therefore, that the overall consanguinity rates in Iran have been increasing since at least the 1940s or since the generation(s) that were married before 1948 as defined by Akrami, et. al.
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[1] Given. B, P., and Hirschman, C. (1994) Modernization and consanguineous marriage in Iran. Journal of Marriage and Family, 56, 820-834.

[2] Saadat, M., Ansari-Lari, M. and Farhud, D.D. (2004) Consanguineous marriage in Iran. Annals of Human Biology 31, 263-269.

[3] Saadat, M and Mohabbatkar, H. (2003) Inbreeding and its Relevance to Early and Pre-reproductive Mortality Rates in Iran, an Ecological Study. Iranian J Publ Health, Vol. 32, No. 2, pp.9-11.

[4] Behanm, D., & Amani, M. (1974). La Population de L’Iran. Paris: CICRED.

[5] Farhud, D.D., Mahmoudi, M., Kamali, M.S., Marzban, M., Andonian, L., and Saffart, R. (1991) Consanguinity in Iran. Iranian Journal of Public Health 20, 1-13.

[6] Aghajanian, A. (2001) Family and Family Change in Iran. (“Paper to be published as a chapter in Diversity in Families: A Global Perspective edited by Charles B. Hennon and Timothy H. Brubaker, Belmont, Ca: Wadsworth Publishing Company.”)

[7] Akrami, S.M.; Montazeri, V.; Shomali, S.R.; Heshmat, R.; and Larijani, B. (2009) Is There a Significant Trend in Prevalence of Consanguineous Marriage in Tehran? A Review of Three Generations. J Genet Counsel 18:82–86.

Genetic relatedness affects social behavior due to kin selection and inclusive fitness.  For instance, organisms are more likely to be altruistic toward genetically related individuals rather than unrelated ones since they share a greater number of genes in common with them and are, therefore, more likely to share in common with them the same gene(s) for altruistic behavior.

Another social behavior likely influenced by inclusive fitness is control of reproduction.  Individuals, and groups of individuals, frequently attempt to control the reproductive activities of other individuals with whom they share genes in common.   Such actions might clearly affect the inclusive fitness of “the influencers.”

In many species, this control often takes the form of the reservation (or attempted reservation) of breeding rights for certain members of a group (e.g. the alpha pairs in wolves and meerkats, or the queen in some of the social insects such as group-living ants).  The ultimate aim, however, is the regulation of which genes get passed on to the following generation and via whom.

It is not difficult to imagine that social behaviors related to the control of reproduction might evolve in many species since inclusive fitness is really all about reproduction.  Just as there is likely an “altruism gene” (or complex of genes), there is probably a complex of genes affecting behaviors related to the social control of reproduction.

In humans, social control of reproduction is frequently achieved via social mores related to mating practices (it is also frequently achieved via more direct means); and the relative stringency of these mores in various societies appears to be connected to the levels of genetic relatedness between the members in each society.  This follows when considered from an inclusive fitness point-of-view.  For instance, if one’s daughter is also one’s cousin in terms of genetic relatedness, the price to one’s inclusive fitness would be greater if she failed to mate successfully than if she is only a daughter.

In Western society, which has for a long time has been characterized by relatively loose genetic relatedness between its individual members, this social control of reproduction has manifested itself in the recent past, for example, in the desire of parents to give formal approval of their children’s (especially daughters’) choice of spouse.

In societies with relatively higher levels of genetic relatedness between family members, social control of reproduction has manifested itself in restrictions of interaction between unrelated men and women. In the Muslim world, for example, this has taken various forms of purdah. In other societies (mainly some sub-Saharan African societies but also in places such as Egypt and Sudan), the social control of reproduction has taken the form of female genital mutilation (FGM). The Chinese practice of foot binding may also have been a way to limit the reproductive options of members of that society.

Below is a map indicating the “global distribution of marriages between couples related as second cousins or closer (F≥0.0156).”  Those societies that today have strong practices of social control of reproduction (i.e. purdah and FGM) are also those societies with the highest levels of genetic relatedness within families, e.g. Saudi Arabia, Sudan, Pakistan, Afghanistan.

Consanguineous marriage (%)

Additionally, those societies which practice FBD (father’s brother’s daughter) or patrilineal parallel cousin marriage (e.g. Saudia Arabia, Pakistan, Afghanistan) possess the most stringent social controls of reproduction. Lineages which consistently practice FBD marriage are even more genetically related than those which simply practice general cousin-marriage since all of the males in the FBD lineages will share the same Y-chromosome.

Korotayev has noted that a majority of societies which practice FBD marriage are Islamic. FBD marriage enables families to keep property and wealth in the paternal family while at the same time fulfilling Islam’s requirement entitling women to a share of any inheritance. It is not unlikely, therefore, that those societies in the past which adopted Islam subsequently adopted FBD marriage practices.

However, FBD marriage practices likely pre-date Islam, at least in the Levant. I suggest that many aspects of Islam, such as its call for submission to an ultimate authority, arose in response to the mating practices and levels of genetic relatedness in these societies.  With greater levels of relatedness comes a greater urgency to control who mates with whom.  Many Muslim mores (like many religious and social mores around the world) are simply social controls of reproduction and reflect the levels of genetic relatedness of the societies from which they emanate.

Update:  05/21/10 – edited for clarity.