Tuesday, January 14, 2020

The costs of outbreeding: what do we know?

Charles Davenport, circa 1929 (Wikicommons)

To produce healthy children, you should marry a third or fourth cousin. Farther out, the genetic costs of outbreeding begin to outweigh those of inbreeding. That was what a cohort study found in examining Icelanders born between 1800 and 1964. Fertility was lower if the woman's husband was either closer in or farther out (Helgason et al. 2008).

That finding is supported by a Danish study. Using data on Danes born in 1954, the authors measured the distance between the wife's home parish and the husband's home parish. Fertility peaked at a distance of around 75 km. The relationship between fertility and marital radius was not explained by education, family income, urbanicity, or mother's age at first birth. The authors concluded that their findings were consistent with those of the Icelandic study, the cause being the same: fertility rises with decreasing relatedness up to a peak and then starts to fall. Inbreeding depression gives way to outbreeding depression (Labouriau and Amorim 2008).

How far can you marry out before the costs of outbreeding become unacceptable? One problem lies with that word. What may be perfectly acceptable to one person may not be to another.

So let me review what is known and what remains to be known. You be the judge.

A half-century ago, Ernst Mayr wrote: "Hybridization between species leads almost invariably to unbalance through deleterious combinations of genes. [...] in Drosophila even the hybridization of races may lead to destruction of well-integrated gene combinations" (Mayr 1970, p. 399). He then went on to say that it is "still uncertain" whether these findings apply to our species. In fact, "all investigations of race mixtures in man have failed to produce any evidence of decreased fitness."

It would be better to say "indisputable evidence." In fact, there is evidence from two major studies of the 1920s.

Davenport’s research in Jamaica

I will begin with a study by Charles Davenport, a Harvard zoology professor. In the early years of the twentieth century, he became concerned about human heredity and the genetic consequences of outbreeding. In 1926, with his assistant Morris Steggerda, he went to Jamaica and examined 100 black, 100 white and 100 “brown” adults of equal social status, together with 1200 children between the ages of 10 and 16. They were given anthropometric, physiological, and psychological tests. The psychological results are worth quoting in full:

Disharmonies in the mental sphere are socially more significant, perhaps, than those in the physical sphere, and such disharmonies are apparently common in the adult Browns [...]. Such disharmony and confusion apparently appear in visualization and reproduction, as in putting together the parts of the manikin. The proportion of failures of the Browns is 9.6 per cent, as opposed to 3.1 per cent in Blacks and 2.1 per cent in Whites. In copying of geometric figures 5 per cent of Browns fail completely, as compared with 3 per cent of the Blacks and 0 per cent of the Whites. In the Army Alpha Test IV (opposites and similars) 41 per cent of Browns got only 3.7 or fewer correct, while only 23 per cent of Blacks did so poorly and none of the Whites. In Test V (pied sentences) 30 per cent of the Browns got fewer than 3 out of 24 questions right, while only 26 per cent of the Blacks and 13 per cent of the Whites did. In Test VII (of analogies) 45 per cent of the Browns got less than 10 per cent of the questions correct, while only 30 per cent of the Blacks and 21 per cent of the Whites did so badly. One gains the general impression that, though on the average the Browns did not do so badly, there was among them a greater number of persons than in either Blacks or Whites who were muddled and wuzzle-headed. The Blacks may have low intelligence, but they generally can use what they have in fairly effective fashion; but among the Browns there appear to be an extra 5 per cent who seem not to be able to utilize their native endowment. There are so many variables, however, and the numbers are so small, that the results merely propose an hypothesis and do not warrant a conclusion.

The question arises: are there any traits in which, on the average, the adult Browns are superior to the Whites? We might, theoretically, expect such, yet when we have tested all of the probably genetically distinct traits between Blacks and Whites, we find only one in which the mean of the adult Browns is clearly higher than that of either parental stock.

On the other hand, if we examine the means for children of 10 to 13 or 13 to 16 years there are a few in which the brown children grade higher than either the black or the white children. [...] The fact that youthful Browns sometimes score higher than youthful Blacks or Whites suggests the conclusion that brown children develop in some mental capacities precociously; and then fall behind in development.

