Sunday, June 01, 2008


According to Perth's Sunday Times, Murdoch University's Professor Alan Bittles has researched the incidence of birth defects concluding that most consanguineous couplings produce healthy babies. And anyway, we might as well get used to blood relatives marrying:
“There is widespread misconception that these marriages [are] rare,” Prof. Bittles said.

“In reality there are over 1000 million people worldwide that live in regions where 20 – 50 per cent of marriages are between blood relatives.”

Prof. Bittles believes as more migrants move into Australian communities there will be a greater incidence of first-cousin marriages.
According to the article, there are 500 such marriages in Western Australia. Using the figures cited by the Sunday Times, such marriages are expected to have a 4% incidence of birth defects as opposed to 2% for the general population. Most people skimming through the article would see the 2% and 4% figures and probably think to themselves, "that's no big deal".

If the 500 WA consanguineous couples produced 500 children it is expected that 20 of these will have a birth defect. 500 non-consanguineous couples would produce 10 children with birth defects. Now I'm no maths whizz but that looks like 100% more for the consanguineous couples. In terms of total numbers it's really no big deal, however.

But for Australia as a whole it could be a big deal if the number of consanguineous couples increases. 265,990 children were born in Australia in 2006. Lets assume none of these children were produced by blood relatives. The expected number of birth defects is 5,320. Now let's assume 1% of births are to blood relatives. The expected number of birth defects is now 5,373. What if Australia had a 20% incidence of blood marriage, as in some countries? Expected birth defects would be 6,385. With 50% blood marriage the figure is 7,980. These numbers, while interesting, are meaningless because the number of Australian consanguineous couplings is not likely to dramatically increase any time soon.

Consanguineous marriage in large, high birth rate countries is downright scary, however. Let's use a country of 160 million plus with 4.5 million births per year, half of which are to blood relatives. That's 135,000 birth defects per year. For 20% blood marriages the figure is 108,000. Assuming no blood marriages the total number of expected birth defects is 90,000.

Even though the risk for individual couples of producing a baby with birth defects is small, marrying a blood relative looks like a bad idea no matter what the experts say.

Note: I won't be surprised if some of my calculations are a bit out, so feel free to point out any errors.


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