An analysis of the world literature on the children of first cousin marriages reveals that the depression of survival in offspring followed from birth (including late miscarriages; at about six months gestation or later) to a median age of 10 years is constant (4.4% ± 4.6) across a wide range of values for population prereproductive mortality. There is thus no evidence for the action of conditional lethals. On the basis of these data, it is calculated that the average human is heterozygous for only 1.4 lethal equivalents capable of acting over this portion of the life cycle. The implications of these results are discussed in the context of genetic counseling, and the biomedical significance of variation in DNA.