The relative underperformance of biracial individuals looks significant, given the sample sizes. Less clear is its magnitude and, most importantly, its cause. If we exclude the hypothesis of incompatibilities during development, there are only two other explanations:

- The “brown” individuals had suffered some kind of deprivation that the black and white individuals hadn't.

- “Brown” individuals with mental problems had been oversampled.

Neither explanation seems likely. In the Jamaica of the 1920s, biracial individuals were if anything a privileged group. They dominated the middle class. Nor is it clear why those with mental problems would be oversampled. 

Mjøen’s research in Norway and Sweden

The second study was by Jon Alfred Hansen Mjøen, a Norwegian who in 1906 founded the Vinderen Biological Laboratory, a center for international research in genetics. He likewise was concerned about the possible negative effects of outbreeding and became a proponent of eugenics. In 1934, he supported a Norwegian law for forced sterilization. He died in 1939. 

With the support of his laboratory, Mjøen did fieldwork in northern Norway and Sweden, where he observed and measured about 600 Norwegians/Swedes, about 600 Sami, and more than 300 people of first-generation mixed ancestry. Most individuals in the last group were described as having "harmonious faces, general good looks, decent stature, height, and strength, as well as a good correlation between volume of lungs and body-size, and muscular strength and body-size." However, "we found more disharmonies, both physical and mental, than in the two parent races."

The disharmonies were: "Relatively large or small ears, disproportionate extremities, unusual length of body in the F1 generation, abnormal range of variation with regard to such characteristics as lung-volume and muscular strength [...], greater prevalence of diabetes [...], loss of balance, and diminished resistance to tuberculosis" (Mjøen 1931).

Mjøen argued that disharmonies are most likely to occur when a trait is determined by large numbers of genes acting together, such as when hormones act on genes for growth and development: "It is highly probable that the frequently observed exaggerated growth of the hybrid and his disproportionately large extremities are due to a glandular disturbance of genetic origin." He also attributed the higher rate of diabetes to errors in hormonal regulation.


There were no subsequent studies. It would be easy to say that research ended with the widespread revulsion against Nazi Germany in 1945. Actually, the end came earlier, in the 1930s with the triumph of Boasian anthropology and behaviorist psychology. Both stressed the plasticity of the human mind and shifted the focus of research from nature to nurture, i.e., to "society" and possibilities for social change. 

Nonetheless, even within academia, important people continued to voice support for hereditarian and eugenic positions. John Maynard Keynes served as the director of the Eugenics Society of London (1937-1944) and in 1946 was still calling eugenics "the most important and significant branch of sociology" (Brignell 2010). The science fiction series Star Trek ran an episode in 1967 on eugenics ('Space Seed'). The Eugenics Review would be renamed The Journal of Biosocial Science only in 1969. Eugenic laws remained on the books and were enforced as late as the 1970s in some jurisdictions. 

All of this may seem surprising because there has been an effort to push the systematic rejection of eugenics farther and farther back in time. The Wiki entry on Charles Davenport, for instance, states that "only his ardent admirers" took his research seriously. The truth is that he was widely respected until the 1930s. His 1911 work, Heredity in Relation to Eugenics, became a college textbook (Wikipedia 2020). Mjøen was likewise well regarded:

His fame was such that he was one of three expert advisors consulted by the US Congress in the appointment of the first President of the American Eugenics Society, he gave a lecture tour of eighteen American universities in 1927, and, in a period in which The New York Times voiced anxieties about the future of the 'Nordic Type' in America, the newspaper of record invoked Mjøen repeatedly (Reinert 2015)

Even on the left, the general attitude was cautious acceptance, as seen in this 1928 report from the Eugenic Society of Leningrad:

P. I. Ljublinsky remarked that there was not much to be said for the two extreme views of this problem—the doctrine of complete indifference to the matter and the system of strict measures. Therefore what must be recognised as most advantageous is something lying between the two extremes,—a certain system of compromise, of which he himself was an adherent. The meeting gave its support to this last standpoint. It was decided that certain legislative prohibitions in this field are undoubtedly of service. (Philiptschenko 1928)

On the specific issue of outbreeding, feelings were more mixed:

After the conclusion of the discussion the meeting decided that, although the question of race crossing is very important, it is difficult to regard is as definitely solved. But in contrast to the attitude adopted in the Norwegian programme it must be stated that, so long as we have no other data available, we cannot express an opinion against the crossing of different races and nations. (Philiptschenko 1928)

There would be no more data. The Soviet Union, like the world in general, looked up to progressive thought in the West, particularly in the United States. By the 1930s, the shift toward blank slate thinking had become overwhelming among American intellectuals.


