Is Finger Length Ratio Evidence Of A Woman’s Fidelity?

Here’s a very thorough and illuminating study into digit ratio (a subject previously covered on this blog) that adds a new angle: the relation of female to male digit ratio by ethnicity, and what it could mean about current human evolution. The study’s findings also have some interesting implications for pickup and game, and for male and female reproductive success and health in general.

This is a Big Info post, so I suggest you read the study at the link provided to get the full impact of its conclusions. Bonus: your diligence will help keep the comments free of non sequitur-ish clutter. To keep things straight in your head, it helps to remember that low digit ratio (a shorter index finger than ring finger) corresponds to exposure to higher levels of testosterone in the womb, and a high digit ratio corresponds to higher estrogen exposure in the womb. Prenatal hormone exposure influences the development of the brain, and thus the personality and behavioral inclinations of the adult.

The 2nd:4th digit ratio, sexual dimorphism, population differences, and reproductive success: evidence for sexually antagonistic genes? […]

We report data on the following. (a) reproductive success and 2D:4D from England, Germany, Spain, Hungary (ethnic Hungarians and Gypsy subjects), Poland, and Jamaica (women only). Significant negative associations were found between 2D:4D in men and reproductive success in the English and Spanish samples and significant positive relationships between 2D:4D in women and reproductive success in the English, German, and Hungarian samples. The English sample also showed that married women had higher 2D:4D ratios than unmarried women, suggesting male choice for a correlate of high ratio in women, and that a female 2D:4D ratio greater than male 2D:4D predicted high reproductive success within couples. Comparison of 2D:4D ratios of 62 father:child pairs gave a significant positive relationship. This suggested that genes inherited from the father had some influence on the formation of the 2D:4D ratio. Waist:hip ratio in a sample of English and Jamaican women was negatively related to 2D:4D. (b) Sex and population differences in mean 2D:4D in samples from England, Germany, Spain, Hungary (including ethnic Hungarians and Gypsy subjects), Poland, Jamaica, Finland, and South Africa (a Zulu sample). Significant sex and population differences in mean 2D:4D were apparent.

It has been known for some time that the ratio between the length of the 2nd and 4th digits (2D:4D) is a sexually dimorphic trait (Baker, 1888; George, 1930). In general, mean 2D:4D has been found to be lower in men compared to women (Phelps, 1952). The differentiation of the digits is under the control of Homeobox or Hox genes (the posterior-most Hoxd and Hoxa genes), which also control the differentiation of the testes and ovaries (Peichel et al., 1997; Herault et al., 1997).

Some people don’t like it when I use the term biomechanics in discussions of love and sex and men and women. They complain it’s reductionist, that I’m somehow violating a cosmic rule by analyzing the component parts of relationship dynamics and the sexes. I think studies like this must give them the hives, for every time one of these babies drops in the lap of intellectual debate, my reductionist worldview is further confirmed. Hey, I love poetry and starlight gazing and flutters of the heart as much as the next romantic sucker, and happily wallow in it, but I also love the truth. Especially when it has real world implications for my own life.

A correlate of maternal levels of testosterone and estrogen is waist:hip ratio (WHR). Women with low ratios have low testosterone and high estrogen, while women with high WHR have high testosterone and low estrogen (Evans et al., 1983). The WHR of women has in turn been found to correlate with the 2D:4D ratio of their children, i.e., women with low WHR have male and female children with high 2D:4D, and mothers with high WHR have low 2D:4D ratio children of both sexes (Manning et al., 1999). We argue that 2D:4D may be a marker for sexually antagonistic genes (Rice, 1996a, 1996b; Rice and Holland, 1997) that exert their effects prenatally.

I’m building up to a blockbuster hypothesis, so read the above again. Women with low WHRs, (that is, women with sexy hourglass figures) give birth to boys and girls with more feminine digit ratios — they pass their femininity on to their kids of either sex — and women with high WHRs (more boyish figures) give birth to more masculine sons and daughters. Scientists refer to this prenatal process as the actions of sexually antagonistic genes, meaning genes that are good for sons are bad for daughters, and vice versa.

On the one hand, low 2D:4D may indicate prenatal exposure to high testosterone and low estrogen levels, a situation that enhances fertility in males but reduces it in females. On the other hand, high 2D:4D ratios may correlate prenatally with low testosterone and high estrogen and be associated with low fertility in males and high [fertility] in females.

