Eye colors (R.A. Sturm, University of Queensland)
Estrogen seems to favor the expression of non-black hair and non-brown eyes during fetal development. The “new” hair and eye colors are not only more frequent among women but also associated, in the case of blue eyes, with feminization of male face shape, female shoulder width, and female waist-to-hip ratio ... and with shyness in young boys.
Europeans
have a surprising variety of hair and eye colors. Their hair is not only black
but also brown, flaxen, golden, or red. Their eyes are not only brown but also
blue, gray, hazel, or green (Frost, 2006; Frost, 2022). This
differentiation from the original black hair and brown eyes seems to have begun
among women and gone farther among them.
Hair
color - Women more often have the new hair colors, particularly red and
blond. Conversely, their hair is less often black—three to five times less
often. This sex difference is natural (Hysi et al., 2018; Shekar et al.,
2008). Among Czechs, 19% of women and 11% of men have the highest gradation of
hair redness (Frost et al., 2017).
Eye color - Women more often have the new eye colors, particularly green and hazel (Frost et al., 2017). Conversely, their eyes are less often brown. The first new eye color seems to have been blue, which then differentiated to create gray, green, and hazel. Thus, “blue” in its narrow sense has lost ground among women to the derived variants of green and hazel.
Population frequencies of eye colors, for men and women (Frost et al., 2017)
The
new hair and eye colors are unusual in two ways. First, they are brighter than
the original black and brown. They thus reflect more light and have a higher
chance of standing out against the visual landscape. Second, they are
“purer”—they occupy thinner slices of the visible spectrum than the original
black and brown. In nature, pure colors are typically found in situations where
an animal or a plant has to catch attention, such as to get pollinated, to warn
predators, or to attract a mate.
This
need for attention may explain how a single hair or eye color evolved into a
diverse palette of hues. A color gets noticed not only for its brightness and
purity but also for its novelty. The last quality is frequency-dependent. If a
noticeable color becomes too frequent in a population, it thereby becomes less
noticeable and, hence, less novel. The desire for novelty is now reoriented
toward less frequent colors, including those that have recently appeared
through mutation. Thus, over successive generations, the population will
accumulate more and more color variants. This is likely how hair and eye color
became polymorphic (see Note #1).
Again,
the evidence seems to point to women being the main target of this selection
for brighter, purer, and more novel colors. One piece of evidence is the higher
frequency of the new hair and eye colors in the female population. Another is
the role of estrogen in this sex-linkage. The female hormone seems to favor the
expression of non-black hair and non-brown eyes during fetal development.
Red
is the hair color that differs the most in frequency between women and men. Red
hair should therefore be most clearly associated with increased exposure to
estrogen during fetal development. This hypothesis is supported by the higher
incidence of estrogen-dependent diseases in redhaired women. According to a
health survey of over seven thousand people, male redheads are as healthy as
other men, doing better on average in three categories and worse in three.
Female redheads, however, do worse on average than other women in ten
categories and better in only three. They are especially prone to four types of
cancer: colorectal, cervical, uterine, and ovarian—three of which are
estrogen-dependent (Frost et al., 2017). Being both female and red-haired
therefore generates the highest level of risk for estrogen-dependent diseases,
probably because of the combined effect of these two risk factors.
In
sum, the new hair and eye colors were favored by a selection pressure that
acted primarily on European women, with European men acquiring them as a
side-effect (since the new alleles are only partly sex-linked). The selection
was specifically for eye-catching qualities—brightness, spectral purity, and
relative novelty.
This
looks like sexual selection, but why would women have a greater need to get
noticed on the mate market? Usually, it is the other way around, both for
humans and for mammals in general. Females are less available for mating
because of the limitations of pregnancy, lactation, and early infant care.
Conversely, males are more available, and thus often have more than one mate at
any one time. That was, in fact, the situation of most humans in prehistory.
But that situation changed as they expanded their range out of the tropics and into
more seasonal environments. At higher latitudes, proportionately fewer men were
available for mating at any one time. There were two reasons:
·
Polygyny
was more costly for men. With men specializing in hunting and women in
gathering, women became dependent on men during winter—since there was little
food to be gathered. Men thus had to bear a greater share of food provisioning,
with the result that polygyny became impossible for all but the ablest hunters.
·
Death
rates were higher for men than for women. Because men had to hunt for more food
and over longer distances, they suffered a higher death rate at younger ages.
They were thus fewer in number overall.
Male
scarcity was most acute in an environment that no longer exists: the
steppe-tundra of the last ice age, essentially the vast plains stretching from
the Baltic to western Siberia. That environment supported large herds of
reindeer and other herbivores, which could in turn support a large human
population. But at a cost: women depended almost entirely on their hunting
husbands for food, and those hunters had to cover long distances without
alternative food sources, thus risking death from starvation or exposure. The
result was an imbalance in the operational sex ratio: too many women for too few
men, and strong selection for women with eye-catching features (Frost,
2006; Frost, 2022; Frost, 2023).
Proposed study
The
aim here is to determine whether the ratio of estrogens to androgens in fetal
tissues influences the development of hair and eye color. One way would be to
measure the “digit ratio”—the length of the index finger divided by the length
of the ring finger. This measure of fetal exposure to the sex hormones is
relatively inexpensive, though disputed by some researchers. The lower your digit
ratio, the more you have been masculinized by androgens during fetal
development; the higher your digit ratio, the more you have been feminized by
estrogens during fetal development. The left-hand digit ratio is associated
with prenatal and postnatal exposure to the sex hormones. The right-hand ratio
is associated much more with prenatal exposure (see Note #2).
