What a coincidence! I was just at the archives in Charlotte, NC reading about eugenics today, so I'll give you an example to think about:

A bit after 1912, Annie Alexander (first female doctor in Charlotte, NC who is widely celebrated for advancing women's rights in science and doing work as a groundbreaking gynecologist) wrote an essay on eugenics, including this paragraph:

“The number of feeble minded, including juvenile delinquents is enormous – Dr. Goddard of the Department of Research of N.J. says there are in the U.S. 300,000. He says 25 per cent of those in reform schools are feeble minded. They are usually insufficiently bright to mate with others of the same mental calibre and begin new lives of degeneracy. They do not have the ability to make a living for themselves and those dependent on them nor willpower to resist the ordinary temptations, consequently they become a burden to the community or state.

She went on to describe "degeneracy" in more detail:

Except in rare instances feeblemindedness, insanity, criminality, inebriety, and pauperism may be looked on as allied conditions with tendencies to be reproduced in one or another form in the children of persons so affected, with a strong tendency for the defects to become more & more marked in each generation. Such defectives mate with others of like nature.

From these quotes I think you can see the "flawed science" start to appear. First, how do we measure the number of "juvenile delinquents"? We know now more than ever that delinquency is linked to ideas about race - disproportionate arrest and suspension rates for example. Delinquency is not about an individual's mental capacity, it is almost always about the rules created by institutions. Second, what is "feeble minded"? Again, these assumptions are very closely linked with racial science, and they also reflect an early understanding of genetics and disease that is flat out incorrect. Both of these assumptions were very widespread, reflected in the US's anti-vagrancy laws and growing prison population in the early 1900s.

You can see all these false assumptions play out together in the second quote, where Annie implies that drunkenness and insanity and criminality and being poor (pauperism) are all inherited through generations. We now know this not to be true, except in some rare circumstances. Instead, Annie and her colleagues were calling for the sterilization of all peoples who fell into these categories.

As for your question about what "consensus" means, that is more complicated. Among the scientists who wrote for leading medical journals (white, elite, European) it did reach something of a consensus. For example, the Dr. Goddard that Annie cited was the same Dr. Goddard who was widely cited in the Journal of the American Medical Association, including in the notes of a "comment" published in JAMA in 1912:

The objection has also been urged that any sterilizing operation would fall under the head of cruel and unusual punishment, and on this account the practice is at present in abeyance in more than one state pending the outcome of supreme court decisions on the constitutionality of the laws which have provided for such operations… When the whole subject is viewed from a practical point of view the arguments for sterilization of the mentally defective seem greatly to outweigh the sentimental reasons advanced against it… Although segregation of this class during the whole of the reproductive period is effective in its results, it carries with it a financial burden which seems unnecessarily large. Considered in all its various aspects it would appear that the most practical plan for the elimination of the feeble-minded strains should judiciously combine the methods of segregation and sterilization.

The United State's foremost medical professionals and politicians were openly calling for the sterilization of hundreds of thousands of Americans. However, this was also a time of severe oppression of Black and disabled people in the United States, so the overwhelmingly (and in many cases exclusively) white groups such as the American Medical Association and statehouses across the US would not have received much pushback.

In other words, they were using "data," however, this was not the same as the consensus that exists today about global warming. The existence of climate change is a global consensus in the true sense of the term, in a way that eugenics never was. In addition the JAMA author's urge to push back against "objections" that eugenics was cruel and unusual suggests that there was not a full consensus, or at least not enough consensus for people to write about it without recognizing its critics.

I hope this helps! I too am grappling with the exact question you have asked and this really helped me think through an answer. I hope some other commenters can provide some suggested readings that might help answer your question on a more macro scale.

There is an entire school (i.e. "group") of criminology theories called the Positivist School. The common theme is that crime is not the result of free will, but the result of some factors (biological, psychological, environmental, sociological, etc.) that can be scientifically identified and then remedied. This was considered similar to medicine, in that you could diagnose and then treat a disease.

Many of these theories have been debunked, some just in the past few decades. We had Atavism, Phrenology, "Super-male" XYY, Morphology and so on. They were all considered serious and scientific in their heyday. Some are still taught for historical purposes. I still remember some being taught as actual working theories, and I'm only 45 years old.

