Alan Bittles on his own meta-study

[...][If] the data [from my meta-study] are censored to exclude physical and mental abnormalities among the male and female parents, and major disparities with respect to young and advanced maternal age, few differences remain in the overall health outcomes recorded for [the children of the consanguineous group and the children of the control group].

- Alan H. Bittles, Inbreeding, Incest, and the Incest Taboo 

Given the strongly judgmental societal attitudes, direct assessment of the biological outcomes of incest may be subject to significant ascertainment bias, as a rigorous examination, including determination of paternity, may be initiated only if a child shows symptoms of physical or intellectual handicap, or both. The net result is a marked lack of data, which probably does not reflect the actual numbers of incestuous pregnancies that successfully proceed to term. Problems also arise in attempting to control for the potentially adverse effects of nongenetic variables, such as very young or advanced maternal and paternal ages, paternal disease, and unsuccessful attempted interruption of the pregnancy.

- Alan H. Bittles, Inbreeding, Incest, and the Incest Taboo