Wednesday, June 17, 2009

Charles and Emma- Benefits and Hazards of First Cousin Marriages

At first glance this title seems to have a rather tenuous link to Charles Darwin unless you were aware that Charles Darwin married his first cousin Emma Wedgewood and was plagued with fears throughout his life that he had left his children with a ‘genetic time-bomb’. Emma and Charles shared a common grandfather Josiah Wedgewood and were born of brother and sister; Josiah Wedgewood II and Susannah Wedgewood respectively. They went on to have ten children but three died in childbirth, Mary Eleanor less than four weeks from birth, Annie (known as Darwin’s favourite child) at 10 years and Charles Waring at 6 months (1). Currently it is generally legal for first cousins to marry in most western societies, with only the USA notably banning the practice, 31 states make this practise illegal (2). Over recent times, the tradition of marrying first cousins, a practice favoured by the elite in early centuries in an attempt to not dilute the blood line and limit the claimants to family money, is becoming less common with increased transport and communication ensuring a larger population of prospective mates (3). However it is still believed 80% of marriages historically have been first cousin marriages, and are still prevalent in some societies; 3 out of 4 marriages in Bradford’s Pakistani community are inter-cousin unions(4).
It seems when debating views on first cousin marriages the scientific hazards and benefits need to be considered along with the ethical and social consequences. Darwin’s Origin of Species (5) explains evolution as the variation, non constancy of species giving them advantageous characteristics allowing them and in turn their offspring to survive, inheriting these characteristics, this principle is natural selection. The result of natural selection is gradual change due to the accumulation of heritable advantageous traits. The principle of descent with modification offers an explanation for some specie becoming extinct whilst others continue, it also explains Darwin’s tree of life; all living organisms come from common ancestors. Many theories of benefits of first cousin marriages relate to the belief that by doing so they are following the principles of evolution. The belief in common ancestors leads to belief in a continuum of animals and humans and the ultimate question, what separates humans from our ancestors, chimps; chimps mate within a community (6) therefore it is highly likely they mate with first-cousins indiscriminately, so at what point does it become our decision to actively choose a mate due to social and legal rules as opposed to the one nature causes us to be attracted to? At what point do the ‘rules’ for us choosing an appropriate mate become different to those of a chimp?
In addition sexual selection (5) is the struggle within the same sex of the same specie to ensure they mate with the partner with the most desired characteristics giving their offspring maximum chance of survival to propagate their specie with characteristics to exploit the resources available. Therefore if an individual has attracted the individual of the opposite sex who could allow them to give their offspring that desired characteristic, why should they be handicapped compared to others in their specie of the same sex as a result of a family relationship with the desired partner? If people are of the same family they may share positive characteristics, thus children born of them may have these positive attributes further exaggerated, should positive eugenics be discouraged?
Furthermore due to evidence from mitochondrial DNA it has been suggested that all humans outside of Africa developed from one branch of the ‘tree’ which left Africa in the ‘African exodus’ 60,000 years ago (5). Therefore it could be argued that as everyone is related anyway and we share more genetic similarities than one sub-specie of chimps there is little point stopping the marriage of first cousins when most marriages are between related people anyway.
From the social aspect, couples will have a stronger network to support their relationship if they are related, they are likely to share the same values and common memories, meaning the relationship is likely to be more successful, therefore more likely to conceive children and continue the specie.
The most common concern related to first cousin marriage is the genetic implications to the offspring. Studies have shown there is an increased risk of congenital malformation caused by homozygosity for autosomal recessive alleles. When estimating the probability a child of first cousins will have an autosomal recessive disease, it is assumed the grandparents each have a recessive mutation, the probability of the child inheriting the grandfather’s mutation is 1 in 64, similarly they have a 1 in 64 chance of getting their grandmother’s, giving a probability the child will be homozygotic for one of the grandparent’s mutations of 1 in 32, which when added to the risk of major congenital mutation in the general public (1 in 40) they have a 1 in 20 risk of developing a congenital abnormality.(7)
