Race and sex differences in infant digit ratios

This recent study examined 2D:4D digit ratios for 364 infants from birth through the second birthday. (Digit ratios are thought to reflect prenatal exposure and/or sensitivity to testosterone.) Six measures were taken: during the first month, the first and second birthdays, and both the left and right hands at each visit. I averaged the six measures for both boys and girls: Cohen's d for the difference between the boys'and girls' means is .167--a small difference.

The authors gave Cohen's d for the black-white differences for each of the six measures. I averaged them and got .383--a moderate difference with blacks being more masculinized. (The number might be an underestimate because they ignored the fathers and based racial classification on the mother's race alone. Mixed-race babies might have blurred the difference.) Anyway, in this study of prenatal testosterone, the size of the racial gap is more than double the gender gap.

Digit ratios and celebrity worship

A new study in Personality and Individual Differences looked at Korean teenagers and found that girls with more feminine digit ratios are much more likely to worship celebrities. The authors interpret this to mean that celebrity worship is a form of submissiveness seen in feminine girls ("feminine" meaning the girls were feminized by sex hormones prenatally):

The current study tested the hypothesis that digit ratios would be associated with celebrity worship, especially among adolescents. Our sample consisted of 45 male and 61 female participants aged 12–14 years (M = 13.20, SD = 0.71). We measured the digit ratios on the participants’ right hands and used the 23 items of the revised Celebrity Attitude Scale (CAS) to explore the degree to which participants engaged in celebrity worship. The average digit ratio was 0.95 (SD = 0.04) among male participants and 0.97 (SD = 0.05) among female participants. The mean CAS scores were 50.06 (SD = 20.63) for male participants and 64.49 (SD = 18.04) for female participants. The digit ratios for the entire sample were positively correlated with CAS scores (r = 0.29, p < 0.005). However, the digit ratios were positively correlated with CAS scores among female (r = 0.51, p < 0.001) but not male (r = −0.13, p = .394) participants. Our research found evidence of a significant positive correlation between 2D:4D ratios and celebrity worship in females but not in males, which indicated that females with lower digit ratios were less likely to worship celebrities.

I thought it was a sexist society that makes girls but not boys go crazy for rock stars.

Digit ratios among ten populations

As I described before, the ratio of the length of the index finger to the ring finger is thought to measure the balance of estrogen and testosterone an embryo/fetus is exposed to. Shorter relative index finger length indicates masculinization. The graph above is from page 19 in John Manning's Digit Ratio: A Pointer to Fertility, Behavior, and Health (2002, Rutgers University Press). It shows the means for males and females from ten different populations. The graph does not show the results for Lithuanians and Latvians who scored in the mid-90s. (They were not included since subjects' hands were measured differently). 

The trend looks curvilinear with "masculine" populations nearer to the equator or at high latitudes (Finns). The two black populations are masculine. Notice how the population differences are much larger than the sex differences. Jamaican women have more masculine ratios than most of the male European populations.

Racial differences in digit ratio

John Manning and colleagues have used the ratio of the length of the index to the ring finger (2D:4D) as a proxy measure of in utero exposure to relatively more estrogen and less testosterone. In other words, a long index finger compared to the ring finger is an indicator of "feminization" in the womb, and a comparatively short index is a sign of "masculinization."

 It turns out that there are large ethnic differences in the 2D:4D ratio. Manning is quoted as saying that, "There’s more difference between a Pole and a Finn than a man and a woman." According to the results of one study, "The Oriental Han had the highest mean 2D:4D, followed by the Caucasian Berbers and Uygurs, with the lowest mean ratios found in the Afro-Caribbean Jamaicans." In plainer language, Hans were the most feminized and Jamaicans the most masculine. You can read about a long list of characteristics associated with digit ratio here.

You gotta love citing studies that have Robert Trivers as a co-author.