Eugenics is just a hop, skip, and a jump away from the Positivist School. If crime is not a matter of free will and we can find the genetic factors that cause crime, all we need is a small slippery slope and some scientific hubris (and some cultural/racial biases for spice) before we reach the obviously logical (he said sarcastically) conclusion that the fastest and most cost effective way to solve crime is to purge the "bad stock" from the gene pool.

I just finished teaching about some of these theories in my Intro to Criminal Justice classes. After all of the students express their disdain that anyone could be so foolish as to actually believe such bad theories, I move into the next set of theories. Spoiler alert: we still have a lot of Positivist School theories in use today. However, they have been tempered with either explaining that they are factors that limit the application of free will (a "diminished capacity a.k.a. Insanity Defense of sorts") or used to support the legalization or decriminalization of certain offenses.

New Ken Burns documentary series you say? Well I know how I'm killing the next six hours. Ooof maybe could have used a different expression. Oh wait I'm Jewish. Seriously though Ken Burns "The Roosevelts" was some of the best 13+ hours I ever killed. Not an answer just wanted to say thanks for bringing this new docuseries to my attention!

Interesting that this thread is particularly focused on a more recent binary of "race" -- apparently Black/White -- which corresponds to contemporary Western and U.S.-centered discourse. It's easier to dismantle the "scientificity" of eugenics by placing it in its former sociological context: A lazy, traditional and unscientific cultural conception of "race" that was far more granular and specific; an idea of "race" that was motivated by White Protestant American anxieties far more focused on other European immigrants than on people of African descent: Italian, Slavic & Jewish immigrants to be precise.

Current critiques of the SAT and Stanford-Binet tests (I believe) focus on the culturally specific identity of the test-makers, and their blindness to their own cultural/environmental influences on the creation of the tests in the first place (most of all regarding their earliest versions, how the test-creators language skills, cultural assumptions and patterns of problem-framing in their native language bias the tests). Take a look at this chart:"

TABLE 2

Showing the Percentage of Men of the Sample Nationalities Receiving Final Letter Ratings Under the Given Classifications

Nationality........ D, D- and E......A and B

Total White Draft.......24.1...................12.1
All Foreign .................45.6....................4.0 England........................8.7...................19.7

Germany......................5.0.....................8.3
Sweden.......................19.4....................4.3[...]
Russia.........................60.4....................2.7
Italy.............................63.4.....................0.8
Poland........................69.9....................0.5

In general the superiority of the Nordic stocks over the others is evident in this table. No assertions of language handicap can be brought against these findings [!!], for the men who were unable to manipulate the verbal test, alpha, were given the performance test, beta, or the individual performance test. "

[Source: Intelligence Tests of Certain Immigrant Groups**.** Author(s): Kimball YoungThe Scientific Monthly , Nov., 1922, Vol. 15, No. 5 (Nov., 1922), pp. 417-434Published by: American Association for the Advancement of Science. https://www.jstor.org/stable/pdf/6403.pdf . My emphasis in BOLD]

Here we have the kind of pseudo-scientific rubbish peddled in America by Madison Grant (The Passing of the Great Race, pub. 1916) and Theodore L. Stoddard, political scientist, Klansman, and Harvard graduate, who published Spenglerian tirades against the imminent overthrow of Nordic colonial empires (The Rising Tide of Color Against White World-Supremacy, pub. 1920) and was a direct influence on Nazism via his 1922 The Revolt Against Civilization: The Menace of the Under-man.

(Oswald Spengler's Decline of the West, published in German in 1918, launched "declinism" as a major intellectual current in all Western countries.)

It's -- shall we say? -- relevant to note here that literacy rates in the countries of origin (and the share of immigrants who had primary and even secondary education before arrival in the U.S) varied enormously. The literacy rate in Tsarist Russia on average in 1897 was only 24%, and even in 1917 adult male basic literacy was estimated at only 37.9% of males above seven years of age. There is no more obvious environmental influence on IQ scores than education, which has to be accounted for -- yet the early 20th century eugenicists often ignored this factor flagrantly, as here with the U.S. army induction data.

