Saturday, October 18, 2014

Gender equality and gene-culture co-evolution

The ratio of index finger length to ring finger length provides an index of sexual differentiation (source: Wikicommons)


Are men and women more alike in some populations than in others? It's possible. First, boys and girls differentiate from each other to varying degrees during adolescence, and this process of sexual differentiation is genetically influenced. There are even conditions, like Swyer syndrome, where an individual is chromosomally male (46, XY) and yet develops externally into a woman.

Second, men and women don't have the same sex roles everywhere. According to a survey of 93 nonindustrial cultures, men were expected to dominate their wives in 67% of them, the sexes were expected to be about equal in 30%, and women were expected to dominate their husbands in 3% (Whyte, 1978). Sex roles differ to varying degrees even among hunter-gatherers, who correspond to the earliest stage of cultural evolution. In the tropics, women provide more food through gathering than men do through hunting. The reverse is true beyond the tropics, where women have few opportunities to gather food in winter (Kelly, 1995, pp. 128-132; Martin, 1974, pp. 16-18).

There has thus been a potential for gene-culture co-evolution. Wherever men and women behave more alike, natural selection will tend to level any innate behavioral differences between them. This can come about in several ways, but a particularly common one is to reduce the sex difference in prenatal hormonal exposure, i.e., the ratio of testosterone to estrogen in the uterine environment of the developing fetus.

We have a "handy" way to measure this prenatal influence. It's called the digit ratio: the length of your index finger divided by the length of your ring finger. The lower your 2nd digit to 4th digit ratio (2D:4D), the more you were exposed to testosterone in the womb and the less to estrogen.

English psychologist John T. Manning has pioneered the use of this digit ratio as a way to measure how prenatal male and female hormones influence various behavioral traits. In a recent study, he looked at how prenatal hormones might influence gender equality in different populations. After measuring the digit ratios of participants from 29 countries, his research team averaged the score for each country and compared it with indices of gender equality: women's share of parliamentary seats; women's participation in the labor force, women's education attainment level; maternal mortality rates; and juvenile pregnancy rates. To ensure comparability, all of the participants were of European descent.

The results?

In short, the more similar the two sexes were in 2D:4D, the more equal were the two sexes in parliamentary and labor force participation. The other variables were not as strongly correlated. (Manning et al., 2014)

In general, women from Northwest Europe have more masculine digit ratios, whereas women from farther east and south have more feminine digit ratios. This geographical trend is more pronounced for the right hand than for the left hand. Since the right-hand digit ratio is associated with social dominance, Northwest Europeans may be less sexually differentiated for that particular trait, as opposed to being less sexually differentiated in general.

Presumably, this isn't a new tendency. Women must have been more socially dominant among Northwest Europeans even before the late 19th century and the earliest movements for women's suffrage. So how far back does the tendency go? To medieval times? To pre-Christian times? It seems to go back at least to medieval times and, as such, forms part of the Western European Marriage Pattern:

The status of women differed immensely by region. In western Europe, later marriage and higher rates of definitive celibacy (the so-called "European marriage pattern") helped to constrain patriarchy at its most extreme level.

[...] In eastern Europe however, the tradition of early and universal marriage (usually of a bride aged 12-15 years, with menarche occurring on average at 14) as well as traditional Slavic patrilocal customs led to a greatly inferior status of women at all levels of society. (Women in the Middle Ages, 2014)

Does this geographic tendency go back to pre-Christian times? There is little consensus on this point, as noted in a study of women in Old Norse society:

The conversion of Iceland raises the problem of the impact of Christianity on the female half of the human race. This, in fact, is one of the most controversial issues in women's history. One point of view argues that Christianity was deeply imbued from the beginning with Jewish and Roman patriarchy, which became intensified by an all-male clergy and resulted in misogyny as the most lasting and profound legacy of Christianity for women. An opposite argument claims that the Christian message was fundamentally a liberating force that included women as well, and although the original radicalism of Jesus on this issue, as on so many others, became diluted with time, women were better off during the Christian period and in Christian countries than they had been before and elsewhere. (Jochen, 1995, p. 2)

Perhaps both arguments are true. As I have argued elsewhere, there may have been a "fruitful encounter" between Christianity and pre-existing behavioral tendencies in Northwest Europe, the result being a significantly different form of Christianity (Frost, 2014).