A further theory is that nature itself discourages a too close familial relationship between parents, in a study performed at Liverpool University (8) two sets of male mice were used each identical apart from their major urinary proteins, mice born of related parents have less varied proteins than those of unrelated parents. In the experiment the female mice consistently chose mice with more complex proteins suggesting they could detect the consanguinity, it is thought women may also have this ability.
Once again social opinion influences the practise, in western society it may be viewed unfavourably and people may become ostracised from society ‘We have to stop this tradition of first cousin marriages’ Keighley MP Ann Cryer (4).
However before finally weighing up the hazards and benefits of first cousin marriages it must be considered the way in which the scientific evidence of hazards is reached. Often the cultures considered are those which are either isolated, so first-cousin marriages are a higher occurrence than in some societies or in cultures where it is actively encouraged, which could lead to biased results as the children considered would not be just one off children of first cousin marriages, but an accumulation of several first cousin marriages over many generations in history.
Additionally social and economical factors may have an effect, as cultures where consanguinity is in higher occurrence may also be poorer to begin with, so the mother’s possible malnourishment could be a confounding factor (3).
Also the fertility of the children is often used to consider the success (9) of the offspring and therefore the marriage of the parents, but although a child does not go on to have children themselves, they may have success in other areas such as intelligence. For example, Darwin’s’ children; where William was a banker, Henrietta edited and published her mother’s letters and Leonard taught at the school of military engineering yet none of the three had children (10). Also one should bear in mind that it is known that there are increased genetic problems in children of older mothers (1) but this is not illegal or particularly socially unacceptable.
It may also have been noticed that many of the theories offered as benefits to first-cousin marriages are not benefits just justification to why it is acceptable and not a hazard, just a component of natural selection.
In conclusion, in my opinion first cousin marriages within our society should be neither actively encouraged or discouraged as to allow evolution and natural selection, social and legal implications should not intervene with the biological attraction people feel to their mate. If the practise is encouraged it will occur more regularly than nature intends, so the hazards will accumulate. However, if it is discouraged, natural selection is not occurring and people cannot necessarily mate with those with the most desirable characteristics. My concluding opinion is only relevant in our society where the occurrence is so small as not to be deemed a serious hazard, in smaller more isolated communities it may occur at a higher incidence, thus the hazards are likely to outweigh the benefits. Finally it is important to remember that evolution rejects determinism. We cannot predict evolution as some situations just happen; people may choose not to have children, so we cannot try to manipulate nature as we are unaware of the final consequences.

(1) (last accessed 16th June 2009)
(2)Brandon Keim- ‘Cousin Marriage OK by Science’- Wired Science December 23rd 2008
(3) Diane B. Paul, Hamish G. Spencer.‘It’s Ok, We’re Not Cousins by Blood’. The Cousin Marriage Controversy in Historical Perspective.
(4) Rowlatt J. The Risks of cousin marriage. BBC Newsnight.
(5)‘The Rough Guide to Evolution’ Mark Pallen, 2009
Page 50- The origin of Species: a one page summary
Page 204- Out of Africa
(6) (last accessed 16th June 2009)
(7) Turnpenny P. Ellard S. Emery’s Elements of Medical Genetics 13th edition.
(8)Brandon Keim- Women, Trust Your Nose: Inbred Men May smell bad- Wired Science April 17th 2008
(9) C.D. Darlington. Cousin Marriage and the evolution of the breeding system in man.
(10) (last accessed 17th June 2009)


  1. A nice overview. I am not sure about the 80% figure for first cousin marriages--this seems very high. Your posting tends to lose focus in the middle. Also, more could be made of the comparison between risks of elderly mothers (which society does not forbid) versus first cousin marriages (which, as you point out, are constrained by law in some societies). More paragraph breaks would improve readability.

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