By virtue of my profession, I do not believe in scientific truths that are independent of space and time. Nonetheless, while I was nearing the completion of my dissertation (aka the first draft of my book project), I had to answer frequently to questions such as the following: Is there a true theory of handedness? What does handedness truly tell us about the brain, mind, and person? Is there any truth in the stereotypes associated with handedness? My highly contingent and situated answers to questions of this sort are the following (copied from my final dissertation chapter):
First, it looks like there are different handednesses. An individual’s writing hand and throwing hand, for example, do not necessarily overlap because both tasks engage different muscle groups and require distinct skills.
Second, there seems to be a difference between the direction (left/right) and the degree/consistency (weak/strong) of handedness. Some researchers have suggested that distinguishing between strong lateralization and weak lateralization, instead of and/or in addition to considerations of the direction of laterality, would be a productive avenue for future studies on handedness.
Third, it is useful to distinguish between manual preference and manual skill/performance/proficiency. It is unclear if manual preference is a choice or an innate characteristic, but it seems clear that manual skill is acquired and a consequence of practice and habit. If this were not the case, the conversion of left-handers into practicing right-handers would not be possible. Separating skill from preference also helps explain why once-converted left-handers find it hard to unlearn violently imposed right-handed practice and to carry out skilled tasks with their preferred (but untrained) left hand after re-conversion.
Fourth, manual preference might lie on a continuum, and the ratio of practicing left-handers might, at least to some extent, “serve(s) as a barometer of wider cultural toleration and permissiveness,” as historian of medicine Howard Kushner has suggested. Although we do not know why we live in a right-handed world, this reality significantly affects our manual behavior. For over a century, scholars have repeatedly argued that most humans are at least partially ambidextrous, and some have even proposed that there are only 10 percent “real” right-handers, like there are 10 percent left-handers. For individuals in the (somewhat) ambidextrous majority, it is relatively easy to adapt to our right-biased world, and they blend in with “real” right-handers. The about 10 percent practicing left-handers in our right-handed world might be the minority of strongly lateralized individuals who are unable to conform to the cultural right-hand bias. In a world entirely free of social, cultural, and infrastructural pressures against left-handedness, significantly more individuals might be practicing ambidexters or left-handers. In a very discriminatory society, as little as zero percent might practice left-handedness, although the underlying distribution of manual preference (as opposed to habit or practice) might be the same. This theory would explain why the percentage of practicing left-handers in the United States has risen from probably 3 percent in the late nineteenth century to 10 percent in the twentieth century, and why the incidence of left-handedness seems so much lower in cultures that seem socially conservative from a “Western” point of view.
Fifth, the sex-related puzzles in handedness research might have cultural explanations. One persistent problem for handedness researchers has been the higher incidence of left-handedness among men than women. A recent meta-study suggested that there are 25 percent more left-handed men than women, but this might be due to the fact that men and boys in industrialized societies find it easier to resist the pressure to be right-handed than girls and women, or boys might be given more leeway and not be pressured into right-hand-usage at all. Similarly, the maternal effect (that is, that children are more likely to inherit left-handedness from their mother than from their father) might be an artifact of gendered education. After unsuccessfully trying to account for this effect in their theories of the inheritance of handedness, psychologists have suggested that the maternal effect might be due to underreported rates of maternal left-handedness or prospective mothers’ cheating on their left-handed partners and secretly conceiving children with right-handed men. An alternative explanation for this effect is that, on average, mothers in heterosexual relationships in industrialized countries still carry more than half of the child-rearing responsibilities. If left-handed mothers encourage left-handedness in their children, the manual preference of an oftentimes absent father would affect the child’s handedness less than that of the oftentimes present mother.
Finally, and of that I am sure, handedness is not in the brain. In contemporary industrialized cultures, it is part identity and part embodied behavior that depends on the number of hands we possess, the habits we have acquired, and the socio-cultural environment in which we live. I do not wish to deny that there are neural processes and maybe even structures that reflect our hand preference and manual habits. However, the idea that what we do with our hands is caused by what our brain is and does is a gross oversimplification. More than that, it is one instance of dehumanizing neuro-centrisms that make us project all things human onto our brains (or onto one facet of the brain multiple). With the idea that handedness is in the brain, we would accept the doctrine of “[b]rainhood, … a historically contingent resource, born to uphold and make plausible a redefinition of personhood” from the late seventeenth century. Admittedly, this doctrine has brought us groundbreaking therapies for neurological conditions and psychiatric illnesses, but also racial science and neurosexism. Let us be weary of claims that something is in the brain when we can clearly see it at work outside of the skull.
