The writings of Scottish deputy commissioner for lunacy Arthur Mitchell claiming that cousin marriage had injurious effects on offspring were largely contradicted by researchers such as Alan Huth and George Darwin.
Children of first-cousin marriages may have an increased risk of genetic disorders, particularly if their parents both carry a harmful recessive mutation, but this can only be estimated empirically, and those estimates are likely to be specific to particular populations in specific environments.
Children of more distantly related cousins have less risk of genetic disorders.
Though contemporaneous, the eugenics movement did not play much of a direct role in the bans.
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.
To many, Morgan included, cousin marriage, and more specifically parallel-cousin marriage, was a remnant of a more primitive stage of human social organization. Briggs appointed a commission to study mentally handicapped people (termed "idiots") in the state.