Genetics

"In April 2002, the Journal of Genetic Counseling released a report which estimated the average risk of birth defects in a child born of first cousins at 1.1–2.0 percentage points over an average base risk for non-cousin couples of 3%, or about the same as that of any woman over age 40.  In terms of mortality, a 1994 study found a mean excess pre-reproductive mortality rate of 4.4%, while another study published in 2009 suggests the rate may be closer to 3.5%.  Put differently, first-cousin marriage entails a similar increased risk of birth defects and mortality as a woman faces when she gives birth at age 41 rather than at 30.  Critics argue that banning first-cousin marriages would make as much sense as trying to ban childbearing by older women.

Higher total fertility rates are reported for cousin marriages than average, a phenomenon noted as far back as George Darwin during the late 19th century. There is no significant difference in the number of surviving children in cousin marriages because this compensates for the observed increase in child mortality.  The total fertility increase may be partly explained by the lower average parental age at marriage, and age at first birth, observed in consanguineous marriages. Other factors include shorter birth intervals and possibly a lower likelihood of using reliable contraception.  There is also the possibility of more births as a compensation for increased child mortality, either via a conscious decision by parents to achieve a set family size or the cessation of lactational amenorrhea following the death of an infant.  According to a recent paper the fertility difference is probably not due to any underlying biological effect.  Studies consistently show a lower rate of primary infertility in cousin marriages, usually interpreted as being due to greater immunological compatibility between spouses."

If cousin marriages were legal and accepted by the medical community, doctors could help to reduce the chances of birth defects.