The problem is worst among children born in Britain's Pakistani community, where more than half of marriages are between first cousins, and children are 10 times more likely than the general population to suffer genetic disorders.
"The medical risks of first cousin marriages include higher rates of infant mortality, birth defects, learning difficulties, blindness, hearing problems and metabolic disorders.
"As adults, the children born from first cousin marriages are at increased risk of miscarriage or infertility. A third of children affected die before their fifth birthday.
"An investigation by Channel 4's Dispatches programme found that although more than 70 British studies have proved the risks, and 700 British Pakistani children are born with associated genetic diseases every year, many people deny the dangers."
On the other hand, they may be breeding faster than native Britons, but they are to a certain extent offsetting that by producing so many unwell children.
Do not question the increased risk, only its degree.
700 is not a particularly large number in a community of this size. If first cousin marriage were as harmful as some claim, it would be more in the range of 70,000 damaged children.
BTW, "increased risk" is often a way of obfuscating rather than enlightening the issue. Is that increase 10%, 100% or 1000%? Inquiring minds want to know.