Although never outlawed in England, during the second half of the 19 century, many states began to ban marriages between first cousins, as part of a larger movement after the Civil War for greater state involvement in a variety of areas, including education, health and safety.
Today, first cousins may not marry in AR, DE, IA, ID, KS, KY, LA, MI, MN, MS, MO, MT, NE, NV, NH, ND, OH, OK, OR, PA, SD, TX, WA and WV.
First cousin marriage is allowed without restriction in 19 states, and with some restrictions in AZ, IL, IN, ME, UT, WI and NC (in North Carolina, while first cousins may marry, “double cousins” may not- more on this one in a bit.) The distinction lies in the debate about whether or not there is an increased risk that the partners’ shared genes will produce an increased chance that their offspring will have recessive, undesirable traits.
A recent report on births in a British-Pakistani community (where first cousin marriage is very common) demonstrated that first cousin children there were twice as likely to be born with “potentially life threatening birth defects” as compared with the children of unrelated parents. [unless a] mother had a brother whose wife was impregnated by [mother’s husband].
Advocates on the other side point out that this resulted in only a 6% chance for the children in the study, as compared with a 3% chance for the population as a whole. This is not very likely to happen in modern societies that practice first-cousin marriage.
In modern western society, marrying your cousin is not well accepted, particularly in the United States.