Frost, P. (2014). A fruitful encounter, Evo and Proud, September 26

Jochens, J. (1995). Women in Old Norse Society, Cornell University Press.

Kelly, R.L. (1995). The Foraging Spectrum. Diversity in Hunter-Gatherer Lifeways, Washington: Smithsonian Institution Press. 

Manning, J.T., B. Fink, and R. Trivers. (2014). Digit ratio (2D:4D) and gender inequalities across nations, Evolutionary Psychology, 12, 757-768.

Martin, M.K. (1974). The Foraging Adaptation - Uniformity or Diversity? Addison-Wesley Module in Anthropology 56. 

Women of the Middle Ages. (2014). Wikipedia 

Whyte, M. K. (1978). The status of women in preindustrial societies, Princeton, NJ: Princeton University Press.


Anonymous said...

Don't forget the female right of property, divorce, and shieldmaiden-hood.

Anonymous said...

You've misspoken. It's the other way around. For men, the index finger is SHORTER than the ring finger, resulting in a LOWER 2D:4D ratio.

Juoni said...

In general, women from Northwest Europe have more masculine digit ratios,

How does Finland fit into this since it's in Northeast Europe and on the "wrong" side of the Hajnal line?

Yet Finns seem to be more like Northwesterners at least in this sense.

Peter Fros_ said...

First and fourth commenters,

Yikes! thanks for telling me.

2nd and 4th,

Yes, I found references to shieldmaidens, but I wasn't sure whether the references were mythical (they were made by later authors from the Christian period).


The Hajnal line runs from Trieste to St. Petersburg (approximately). So Finland is north of that line.

Juoni said...

The Hajnal line runs from Trieste to St. Petersburg (approximately). So Finland is north of that line.

According to Wikipedia Finland is concidered to be outside of the
Northwesteuropean marriage mattern.

Anonymous said...

Starting at 2 minutes, the following video analyzes the hands and 2D:4D ratio of Michelle Obama. She has a very low 2D:4D ratio. Her ring finger seems to be longer than her index finger.

Peter Fros_ said...


James Foreman-Peck wrote a paper "The Western European Marriage Pattern and Economic Development" (2009), and he found that Finland tended to cluster with the WEMP. In 1900, for instance, 68% of women were single in the 20-24 age group and 40% were still single in the 25-29 age group.

Luke Lea said...

You are really a remarkable writer/researcher. I hope you get the recognition you deserve.

Robertus said...

How do you interpret studies showing Caucasian women have lower measured estradiol levels than African American and Asian American women?
("Pinheiro 2005 Racial Differences in Premenopausal Endogenous Hormones")
This contradicts earlier studies from decades ago showing higher levels, and contradicts the higher digit ratios of Caucasians compared to the other two groups.

Peter Frost said...

Hi Robertus,

Several points:

- Pinheiro et al. (2005) looked at estradiol levels in adult premenopausal women. Digit ratios are determined primarily by sex hormone levels before birth, this being more true for the right-hand ratio than for the left-hand ratio.

- Pinheiro et al. (2005) found higher estradiol levels in East Asian women than in European women. This finding contradicts the findings of two earlier studies. It is also inconsistent with the lower breast cancer rate of East Asian women. I believe that more work is needed on this point.

- Digit ratios are a product of estrogen levels and androgen levels. Although African women have higher estrogen levels at birth, they also have higher androgen levels, see:

Agurs-Collins, T., Rohrmann, S., Sutcliffe, C. et al. Racial variation in umbilical cord blood sex steroid hormones and the insulin-like growth factor axis in African-American and white female neonates. Cancer Causes Control 23, 445–454 (2012).