Abstract The discovery of nearly 180-year-old cranial measurements in the archives of 19th century American physician and naturalist Samuel George Morton can address a lingering debate, begun in the late 20th century by paleontologist and historian of science Stephen Jay Gould, about the unconscious bias alleged in Morton’s comparative data of brain size in human racial groups. Analysis of Morton’s lost data and the records of his studies does not support Gould’s arguments about Morton’s biased data collection. However, historical contextualization of Morton with his scientific peers, especially German anatomist Friedrich Tiedemann, suggests that, while Morton’s data may have been unbiased, his cranial race science was not. Tiedemann and Morton independently produced similar data about human brain size in different racial groups but analyzed and interpreted their nearly equivalent results in dramatically different ways: Tiedemann using them to argue for equality and the abolition of slavery, and Morton using them to entrench racial divisions and hierarchy. These differences draw attention to the epistemic limitations of data and the pervasive role of bias within the broader historical, social, and cultural context of science.

Citation: Mitchell PW (2018) The fault in his seeds: Lost notes to the case of bias in Samuel George Morton’s cranial race science. PLoS Biol 16(10): e2007008. https://doi.org/10.1371/journal.pbio.2007008 Published: October 4, 2018 Copyright: © 2018 Paul Wolff Mitchell. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The author received no specific funding for this work. Competing interests: The authors have declared that no competing interests exist. Abbreviation: IC, internal capacity Provenance: Not commissioned; externally peer reviewed.

Morton’s measures and Gould’s argument Morton published three major works documenting his cranial collection: Crania Americana (1839) [60], Crania Aegyptiaca (1844) [61], and a Catalogue of Skulls of Man and the Inferior Animals, in three editions (1840, 1844, and 1849) [59,62,63]. Each successive edition of the Catalogue documented every skull in Morton’s growing collection at the time of publication. In Crania Americana, Morton published IC measures for the five principal races he recognized (Ethiopian [i.e., African], [Native] American, Caucasian, Malay, and Mongolian, Fig 3, S1 Text), taken by pouring white pepper seed into the brain cavity, then measuring the volume of seed in cubic inches necessary to fill the skull [60]. (Later, Morton [63] claimed that he had used “white mustard seed.” In recent years, white peppercorns, Piper nigrum, have been found still lodged in skulls in Morton’s collection, suggesting that peppercorns were used for measurement.) In Crania Aegpytiaca and the last edition of the Catalogue of Skulls, Morton published new IC data taken with lead shot because he had found inconsistencies in measuring IC with seed [63]. While Lewis and colleagues and Michael showed the accuracy of Morton’s lead shot measurements, Gould assumed the accuracy of the shot ICs [58]. (“I will assume, as Morton contends, that measurements with shot were objective and invariably repeatable to within 1 in3” [36].) The bias that Gould attributed to Morton was to be found in the difference between seed and shot ICs for racial groups. Gould contended that seed, but not shot, could easily be manipulated as Morton packed and settled the seeds into the cranial cavity in the measurement of each skull. Morton, Gould supposed, had likely unconsciously overstuffed Caucasian crania with seed and only sparingly filled the crania of other races, leading to a systematic underestimation of non-Caucasian IC with seed. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 3. Morton’s cranial capacity by race in Crania Americana (1839) [ Morton’s cranial capacity by race in Crania Americana (1839) [ 60 ], page 260. Note Morton’s handwritten correction of the American mean in printed copies of Crania Americana, which he changes to 80 in3 from the incorrectly printed 82 in3. Image available from: https://archive.org/details/Craniaamericana00Mort. https://doi.org/10.1371/journal.pbio.2007008.g003 Demonstrating this supposed bias is not straightforward, however, because Crania Americana records IC (seed) only as an average, minimum, and maximum for all races but one, the American, for which the full data for all 147 crania measured are published. In contrast, the 1849 Catalogue lists the IC (shot) of every cranium measured, for all races. Thus, direct comparisons of seed and shot ICs from Morton’s published works are possible only for the American crania. Of the 147 American crania measured in Crania Americana, 111 were later re-measured with shot and recorded in Morton’s 1849 Catalogue (See Supporting Information Data 3: Native Am Seed Shot, from [52]). These 111 comparisons show a mean difference between seed and shot (“seed-to-shot correction”) of +2.2 in3 for Americans. Inferring the mean seed-to-shot correction for the other four races requires reconstructing which crania were measured in 1839, which Gould did by working from the information recorded in the third edition of the Catalogue (1849) about the timing and ordering of Morton’s acquisition of crania. Thereby, he could presumably account for the crania in Morton’s collection by the time of Crania Americana’s publication. Comparing the seed IC means for Africans and Caucasians published in 1839 to the shot IC means of Gould’s reconstructions of which crania were re-measured in each racial group, Gould produced a mean seed-to-shot correction of +5.4 in3 for Africans and +1.8 in3 for Caucasians (Gould did not analyze the Mongolian or Malay seed-to-shot corrections). Gould suggested that such a difference between Africans and Caucasian corrections is most likely explained by Morton’s underestimation of African crania with seed, prima facie evidence of Morton’s bias [36]. Gould suggested that this bias was unconscious because Morton, commendably, openly published his data [36] (as this paper shows, contra Gould, Morton published many—but not all—of his data). Although he proffered unconscious bias as an explanation, Gould also mentioned what now appears the likelier source of the seed-to-shot correction differences: Morton “borrowed some skulls from friends” in 1839 [36]. Gould, although probably not entirely accurately (S2 Text), identifies only 18/29 (or 62%) of the African crania and 19/49 (or 39%) of the Caucasian crania as having been re-measured, while the rest were borrowed from Morton’s associates in 1839 and not later re-measured. Thus, in calculating his “seed-to-shot corrections,” Gould compares the shot IC means of the 18 re-measured Africans and 19 re-measured Caucasians to the seed IC means for 29 Africans and 49 Caucasians measured in Crania Americana. In so doing, he does not account for the other 11 Africans and 30 Caucasians measured in Crania Americana, which comprise approximately 40% to 60% of their respective samples, as has been briefly noted before [52]. Given that borrowed skulls that were not re-measured comprise such a significant proportion of Morton’s 1839 samples, it is likely sample differences, rather than systematic bias in seed measurements, that accounts for the differences in seed and shot IC (S3 Text). Moreover, an enlarged sample of direct seed—shot comparisons possible with the recovered seed data presented here does not support Gould’s claim of Morton’s unconscious bias.