It's easy to see why Imperial Germany (highest scoring non-English language country of origin) comes out looking "Nordic" and intelligent consequently. By 1900 unified Germany was estimated to have 99% literacy rates for men and women both. It's educational system was legendary and prestigious; near-universal primary education of children, even for peasants, was normal by the 1830s in Prussia, Imperial Germany's core and largest subcomponent.

In Italy, by contrast, also a newly unified state but far less prosperous outside of Lombardy in its North, literacy rates only attained 50% in 1900; the newly unified Kingdom of Italy only required (and funded) 2 to 4 years of primary education prior to 1900 in fact (and not all peasant communities had schools of any kind). Literacy rates between 1860 and 1950 for the countries in the U.S. Army studies summarized in the 1922 article that provided Table 2 (except Poland and Russia) can be seen here: https://link.springer.com/article/10.1007/s11698-020-00201-6/tables/1

Italians, Poles, Russians and -- in the U.S. broken out separately in many studies were Jews from Tsarist Poland and Russia -- "scored consistently at or near the bottom" of U.S. Army administered "mental aptitude" tests in 1917-1918. Jews from the Tsarist Empire given non-verbal "IQ" puzzle tests to sort out and reject the "feeble-minded" at Ellis Island 'performed' at abysmal levels, the worst of any ethnicity. Burns may have directly covered Goddard's conclusions that Jews, Russians and Hungarians were all genetically predisposed to be "morons" or worse based on the Ellis Island data (I haven't seen the documentary).

Note that this on-the-face-of-it absurd finding, given the educational and other attainments Russian and Polish Jews managed later in America once able to access high-quality education, as well as cleaner water, better nutrition, libraries, etc., gives absolutely no pause to today's continuing eugenicists like Charles Murray -- who now argues for Jewish hereditary superiority in intelligence, simply inverting the conclusions of a century ago.

That these results were sheer nonsense scientifically (with implications for the even more dismal testing results for native-born African-Americans provided all too often with either no public education or segregated public education of low quality) was shown very clearly in the consistent finding that IQ scores of immigrants to the United States rose in correlation to the number of years of residence in the United States (i.e. in direct proportion to the acquisition of advanced English language skills and acculturation to American life).

Eugenicists were so embedded in the very widespread (and psychologically self-validating) pre-existing beliefs in WASP and/or Nordic intellectual (and not just intellectual!) superiority of the 1900-1939 era that they refused to recognize the import of this kind of data:" Instead of considering that our curve indicates a growth of intelligence with increasing length of residence, [!!] we are forced to take the reverse of the picture and accept the hypothesis that the curve indicates a gradual deterioration [!!] in the class of immigrants examined in the army, who came to this country in each succeeding five-year period since 1902...." wrote Princeton Prof. Carl C. Brigham in 1923 (it's no accident ETS is headquartered in Princeton, NJ. Source: https://www.researchgate.net/publication/230435620_Intelligence_Tests_and_Immigration_to_the_United_States_1900-1940 )

I think this kind of refusal to see the clear implications of the data these 'scientists' were themselves collecting and then using to argue for eugenicist laws and policy are exactly the "flaws" that are referred to in summaries of the period. If intelligence is primarily hereditary it CAN'T increase purely based on years of mere residence in a given country. Yet it did in their tests. In fact, if we had the data that showed how many years of schooling in their native language the test-takers had received, we would almost certainly see strong correlations to their results on the test, just as were found from years of U.S. residence. What's obvious is that eugenics was already a conclusion, cherry-picking 'evidence' to support a belief, not science.

As for scientific consensus on global warming or climate change, we can observe that at least the global scientific community is no longer overwhelmingly composed of white men of English, German and French ethnicity, with a smattering of Italians and others, during an age in which their countries practiced global colonialism. Scientists today aren't perfectly representative of humanity's diversity, but they are vastly more diverse in cultural background, language, experience and gender than they were in 1910.

There were those back in the early 20th century like the influential columnist Walter Lippman who attacked the quality of the eugenicists' thinking (also the Swiss-born anthropologist Franz Boas at Columbia, notably). But unfortunately they were swimming against the tide. In both Europe and the United States following the shock of the First World War, anti-liberal, racist and xenophobic nationalism were on a global upswing that was to culminate swiftly in the arrival of Fascism and Nazism. Science isn't immune to these kinds of vast transformations in the cultural landscape.