Marriage between relatives is seen as distasteful within some cultures, but it has been a common feature in many others for thousands of years.
However, growing evidence has shown that children born to parents from the same extended family face a higher risk of developing a range of health problems.
Research from 2008 shows that marriage between cousins in the US, Europe, Russia and Australia is less than one per cent.
In countries such as Saudi Arabia, the United Arab Emirates and Qatar, more than half of the population marry a spouse who is considered a relative.
Some of these countries and a number of African and Asian countries have the world's highest rates of birth defects - up to 69 cases in every thousand people.
Some experts say the real figure is much higher.
Like its Gulf neighbours, Qatar has now made pre-marital medical tests mandatory.
Charles Stratford has this report.