Conclusion Gould’s major argument about Morton’s biased data is rendered highly problematic by the seed data presented here, but, even so, Morton’s work cannot be regarded as unbiased science (S6 Text). Morton’s conclusions cannot be extricated from his biases, no matter the fault in his seeds. Morton, Tiedemann, and other 19th century investigators sought to empirically ground ethical and political questions about the meaning of human difference in the measurement of skulls. The divergence of Morton’s and Tiedemann’s conclusions despite the similarity of their results shows not only that cranial race science could accommodate diametrically opposed interpretations, both for and against racial equality [86,87,88,89] but that those interpretations were underdetermined by the data. Whether as a tool to bulwark or to undermine hierarchy and oppression, this science was inevitably bound up with questions about slavery, colonialism, and differential human worth. Not inevitable, however, were the answers to these questions read from careful records of seed and lead shot packed into and poured out of a few hundred skulls. Attention to the history and broader social context of race science reveals that such measurements are never innocent of an armature of assertions and assumptions that give them meaning, making bias much more than just a potential property of data [57]. As science is a historically, culturally, and socially situated endeavor [90,91], bias is an abiding factor in framing inquiry, forming concepts, generating questions, and designing and implementing methods, as well as interpreting results. Countervailing these forces requires “vigilance and scrutiny” [36], as Gould suggested, and a critical and diverse community of investigators [92,93] in which the open presentation of data and procedures is a norm [36,58]. As for Dr. Samuel George Morton, the accuracy of his cranial measurements neither explains nor excuses the racism constitutive of his thought and its legacy, cautioning us to remember that “unbiased data” cannot be equated with unbiased science.

Acknowledgments The author thanks John S. Michael, Wolfgang Böker, Hanna Polasky, Sharon Ashok, Jean Henry, Michael Weisberg, Deborah Thomas, Adriana Petryna, Sebastián Gil-Riaño, Nicolaas Rupke, M. Susan Lindee, and Janet Monge for assistance and lively discussion. The author is also grateful to the Penn Museum, Stanford University Archives, and Academy Archives of the Academy of Natural Sciences of Drexel University for access to their collections.