Marriages of blood relatives give rise to certain increased risks in the offspring. These risks arise both for traits controlled by recessive genes and those determined by polygenes. In either case, the result is to expose a proportion of the otherwise largely hidden component in human genetic variability.
Studies have shown little difference between the outcome of consanguineous and control marriages up to and including birth. From birth onwards, however, the findings are different; in a certain Japanese city a death-rate of 116 per 1000 was found during the first 8 years of life amongst the offspring of first cousins, against 55 amongst the controls. The proportion of major congenital abnormalities for children born of consanguineous parents was almost twice that of the others. In an American city, with a lower total death-rate, amongst the offspring of consanguineous unions the death-rate by the age of 10 years was 81 per 1000 compared with 24 per 1000 in the controls. The problem is not an urgent one in communities in which the rate of first-cousin marriages has already fallen to a low figure (below 3 per 1000), but in many parts of the world consanguineous marriages represent a rather high proportion of the total marriages. The likelihood of such marriages occurring increases in societies where the family size is large, in physically or culturally isolated communities (including small islands, inaccessible mountain valleys, sectarian communities).