Criticisms of Race Crossing Theory
As with other major tenets of eugenics, the danger of race crossing
was eventually debunked by geneticists who did not let their prej-
udices stand in the way of clear analysis. One such geneticist was
William E. Castle, who challenged the idea that physical traits were
inherited as separate Mendelian characters, as Davenport and
others had claimed. From his crosses between purebred large and
small rabbits, Castle concluded that rather than separate factors
controlling individual physical traits, the effect of genes on stature
was more general. Hence, long legs and large internal organs would
tend to be inherited together, not independently. The “disharmo-
nies” that Davenport had cautioned against would not, therefore,
be expected to occur. Castle argued instead for the apparent blend-
ing of characters in the offspring of race crosses, with intermediate
values of traits to be expected in the offspring of “wide crosses.”
Castle wrote in 1930:
We like to think of the Negro as inferior. We like to think
of Negro-white crosses as a degradation of the white race.
We look for evidence in support of the idea and try to
persuade ourselves that we have found it even when the
resemblance is very slight. The honestly made records
of Davenport and Steggerda tell a very different story
about hybrid Jamaicans from that which Davenport and
Jennings tell about them in sweeping statements. The
former will never reach the ears of eugenics propagandists
and Congressional committees; the latter will be with us
as the bogey men of pure-race enthusiasts for the next
hundred years. (Castle, 1930)
Charles Davenport’s paper warning of the threat of race crossing was
published a century ago, and from this vantage point we can easily
identify Davenport’s mistakes and the flaws in the entire eugenics
program (Allen, 2000).
First of all, Davenport, along with every other prominent eugenicist, assumed that because a particular trait appeared in consecutive
generations ( i.e., was familial), the trait must be genetically determined. In almost every case, they ignored all possible social and
environmental influences on the trait. Second, the eugenicists
naively assumed that even those traits that were clearly inherited
were simple Mendelian characters. In other words, they believed that
each trait was determined by a single gene with two alleles, one dominant and the other recessive. They ignored the possibility that the
trait could be influenced by multiple genes acting together, with
each making a small contribution to the trait. (Davenport himself
had concluded that skin color and height were polygenic, controlled
by two or more genes.)
Additionally, Davenport and other American eugenicists allowed
their prejudices to affect the interpretation of their honestly made
research findings. Because they assumed blacks to be inferior to
whites, they were unable to evaluate their data without bias. Even
when the evidence failed to support their racist assumptions, they
refused to abandon those assumptions.
This episode may serve as a cautionary tale even for 21st-century
biologists. In the study of human genetics, simple answers must be
viewed with extreme caution, as they can often lead us astray. Data
must be interpreted without bias so that conclusions don’t simply
reinforce existing social prejudices. Finally, it should be clear
that using the latest findings in genetics – or any other branch of
biology – to solve social problems is fraught with danger. The
eugenicists thought they were wise enough to take control over
human reproduction, based on their understanding of Mendelian
genetics. We might ask ourselves whether we as a society are about
to display the same hubris in the application of gene editing and
other new genetic technologies. Heeding the lessons of the eugen-
ics movement may help us navigate the treacherous waters of the
new genetic age we are about to enter.
I would like to express my appreciation to Dr. Randy Moore at the
University of Minnesota for his encouragement.
1. “Feeblemindedness” was an ill-defined category that encompassed a
wide range of mental deficiencies and socially deviant behaviors. See
Kevles (1998, pp. 77–79).
2. Davenport’s examples were “the Scotch” and “the South Italian,”
respectively (Davenport, 1917, p. 366).
3. Davenport wrote: “Again it seems probable, as dentists with whom I
have spoken on the subject agree, that many cases of overcrowding or
wide separation of teeth are due to a lack of harmony between size of
jaw and size of teeth . . . ” (Davenport, 1971, p. 366).
4. Note that Davenport did not diagram such a cross, but it is clear that
this is what he was referring to on pages 366 and 377 of his 1917
5. Davenport (1917, p. 367).
6. 0.46 cm for arm length (Davenport & Steggerda, 1929, p. 88) and 0.52 cm
for leg length (Davenport & Steggerda, 1929, p. 119).
Allen, G.A. (2000). Flaws in eugenic research. Cold Spring Harbor
Laboratory’ sEugenicsArchive.Re trieved from http://www.
Castle, W.E. (1924). Biological and social consequences of race-crossing.
Journal of Heredity, 15, 363–369.
Castle, W.E. (1930). Race mixture and physical disharmonies. Science,
Corcos, A.F. & Monaghan, F.V. (1993). Gregor Mendel’s Experiments on Plant
Hybrids: A Guided Study (pp. 100–120). New Brunswick, NJ: Rutgers
Davenport, C.B. (1913). Heredity of Skin Color in Negro-White Crosses.
Publication No. 188, Carnegie Institution of Washington, Washington,
Davenport, C.B. (1917). The effects of race intermingling. Proceedings of the
American Philosophical Society, 56, 364–368.
Davenport, C.B. & Steggerda, M. (1929). Race Crossing in Jamaica.
Publication No. 395, Carnegie Institution of Washington,