Cousin marriage and late pregnancy both double chances of birth defects
Guardian article which says:
General rate of birth defects in Pakistanis: 3%
Rate where couple are first cousins: 6%
General rate of birth defects in Brits: 2%
Rate when the woman is over 34: 4%
(The article headline only covers the cousin marriage aspect.)
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My reading of the article does suggest a naturally higher chance birth defects - 2% Vs 3%.