All human beings are related, and some are more closely related than others. In medical and demographic literature, consanguineous marriage is usually defined as marriage between a man and a woman who are related as second cousins or closer.1 First-cousin marriage is supposed to be the most prevalent form globally. Effects on disease and death have been demonstrated primarily among children of parents who are related as first cousins or closer, whereas it is not clear that similar effects are seen among children of parents who are second cousins or more distant relatives. Recent studies on the frequency of consanguinity have shown that it is not rare and declining in human populations, but prevalent and perhaps increasing globally. To date, studies on the effects of consanguinity at a population level have not been successful in establishing consensus about the kinds and sizes of these effects. However, as other causes of disease and death are declining in many regions of the world where consanguinity is prevalent, the relative importance of consanguinity as a risk factor for disease and death increases.2–4 A project to determine the global burden of disease due to consanguinity has been established under the leadership of Alan H Bittles, and the results of this project will be important for evaluating the public health impact of consanguinity worldwide (Alan H Bittles, personal communication).