In Lasker, Pesta, Fuerst, and Kirkegaard (2019), we found an unstandardized beta for European genetic ancestry, when predicting g, of .85 among African Americans (model 2; Table 6). Simply put: a 100% increase in European (vs. African) ancestry was associated with a 0.85 d increase in intelligence. We interpreted these results as strong support for a partial hereditarian model. As did others in the HBD sphere.
Bird (2021a), in contrast, argued that our regression analyses suffered from omitted variable bias. Notably, he did not disagree that the results would support a hereditarian model were they robust.
Given the 2.053 d (or 30.8 point) measured test score difference between continental Africans and Europeans which Bird (2021a) adopts, genetic effects alone, based on our results, would represent .85 d /2.053 d = 41% of the phenotypic difference. Expressed in terms of variance explained, this would be (.85 d)^2/(2.053 d)^2 = 17.14%. [1] However, this is relative to an average within-groups heritability for g of 66.5% for this specific sample (Mollon et al., 2018; Pesta, Kirkegaard, te Nijenhuis, Lasker, & Fuerst, 2020). Since the expected differences are proportionate to the within-groups heritability, the variance explained would be predicted to be around 17.14%/66.5%*50% = 12.88% conditioned on a heritability of 50%.[2]
Now, based on his analysis of SNP data, Bird (2021a) estimated a variance explained of 12% given a heritability of 50%. Thus, these two very different methodologies (global admixture analysis & SNPS Fst comparisons) derive very similar estimates conditioned on the same heritability coefficient.[3]
But Bird (2021) goes on to interpret his result as “no support for a hereditarian hypothesis”. Well, one could define a ‘hereditarian hypothesis’ such that these magnitudes do not support it. But, in that case, one could just cite our own widely discussed research results against it. In this case, Bird (2021b) should then also state that, “Lasker et al. (2019) also found ‘no support for the hereditarian hypothesis of the Black–White achievement gap’ and, in fact, Fuerst is strongly supportive of an environmental model, despite what some disreputable sites claim.”
I won’t complain. I am sure that being labeled an environmentalist will not hurt my career prospects. However, don’t call me a hereditarian for arguing X but then go on to argue X and also call that ‘no support for a hereditarian hypothesis’.
Note:
[1] To convert between variance metrics, such as R^2, and linear metrics such as r, you take the square-root of the former or the square of the latter. The difference between variance and linear metrics can lead to misinterpretations, since variance metrics do not align with our intuitive sense of distance. Phil Birnbaum (2006) gives the following example: if you were playing baseball and you made it to second base, you might think you made it 2/4 = .5 or one-half of the way home, but in terms of variance metrics you really only ran 2^2/4^2 = 4/16 =.25 or one-quarter of the total variance to home base. This is why, in context to the continental African and European differences discussed, a between-group variance of 17.14% is equivalent to a real-world percent explained of sqrt(17.14%) = 41%.
[2] Originally, I reported an average heritability for g in the TCP sample of 81.5; the correct value was 66.5 (White = 72%; Black = 61%). The text has been updated.
[3] As for which estimates to use, a point which Bird (2021b) touches on, ideally one would employ both within-groups broad-sense heritability and total genetic variance between populations so to calculate the broad-sense between-group heritability and the total expected differences. This is insofar as one is interested in the overall differences, not predicting offspring values from parental ones or testing specific evolutionary models. Now Bird (2021a) cites Polderman et al. (2015). For adults (age 16 to 65), Polderman et al. (2015) give meta-analytic MZ and DZ correlations of .68 and .28 (Figure 3; High-level cognitive functioning), which, using Falconer’s formula, yields a meta-analytic broad-sense heritability of 80%.
Of this, most of the variance is additive genetic; nearly all the remainder is due to an unknown mix of active gene-environmental covariance and dominance variance. Now, if for methodological or theoretical reasons, one uses within-groups narrow-sense heritability and additive genetic variance between populations, one simply derives the expected differences due to additive genetic differences. That can be useful for certain purposes, however, it will underestimate total genetic differences (unless, unexpectedly, in this case, the genetic variance components go in discordant directions between populations). Regardless, since global admixture results will relate to broad-sense heritability, one needs to adjust the heritability when comparing the results of Bird (2021) to those of Lasker et al. (2019).
References
Bird, K. A. (2021a). No support for the hereditarian hypothesis of the Black–White achievement gap using polygenic scores and tests for divergent selection. American Journal of Physical Anthropology.
Bird, K. A. (2021b, February 12). Still No Support For Hereditarianism. Accessed at: https://kevinabird.github.io/
Lasker, J., Pesta, B. J., Fuerst, J. G., & Kirkegaard, E. O. (2019). Global ancestry and cognitive ability. Psych, 1(1), 431-459.
Mollon, J., Knowles, E. E., Mathias, S. R., Gur, R., Peralta, J. M., Weiner, D. J., … & Glahn, D. C. (2018). Genetic influence on cognitive development between childhood and adulthood. Molecular psychiatry, 1-10.
Pesta, B. J., Kirkegaard, E. O., te Nijenhuis, J., Lasker, J., & Fuerst, J. G. (2020). Racial and ethnic group differences in the heritability of intelligence: A systematic review and meta-analysis. Intelligence, 78, 101408.
Polderman, T. J., Benyamin, B., De Leeuw, C. A., Sullivan, P. F., Van Bochoven, A., Visscher, P. M., & Posthuma, D. (2015). Meta-analysis of the heritability of human traits based on fifty years of twin studies. Nature genetics, 47(7), 702-709.