 See, e.g., Anthony Lambert and Charlene Hallett, “Hand Preference for Sending Mobile-Phone Text Messages: Associations with Sex, Writing Hand, and Throwing Hand,” Laterality 14, no. 4 (2009), doi:10.1080/13576500802396545; Ian C. McManus et al., “Eye-Dominance, Writing Hand, and Throwing Hand,” Laterality 4, no. 2 (1999), doi:10.1080/713754334. See also the literature on other lateralities, including hand clasping, arm folding, eyedness, earedness, and footedness; e.g., D. C. Bourassa, Ian C. McManus, and Mark P. Bryden, “Handedness and Eye-Dominance: A Meta-Analysis of Their Relationship,” Laterality 1, no. 1 (1996), doi:10.1080/713754206; Miriam Ittyerah, “Hand and Foot Preference in Two Cultures,” Laterality, 2019, doi:10.1080/1357650X.2019.1575389; Imhokhai Ogah et al., “Hand Clasping, Arm Folding, and Handedness: Relationships and Strengths of Preference,” Laterality 17, no. 2 (2012), doi:10.1080/1357650X.2010.551126; Chiara Sacco et al., “Joint Assessment of Handedness and Footedness Through Latent Class Factor Analysis,” Laterality 23, no. 6 (2018), doi:10.1080/1357650X.2018.1435675; K. Saudino and Ian C. McManus, “Handedness, Footedness, Eyedness and Earedness in the Colorado Adoption Project,” British Journal of Developmental Psychology 16, no. 2 (1998), doi:10.1111/j.2044-835X.1998.tb00916.x; Kunitake Suzuki and Juko Ando, “Genetic and Environmental Structure of Individual Differences in Hand, Foot, and Ear Preferences: A Twin Study,” Laterality 19, no. 1 (2014), doi:10.1080/1357650X.2013.790396; Ulrich S. Tran et al., “Lateral Preferences for Hand Clasping and Arm Folding Are Associated with Handedness in Two Large-Sample Latent Variable Analyses,” Laterality 19, no. 5 (2014), doi:10.1080/1357650X.2014.891607; Diane M. Warren et al., “Heritability and Linkage Analysis of Hand, Foot, and Eye Preference in Mexican Americans,” Laterality 11, no. 6 (2006), doi:10.1080/13576500600761056.
 See, e.g., Kristen A. Kaploun and Christopher A. Abeare, “Degree Versus Direction,” Laterality 15, no. 5 (2010), doi:10.1080/13576500902958871; Eric C. Prichard, Ruth E. Propper, and Stephen D. Christman, “Degree of Handedness, but Not Direction, Is a Systematic Predictor of Cognitive Performance,” Frontiers in Psychology 4 (2013), doi:10.3389/fpsyg.2013.00009. See also Ian C. McManus, “The Determinants of Laterality in Man” (Dissertation, Christ’s College, University of Cambridge, 1979), accessed March 15, 2019, http://www.ucl.ac.uk/medical-education/publications/phd, 42–48; Ian C. McManus, “Ultrasonography and Handedness: Don’t Confuse Direction with Degree,” British Medical Journal 307, no. 6903 (1993); Ian C. McManus and Mark P. Bryden, “The Genetics of Handedness, Cerebral Dominance and Lateralization,” in Child Neuropsychology: Part 1, ed. I. Rapin and Sidney J. Segalowitz, Handbook of Neuropsychology 6, Section 10 (Amsterdam: Elsevier, 1992), 116–17; Ian C. McManus et al., “The Development of Handedness in Children,” British Journal of Developmental Psychology 6 (1988). See also Ian Christopher McManus, interview by Tabea Cornel, July 12, 2016, London.