Digit ratio means something. It has real ramifications for your reproductive success, if this study is on the right track. Naturally, men and women don’t go around examining each others’ hands for mate suitability. Rather, we are attuned to more conspicuous behavioral and physical characteristics which act as a proxy for the genes that influence mate value and, by extension, digit ratio.

(Though it should be noted that there is such a thing as objectively appealing feminine and masculine hands. “Manhands” — hi Sandy! — are unattractive to most men, and strong bear claw hands on men are attractive to most women.)

I’m going to skip through a lot of tables and analysis on marriage rates, fecundity and digit ratio to get to the game-relevant meat of this study. (If you’re interested, some data I’m passing over for discussion include the results that male digit ratio had no effect on male marriage rate, but female digit ratio did have an effect — women with higher ratios (more feminine) were more likely to get married. Fecundity — large family size — was also positively correlated to high female digit ratio and low male digit ratio.)

We do not argue that the 2D:4D ratio is important mechanistically or as a display trait in mate choice. Most probably it affords us a window into prenatal hormonal conditions. […]

Our results indicate significant differences in mean 2D:4D between populations and confirm that the trait is sexually dimorphic. There was also a trend for 2D:4D to be negatively related to reproductive success in males (the English and Spanish samples) and positively in females (the English, German, and Hungarian Caucasian and Gypsy samples). There are many cultural and biological factors that intervene between fertility and reproductive success. Perhaps the most important is the fertility of the long-term partner. In an English sample we found high reproductive success in partnerships in which the male had lower 2D:4D than his partner and low reproductive success when 2D:4D was higher than that of his partner. When English and Jamaican data were pooled, there was some evidence of a weak negative association between WHR and 2D:4D in women. […]

The 2D:4D ratio is negatively related to testosterone and to sperm numbers (Manning et al., 1998). Our Finnish sample had very low male 2D:4D, and it is known that Finnish men have sperm counts that are nearly double that of men worldwide (Suominen and Vierula, 1993; Vierula et al., 1996).

Go Finns! The intra- and interethnic and interracial comparisons of 2D:4D digit ratio in men and women are the most interesting part of this study. The graph below is chock full of potential insights into ethnic differences in sexual behavior.

Poles of both sexes (on the far left) have the highest overall digit ratios in this sample, and Finnish men (far right) the lowest. Poles also show the least within-sex variance in ratios, and Finns and Hungarians the most. Intersex comparisons show that Polish women and men have nearly identical digit ratios and variance, and Finnish women are significantly more feminine relative to their male co-ethnics. Steady on, because this is leading to something.

We suggest the following model. Consider a man who has had high testosterone and low estrogen exposure in utero, i.e., he has a low 2D:4D ratio. It would be of advantage to him if his sons shared these characters. They may therefore make many grandchildren to him. However, what of his daughters? High testosterone and low estrogen could compromise the development of their reproductive system and therefore reduce their fitness. Similarly, a woman with low testosterone and high prenatal exposure to estrogen may produce fertile daughters but low-fertility sons. In such a situation, modifiers of genes controlling sex-limited prenatal testosterone and estrogen exposure may arise and spread. Eventually, we may expect complete sex dependence to characterize the expression of genes that influence prenatal hormonal levels.

The distribution of the 2D:4D ratio shows a high degree of overlap between males and females. This suggests that sex-limited expression is incomplete. Why is this so? Sex limitation is a complex adaptation, involving the evolution of sex-specific regulatory sequences (Rice, 1996a). It will therefore evolve slowly.

The fact that male-female digit ratios for many ethnic populations overlap and thus disadvantage the opposite sex children of the dominant gene expression suggests, tantalizingly, that evolution on these sex-dependent genes is in an incomplete stage. That is, we humans are a sputtering work in progress, and our current beta testing blueprint is rife with bugs and unintended algorithms. In his image, my ass.

Now we may begin the impolite hypothesizing. Are Finns, by virtue of their non-overlapping intersex digit ratios, a more evolved race than Poles? Not so fast. The study authors offer an alternative explanation to incomplete evolution.