If we go back to the recent studies on fertility and outbreeding, we see that fertility progressively declines when outbreeding takes place beyond one's fourth cousin. A growing proportion of embryos or fetuses fail to develop beyond a certain point, apparently because of some incompatibility.

Outbreeding depression seems to be a real phenomenon, but how serious is it? Davenport examined perhaps the maximum degree of outbreeding possible, and only a minority of individuals showed a degree of dysfunction that was higher than expected. The proportion varied according to the mental test, from a low of about 5% to a high of 30%. It is likely that subsequent generations would show progressively lower proportions, with natural selection removing the least functional individuals through reduced fertility, inability to find a mate, or higher risk of illness. 

Davenport was cautious in interpreting his results, noting the difficulty in estimating the size of the adverse outbreeding effect. His self-criticism was taken up by his critics, and the current prevalent view is that his study has been thoroughly discredited, if not by methodological problems, then by Hitler.


Brignell, V. (2010). The eugenics movement Britain wants to forget. New Statesman. December 9 http://www.newstatesman.com/society/2010/12/british-eugenics-disabled 

Davenport, C.B. and M. Steggerda: (1928). Race Crossing in Jamaica. Washington: Carnegie Institution, Publication no. 395.

Helgason, A., S. Pálsson, D.F. Guðbjartsson, þ. Kristjánsson, K. Stefánsson. (2008). An association between the kinship and fertility of human couples. Science 319(5864): 813-816.

Labouriau, R., and A. Amorim. (2008). Comment on "An Association Between the Kinship and Fertility of Human Couples" Science 322(5908): 1634

Mayr, E. (1970). Populations, Species, and Evolution. Cambridge (Mass.): Belknap Press.

Mjøen, J.A. (1931). Race-crossing and glands. Some human hybrids and their parent stocks. The Eugenics Review 23(1): 31-40.

Philiptschenko, J.A. (1928). The Norwegian eugenic programme discussed at meetings of the eugenic society of Leningrad. The Eugenics Review 19(4): 294-298.

Reinert, S.A. (2015). The economy of fear:  H.P. Lovecraft on eugenics, economics and the Great Depression. Horror Studies 6(2): 255-281.

Wikipedia. (2020). Charles Davenport.


  1. This is exactly the topic I've been thinking about lately. Peter, so is it correct to say that there are evolutionary inhibitors of interracial mixing? The reason I ask is, I've been seeing a sharp rise in white-black couples recently in the US, which was unusual as recently as 5-10 years ago. The latest stats are that interracial marriage in the US is skyrocketing, with 1 in 7 marriages being interracial, between all kinds of races. What are your thoughts on all that? This is a new trend, but is it indeed "the new normal," as the liberal world insists (their idea is that there soon will be no race; but they have a leftist "social justice" agenda)? Will something inhibit this interracial tsunami for biological reasons? One other question: is there something fragile about white DNA that makes interracial marriage particularly unsuitable for whites, but less so for other races? The reason I ask is, if there were, that would be the biological basis for white nationalism or white separatism. In terms of white Europeans, Spaniards and Portuguese have gone down the path of interracial mixing, as we see in Latin America. But Nordics in Northern Europe and North America have somehow avoided that path until now. Yet now there's a surge in interracial offspring even among Nordics. Will the white core survive?

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  4. The whole point of outbreeding is to produce "disharmonies" or greater variability and novel combinations. Any farmer knows that inbreeding "breeds true" i.e.produces offspring with regular, predictable traits, and crossbreeding produces less predictable traits and greater variability. Farmers and breeders crossbreed when trying to develop new types or when trying to adapt to novel environments with greater variability that render the older breeds at a disadvantage.

    Human outbreeding generally isn't random but happens in a similar context, such as migrations and invasions, in which a novel, newly variable environment with different selective pressures is introduced. An Aztec marrying a third or fourth cousin during the Spanish conquest of the Americas in which a different environment with novel pathogens and bias against Amerindians was introduced would have had offspring at a disadvantage relative to mestizos. Iceland especially and Denmark were isolated and homogeneous populations and environments and the opposite of the typical environment in which outbreeding takes place and becomes advantageous.

