Grandma plays favourites: X-chromosome relatedness and sex-specific childhood mortality
Molly Fox, Rebecca Sear, Jan Beise, Gillian Ragsdale, Eckart Voland and Leslie A. Knapp
Proc. R. Soc. B 22 February 2010 vol. 277 no. 1681 567-573


Biologists use genetic relatedness between family members to explain the evolution of many behavioural and developmental traits in humans, including altruism, kin investment and longevity. Women’s post-menopausal longevity in particular is linked to genetic relatedness between family members. According to the ‘grandmother hypothesis’, post-menopausal women can increase their genetic contribution to future generations by increasing the survivorship of their grandchildren. While some demographic studies have found evidence for this, others have found little support for it. Here, we re-model the predictions of the grandmother hypothesis by examining the genetic relatedness between grandmothers and grandchildren. We use this new model to re-evaluate the grandmother effect in seven previously studied human populations. Boys and girls differ in the per cent of genes they share with maternal versus paternal grandmothers because of differences in X-chromosome inheritance. Here, we demonstrate a relationship between X-chromosome inheritance and grandchild mortality in the presence of a grandmother. With this sex-specific and X-chromosome approach to interpreting mortality rates, we provide a new perspective on the prevailing theory for the evolution of human female longevity. This approach yields more consistent support for the grandmother hypothesis, and has implications for the study of human evolution.


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.