In a previous post on human variation in testosterone levels, I discussed the Rohrmann et al. study and its methodological flaws. This study tested black Americans, white Americans, and Mexican Americans in three age classes: 20-44; 45-69; and 70+. It found that even in the youngest age class the race differences were minor, with the Mexican Americans having the highest testosterone levels.
The main flaw here is that the black-white difference in testosterone levels shrinks after 24 years of age, is gone by the early 30s, and seems to reverse at older ages. So it should not show up in the average t levels of 20 to 44 year-olds. As for the Mexican Americans, their t levels were higher probably because they were younger on average. The median age of the 20-44 year-olds was 31.8 for the whites, 29.5 for the blacks, and 28.6 for the Mexican-Americans. This age disparity is probably understated by the use of medians rather than means, given the youthful age pyramid of the Hispanic-American population.
But another thing bothers me with this study. The 45-69 year old black men had higher t levels (5.62) than the 20-44 year old black men (5.35). This is more than a bit odd. Testosterone declines with age and the large number of participants rules out random sampling error.
Nor could recruitment bias be responsible. The authors of this study did not recruit their participants. In fact, they never even saw them. They used serum samples that the National Center for Health Statistics had earlier collected as part of its Third National Health and Nutrition Examination Survey (NHANES III). The authors state they used 1,479 samples that were still available out of an initial total of 1,998.
Uh, why were 519 samples no longer available? That comes to over 25% of the original total. What happened to them?
After some googling, I discovered that another study had used the NHANES-III serum bank for research on a sexually transmitted disease: Herpes Simplex virus type 2 (HSV-2).
The National Health and Nutrition Examination Surveys (1988–1994) (NHANES-III) reported that more than 25% of adults between 30 and 39 years of age were positive on serology for HSV-2 in those years. (Chorba et al., 2007, p. 655)
More than 25% ... Hmm, could these be the missing samples we’re looking for? If so, removing them would have disproportionately depleted the black American component of the study population:
Although HSV-2 infection is increasing among young Caucasians, who have a seroprevalence of approximately 17%, infection is more common among African-Americans, who have a seroprevalence of 45%. (Chorba et al., 2007, p. 656).
The study is written up in Fleming et al. (1997). The ‘Methods’ section describes why some serum samples were no longer available, including a “need to use serum for other tests”:
Of the persons originally selected for NHANES III, 82.5 percent were interviewed, and HSV-2 test results were available for 60.2 percent. The reasons that results were unavailable included the inability to locate the selected subject, refusal by that person to be interviewed or to have blood drawn, unsuccessful venipuncture, the need to use serum for other tests, and the loss of serum samples during transportation, storage, or processing.
When any serum samples tested positive for HSV-2, they were probably set aside for the duration of the study, with a view to further analysis. If, as seems likely, these samples were absent from the NHANES III serum bank when Rohrmann et al. undertook their study, their study population would have had disproportionately fewer participants with a polygynous sexual orientation and with correspondingly high testosterone levels. As Chorba et al. (2007, p. 687) point out, “The major known risk factors for acquiring genital HPV infection include having multiple sex partners and having sex partners who have had multiple partners.”
This data bias would explain Rohrmann et al.’s paradoxical finding of higher t levels in older black Americans. Young black Americans with high t levels were likely to test positive for HSV-2 and to have their samples removed from the NHANES III serum bank.
Chorba, T., Guoyu, T., and Irwin, K.L. (2007). Sexually transmitted diseases. In: Litwin MS, Saigal CS, editors. Urologic Diseases in America. US Department of Health and Human Services, Public Health Service, National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases. Washington, DC: US Government Printing Office, 2007; NIH Publication No. 07–5512 [pp. – 649-695]
Fleming DT, McQuillan GM, Johnson RE, Nahmias AJ, Aral SO, Lee FK, St Louis ME. (1997). Herpes simplex virus type 2 in the United States, 1976 to 1994. New England Journal of Medicine, 337, 1105-11.
Rohrmann, S., Nelson, W.G., Rifai, N., Brown, T.R., Dobs, A., Kanarek, N., Yager, J.D., Platz, E.A. (2007). Serum estrogen, but not testosterone levels differ between Black and White men in a nationally representative sample of Americans. The Journal of Clinical Endocrinology & Metabolism, 92, 2519-2525