  5. Truth,

    It will become more common as white Americans become a minority. When they made up 90% of the population, the likelihood of mixed marriages was a lot lower. I don't know if there are "biological inhibitors." There used to be cultural inhibitors against mixed marriages of any kind (e.g., Catholic/Protestant, upper class/lower class), but we increasingly live in an atomized, acultural environment.

    Spanish and Portuguese colonists were overwhelmingly male. So there was a strong incentive for sexual relations with African slaves and Amerindians. English colonists were mostly families. They also encountered an Amerindian population that was much smaller than it was in Mesoamerica and South America, i.e., the Aztec and Incan empires.

    Will Europeans survive in Europe? I really don't know. The main problem is that the United States occupies a central place in the production of culture, and that culture is conditioning European people to see themselves as problematic, not just in the U.S. but even in places like Finland.


    It looks like the adverse consequences of outbreeding are observable only in a minority of offspring. In Davenport's study, the proportion varied between 5 and 30%. In the case of an Irish/Greek marriage, the proportion would be a lot smaller. It would probably be observable only in slightly reduced fertility.

    There's been a lot of speculation about the Flynn effect. My opinion is that it's not a real increase in intelligence. I don't believe that people today are smarter than they were in my grandparents' generation. In fact, I'm sure they aren't. The Flynn effect is probably increased familiarity with test-taking and, more broadly, with the process of framing things in terms of questions and answers.


    Outbreeding creates more variability. You get interesting combinations, but at a cost.

  6. My view is that all reproductive strategies have costs/benefits. For example, homosexuality has benefits and costs that have together stabilized it at ~5% in the population. I apply the same principle to interracial marriages. Interracial marriage may confer certain benefits, but also extract certain costs. What are those costs? One study from 2007 says that interracial parents invest more in their children to counteract some "disadvantages that their children will face in their lives": https://news.softpedia.com/news/Interracial-Couples-Invest-More-on-Their-Children-52866.shtml. Apparently there are some social costs that can reduce their kids' success, but will those costs remain the same if the entire society undergoes interracial mixing? We don't know. I also consider some of the examples you posted to be an example of those costs, although you mention they're very small, 5-10%. But there may be other costs that we don't know yet.

  7. Hi Peter,

    Two questions on this topic.

    1) Behavioral geneticist Glayde Whitney argued that the higher mortality rate of African-Americans might result from hybrid incompatibilities, since most American “blacks” have considerable European ancestry (see final paragraph at the end of this piece). Do you think this is plausible?

    2) Regarding a topic you have blogged about last year, to what extent do you think that either hybrid incompatibilities or hybrid vigor could complicate the assumption of hereditarians that biracial people should score, on average, midway between the average scores of the two parent races in IQ tests? (I’m aware of one study suggesting a hybrid vigor effect on g for people of biracial ancestry).


  8. Vel,

    Regarding your first point, I'm somewhat skeptical, except in cases where biracials are first generation. Over successive generations, the dysfunctional effects of crossing would be weeded out, if only because the most dysfunctional offspring have reduced fertility.

    Regarding your second point, yes, that could be a complicating factor. On the other hand, the Davenport study suggests that dysfunctional effects are spread unevenly among biracials. Most show no such effects, and Davenport concluded that such effects are most evident in 5% of first-generation crosses. It's difficult to say, because the sample sizes were too small to measure the magnitude. Perhaps 5% show a high level of dysfunction, whereas a larger group shows low levels of dysfunction. I don't know.

  9. It is interesting to note that when Count Richard Nicolaus Coudenhove Kalergi wrote in his book Practical Idealism (Praktischer Idealismus) "The man of the future will be of mixed race. Today's races and classes will gradually disappear owing to the vanishing of space, time, and prejudice. The Eurasian-Negroid race of the future, similar in its appearance to the Ancient Egyptians, will replace the diversity of peoples with a diversity of individuals". that the deleterious effects of race mixing were actively being discussed. Count Kalergi was no lightweight. He was given 60000 gold marks (equivalent at todays gold price to $140m) by Max Warburg to implement the plan. During WW2 he worked with nascent CIA to have assist the founding of the EU after the wars end.