An
unpublished study, using a sample of 644 British participants, found that the
left-hand digit ratio was significantly higher on average among individuals
with blond hair than among those with brown, red, or other hair colors. For eye
color, there was a similar but weaker relationship: the left-hand digit ratio
was higher on average among individuals with blue eyes than among those with
other eye colors.
That
study was not published because of two objections from the referees: hair
dyeing could not be excluded as a possible factor; and identification of hair
and eye color was too subjective. Yet it is difficult to see how hair dyeing or
misidentification can explain the digit ratio differences. Such methodological
problems would introduce more noise into the data and make any differences less
significant.
I
wish to see that study replicated with a more rigorous experimental design,
specifically a larger sample and narrower age range. Age interacts with the
effects of the sex hormones, i.e., prenatal effects on hair color are the
opposite of pubertal effects. Whereas women are lighter-haired than men from 17
onward, they are actually darker-haired up to the age of 14 (Steggerda, 1941).
The right-hand digit ratio should thus be better at predicting the darkening of
hair color before puberty, and the left-hand digit ratio better at predicting
the lightening of hair color after puberty.
In
addition, I wish to see whether the relationship between fetal estrogenization
and eye color explains three other relationships between non-brown eyes and
certain behavioral/physical traits:
·
Blue-eyed
boys tend to be shy. This is the “little boy blue” effect. A study of
preschoolers found more social wariness in blue-eyed boys than in brown-eyed
boys. The difference was greatest at the extremes of wariness. Among the very
inhibited boys, 13 out of 14 were blue-eyed. Among the very uninhibited, only 4
out of 10 were. There was no such relationship among the girls, whose eyes were
blue in 5 out of 9 among the very inhibited and in 6 out of 11 among the very
uninhibited (Coplan et al., 1988).
·
Blue-eyed
women tend to have narrower shoulders and lower waist-to-hip ratios. A Latvian
study found small but significant correlations between female eye color and
certain sexually dimorphic features. Shoulders were narrower and waist-to-hip
ratios lower in blue-eyed women than in brown-eyed women (Kažoka and Vetra,
2011).
· Blue-eyed men tend to have more feminine faces. This was an unintended finding of two Czech studies whose participants were asked to rate male and female facial photos. Initially, the brown-eyed male faces were rated as more dominant than the blue-eyed male faces. When, as a control, the brown-eyed faces were photoshopped to make them blue-eyed, they were still rated as more dominant. On careful examination, the originally brown-eyed faces were found to be more masculine with broader and more massive chins, broader mouths, larger noses, larger eyebrows, and closer-set eyes. The originally blue-eyed faces had smaller and sharper chins, narrower mouths, smaller noses, and greater distance between the eyes. Blue eyes were associated with a more feminine face shape only in male participants. This is perhaps because a male fetus normally does not have enough estrogen to feminize the face. If enough estrogen is present to feminize the face, there is probably enough to influence the development of eye color (Kleisner et al., 2010; Kleisner et al., 2013).
Were brown eyes associated with a different face shape because some of the brown-eyed men were partly Jewish or Roma and had a more Mediterranean appearance? In that case, face shape would have been more variable in the brown-eyed men. It was not. This explanation also fails to explain the effect of gender: why were blue eyes associated with facial feminization in men but not in women?
Averaged faces: blue-eyed men (left), brown-eyed men (right), Czech population (Kleisner et al., 2010).
The above studies suggest that the association between the "new" colors and physical/behavioral feminization is largely confined to men. (There is only a weak association between them and shoulder breadth or waist-to-hip ratio). This is probably because the feminization effects are triggered when the estrogen level has risen above a certain threshold. That threshold would already be surpassed by almost all female fetuses.
Notes
1.
Preference for rare hair colors was demonstrated by Thelen (1983), who
showed pictures of attractive women to male participants and then asked them to
choose the one they most wanted to marry. There were three series of pictures:
the first had equal numbers of brunettes and blondes; the second had one
brunette for every five blondes; and the third had one brunette for every
eleven blondes. The scarcer the brunettes were in a series, the more attractive
they seemed, i.e., each brunette had a better chance of being chosen.
Thelen’s
findings were not replicated by Janif et al. (2015), whose male
participants made their choices online, i.e., in private and on their home
computers. There was thus no control over the female images they may have
previously viewed on the same computer screen or might still be viewing on an
alternate screen or split screen. This source of unwanted female imagery
introduces noise into the data, thus increasing the minimum number of online raters
to produce replicable ratings of female facial attractiveness. Devcic et
al. (2010) report that their mean ratings of facial attractiveness did not
become stable until they had recruited 857 online raters. Popenko et al.
(2012) state that they needed a minimum of 992 online raters to achieve
stable ratings. By comparison, Janif et al. (2015) used 658 male
raters, while making their data even noisier by recruiting an ethnically
diverse pool of raters, i.e., over a third were of non-European descent. Those
raters would have tended to perceive female faces with black hair as ethnic
insiders and female faces with non-black hair as ethnic outsiders.
2.
Using a meta-study, Sorokowski and Kowal, 2023) concluded that the digit
ratio indicates only an individual’s prenatal exposure to testosterone (and
only in amniotic fluid, not in core blood). The authors, however, did not look
at the ratio of estrogens to androgens. Their exclusion of data on estrogen
levels is puzzling, since fetal exposure to estrogens is no less important than
fetal exposure to androgens.
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