We are all members of one species, Homo sapiens. Human beings from any part of the world can mate with each other to produce offspring. Historically, people chose mates who lived within a 15-mile radius, but by the beginning of the 20th century, Americans were marrying people of different races, religions, and ethnicities. Demographic changes — including the exodus from farm to city and the influx of new immigrants — and the increasing use of motor vehicles and telephones encouraged mixing.
In 1908, George Shull at the Station for Experimental Evolution, Cold Spring Harbor, showed that crossing different corn strains produced a more vigorous hybrid. Applying this model to human biology stirred a debate among eugenicists. Shull's advisor, Charles Davenport, could not ignore completely his work, and he even found some evidence for increased vigor among the mixed race people of Jamaica. However, he ultimately concluded that race crossing led to behavioral "disharmony."
However, Shull's work was lost on the vast majority of lay eugenicists who subscribed to the Biblical notion of "like with like" and who believed that miscegenation (race mixing) produced undesirable mongrels. In his influential book, The Passing of the Great Race, Madison Grant warned that racial mixing was "a social and racial crime" that would lead to the demise of white civilization. Eugenicists emphasized the supposed hereditary differences among races and ignored the social-economic variables that might account for differences in behavior and customs. Thus, the eugenic concept of degenerate heredity provided a pseudo-scientific gloss to age-old prejudices.
Laws against interracial marriage had existed in some states since colonial times, but the number increased after the Civil War. Charles Davenport's compilation of State Laws Limiting Marriage Selection, in 1913, showed that 29 states had laws forbidding mixed-race marriages. Twenty-two states had stiff penalties for miscegenation — fines of up to $2,000 and/or prison terms of up to 10 years. Eugenicists actively supported the strengthening of old laws and the enactment of new ones. The eugenicist-inspired Virginia Integrity Act of 1924 prohibited marriage between a white person and anyone with a trace of blood other than Caucasian. The Act was struck down, along with all other anti-miscegenation laws, in 1967.
A few genes determine the skin, hair, and eye characteristics that delineate "races." Human beings are remarkably similar at the DNA level — with a level of homogeneity usually found in a subspecies group. Modern DNA analysis shows that there are fewer genetic differences within the human race than there are within an average band of chimpanzees. This striking DNA evidence has moved many scientists to conclude that race is a social construct with virtually no biological meaning.