 E.g., Albrecht Bethe, “Zur Statistik Der Links- Und Rechtshändigkeit Und Der Vorherrschaft Einer Hemisphäre,” Deutsche Medizinische Wochenschrift 51 (1925): 681–83; Baris Ozener, “Extreme Behavioral Lateralization and the Remodeling of the Distal Humerus,” American Journal of Human Biology 24, no. 4 (2012), doi:10.1002/ajhb.22240; Adam R. Reddon and Peter L. Hurd, “Acting Unilaterally: Why Do Animals with Strongly Lateralized Brains Behave Differently Than Those with Weakly Lateralized Brains?,” Bioscience Hypotheses 2, no. 6 (2009), doi:10.1016/j.bihy.2009.06.007.
 See, e.g., Sara M. Scharoun and Pamela J. Bryden, “Hand Preference, Performance Abilities, and Hand Selection in Children,” Frontiers in Psychology 5 (2014), doi:10.3389/fpsyg.2014.00082; J. I. Todor and T. Doane, “Handedness Classification: Preference Versus Proficiency,” Perceptual and Motor Skills 45, no. 3 (1977), doi:10.2466/pms.1977.45.3f.1041.
 Howard I. Kushner, On the Other Hand: Left Hand, Right Brain, Mental Disorder, and History (Baltimore: Johns Hopkins University Press, 2017), xiii.
 See, e.g., Bethe, “Zur” Ernst W. T. Gaupp, “Über Die Rechtshändigkeit Des Menschen,” Sammlung anatomischer und physiologischer Vorträge und Aufsätze 1, no. 1 (1909); John Jackson, Ambidexterity or Two-Handedness and Two-Brainedness: An Argument for Natural Development and Rational Education (London: Kegan Paul, Trench, Trübner & Co., 1905); Rolf W. Meyer, Linkshändig? Rat & Information, Tipps & Adressen, 9th ed. (Hannover: Humboldt, 2008); Marina Neumann, Natürlich Mit Links: Zurück Zur Linkshändigkeit – Befreiter Leben Mit Der Starken Hand (Munich: Ariston, 2014); Miloš Sovák, Pädagogische Probleme Der Lateralität, Beiträge zum Sonderschulwesen 16 (Berlin: Verlag Volk und Gesundheit, 1968); Hermann-Josef Zoche, Ich Sehe Die Welt Auch Von Der Anderen Seite! Die Besonderen Talente Der Linkshänder (Kreuzlingen, Munich: Ariston, 2002). See also Bernard Mazoyer et al., “Gaussian Mixture Modeling of Hemispheric Lateralization for Language in a Large Sample of Healthy Individuals Balanced for Handedness,” PLOS One 9, no. 6 (2014), doi:10.1371/journal.pone.0101165. McManus called the argument that there are as many left-handers as right-handers “a statistical game,” which is often based on specifically developed questionnaires and strict inclusion and exclusion criteria; Ian C. McManus, E-mail to Tabea Cornel, February 28, 2019.
 Ian C. McManus et al., “Science in the Making—Right Hand, Left Hand III: Estimating Historical Rates of Left-Handedness,” Laterality 15, 1–2 (2010), doi:10.1080/13576500802565313.
 For an overview of investigations of the incidence of left-handedness in other cultures and/or previous generations, and on the ways in which such studies are problematic, see Howard I. Kushner, “Why Are There (Almost) No Left-Handers in China?,” Endeavour 37, no. 2 (2013), doi:10.1016/j.endeavour.2012.12.003; Kushner, On, ch. 3.
 Iris E. C. Sommer, “Sex Differences in Handedness, Brain Asymmetry, and Language Lateralization,” in The Two Halves of the Brain: Information Processing in the Cerebral Hemispheres, ed. Kenneth Hugdahl and René Westerhausen (Cambridge: MIT Press, 2010).
 Marian Annett, “Left-Handedness as a Function of Sex, Maternal Versus Paternal Inheritance, and Report Bias,” Behavior Genetics 29, no. 2 (1999), doi:10.1023/A:1021608522152; Marian Annett, interview by Tabea Cornel, July 11, 2016, Coventry.
 Ian Christopher McManus, interview by Tabea Cornel.
 Fernando Vidal and Francisco Ortega, Being Brains: Making the Cerebral Subject (New York: Fordham University Press, 2017), 231.