Perhaps most important was the report of physician Samuel Merrifield Bemiss for the American Medical Association, which concluded cousin inbreeding does lead to the "physical and mental depravation of the offspring".Despite being contradicted by other studies like those of George Darwin and Alan Huth in England and Robert Newman in New York, the report's conclusions were widely accepted.
Opinions and practice vary widely across the world.
Professors Brent Shaw and Richard Saller, however, counter in their more comprehensive treatment that cousin marriages were never habitual or preferred in the western empire: for example, in one set of six stemmata (genealogies) of Roman aristocrats in the two centuries after Octavian, out of 33 marriages, none was between first or second cousins.
Such marriages carried no social stigma in the late Republic and early Empire.
George Louis Arner in 1908 considered the ban a clumsy and ineffective method of eugenics, which he thought would eventually be replaced by more refined techniques. Since that time, Kentucky (1943) and Texas have banned first-cousin marriage and since 1985, Maine has mandated genetic counseling for marrying cousins to minimise risk to any of serious health defect to their children.
The National Conference of Commissioners on Uniform State Laws unanimously recommended in 1970 that all such laws should be repealed, but no state has dropped its prohibition.
Writers such as Noah Webster (1758–1843) and ministers like Philip Milledoler (1775–1852) and Joshua Mc Ilvaine helped lay the groundwork for such viewpoints well before 1860.