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).