However, other things being equal, it will eventually evolve. So do we simply need more time or are there other factors operating here? One possibility is the occurence of cycles of intragenomic conflict. Males, because they produce low-cost sperm, are able to fertilize many eggs. Females, because they produce high-cost ova, are limited to smaller numbers of offspring. In populations with polygyny or frequent extra-pair copulations (EPCs), the variance of male reproductive success is high. That is, a small proportion of successful males may fertilize a high proportion of eggs. When strict monogamy is practiced by most females, the variance in male reproductive success is similar to that of females. In such a situation, polygyny or EPCs may be a successful female strategy if there is substantial heritable variance in male fitness. If there is little such variance, female monogamy would be favored.

Suppose there are two loci controlling in utero hormonal exposure: one influencing testosterone levels and the other estrogen. A mutation arises at the testoster- one locus of a male, which increases in utero exposure. He has high testosterone levels and sperm counts, and these traits are passed on to his sons. However, because sex limitation is incomplete, his daughters have reduced fertility. The existence of such a male or small numbers of such males increases the variance in male fitness and favors a polygynous or EPC strategy in females. The high testosterone mutation will spread and with it the frequency of polygyny or EPCs. However, as the mutation becomes common, the variance in male fitness declines and females switch to increasing frequencies of monogamy. Now conditions favor the spread of a mutation at the estrogen locus, which increases in utero estrogen exposure. Alternating cycles of high prenatal testosterone and high prenatal estrogen will ccur. This is interlocus coevolution of sexually anagonistic genes. Such coevolution has the characteristics of the Red Queen process (Rice and Holland, 1997). Sexually antagonistic genes should affect fertility and, because of population cycles, may be at different frequencies in different populations. In populations with high prenatal exposure to testosterone in both males and females, there may be substantial differences in the variance in male and female reproductive success.

A negative relationship between 2D:4D and offspring number would be expected in males and a positive association in females. In addition, there will be selective pressure for the accumulation of modifiers that cause sex-limited expression. A population that is highly estrogenized in utero would have no marked difference in variance of male and female repro- ductive success, no strong correlation between 2D:4D ratio and offspring number, and little selective pressure for sex-limited expression of prenatal genes.

There is little variance in Polish male fitness, as judged by their tight gradient digit ratios. Or, to put it another way, Poland is filled with beta males and not too many omegas or alpha male cads. Poland is a place where female monogamy is favored. The fact that Polish men also have relatively high digit ratios suggests that the men are, like their women, more favorable to monogamy.

Now compare that to Finns. Finnish men have a lot of variance in digit ratio, and a very low (masculinized) overall ratio. We can then surmise that Finland is filled with a wide variety of men (relative to their population), from omegas to betas to alphas, who are, nonetheless, more masculine than men from most other ethnic backgrounds. Finnish women would be open to alpha cad flings, cheating and using betas as emotional tampons. Presumably, some Finnish men would be glad to oblige. Both sexes would be less disposed to monogamy.

That is, at least, what a digit ratio hypothesis into sexual behavior differences would tell us. Is it true?

Roosh recently wrote a post about how the Polish women in Poland were much more open to “beta male game” than American women are. He said Polish chicks loved being courted in the traditional sense, didn’t play “I’m the princess, here” games, and were inclined to long term relationships. His experience in Poland precisely matches my experience with Polish chicks. They really are sweeter, more feminine and less interested in short term flings than women from other backgrounds.

And both our experiences with Polish chicks corroborate digit ratio analysis; the high overall digit ratio and low intrasex digit ratio variance of Polish girls predisposes them to LTRs and preferring the company of more attentive, “traditional” men.

My hypothesis then, based on digit ratio analysis, is that in countries where the women have a high overall digit ratio (longer index finger than ring finger) and a low intrasex variance in digit ratio (where most women and men have pretty much the same digit ratio), monogamy will be the preferred relationship norm of the women and aloof alpha male game will need to be distilled with a heavy dose of beta provider game.

I therefore predict, based on the above ethnic comparison graph, that should Roosh go to Finland, he will have to run some seriously hardcore push-pull alpha cad game on the local Finnish women. If my digit ratio hypothesis is correct, he will find Finnish girls to be very similar to coastal city American girls, and very different from Polish girls.

Here’s hoping Roosh takes a detour to Finland and either confirms or falsifies my hypothesis. If he won’t, then maybe I will have to. In the interest of science, of course.