﻿Eye Shape Differences Between Genders There is a clear difference between the
features of the eye for men and for women, and this difference can be
used to maximise the aesthetics of the face of an individual from either gender.
Eyes are a crucially defining element of an individual,
with recognition of even famous celebrities being much harder without reference to the eyes.
Throughout popular literature, eyes are constantly referred to as a concept of beauty, power, and
even mystery, from Van Morrison’s Brown Eyed Girl, to Shakespeare’s famous quotes. With that in mind,
it is important to conceptualize the eyes fit into our wider understanding of facial aesthetics.
Today we will work to deeper understand some of the mystery behind the eye, specifically,
the shape and size of eyes, the differences in this shape and size between males and females,
and the role of eye features on aspects of facial attractiveness. Throughout the video,
I will refer to ‘ocular features’, to describe features relating to the eye,
as ocular is the scientific term meaning “relating to the eye”. Sexual Dimorphism is the phenomenon
in which males and females from the same species have systematic differences in the size and shape
of certain features of their body, beyond the sexual organs. Some examples of sexual
dimorphism in humans include facial hair, height, skull shape, jaw and the list goes on.The roots
of sexual dimorphism come from Darwinism, who was a pioneer in recognising marked differences
between the same species. Most importantly for today, extensive scientific and sociobiological
research has shown that sexual dimorphism extends into the ocular morphology of humans, that is,
that men and women have marked differences in the shapes and sizes of their eyes.
To explore the sexual dimorphism of ocular features, or the differences in eye shape between
genders, we will look towards a European study by Darius Daniel and colleagues, published only last
year. First, facial photographs were taken from 100 European participants, with 50 men, and 50
women, all between the ages of 19-36 used to be analysed. Importantly, to appropriately measure
eye shape and size, as well as sexually dimorphic features, no facial cosmetics, or even facial
expressions, were allowed. To measure the ocular morphology of the eye, 3 different measurements
were used. First, a ‘width to height ratio of the eye outline’, or WHR measurement was used.
This measured the width between one corner of the eye (closer to the ear) to the other corner of
the eye (closest to the nose), seen here by the blue dotted line, and compared it in a ratio to
the height of the eye, measured as the longest possible distance between the bottom eyelid and
the top eyelid, shown here as a solid red line. Next, a ‘Sclera Size Index’, or an SSI, was used.
The SSI is another ratio, this time comparing the width of the exposed eyeball, shown in the
diagram as a dotted orange line, in ratio to the Diameter of the Iris, shown in the diagram as a
solid horizontal yellow line. For the purpose of all scientific measurements, the Iris is the thin
structure around the pupil of the eye that gives an eye its characteristic, distinct colour.
The final measure used to define the ocular morphology was the overall surface of the visible
sclera, represented as the ‘Sclera Surface Ratio’, or SSR, A ratio defined as the total surface of
the visible eyeball, as measured by pixels on a computer screen, represented here as pink shading,
compared to the surface of the iris and the pupil, represented by the green shading. The pink
dots within the green shading here represent that there is overlap, such that wherever the iris and
pupil surface area are being measured, so is the overall eyeball surface area being measured. On
top of all of these measurements, a luminance measurement, or effectively, a brightness contrast
measurement, was used to test the difference in brightness between the whites of the eyes,
known as the sclera, and the iris. This measurement was referred to as a ‘relative Iris
luminance’ or RIL. An example of a high contrast between sclera and Iris can be seen here.
Now, before we get onto the (exciting) slog of statistics that will objectively prove
the differences between men and women’s eye shape, make sure to li ke and subscribe,
and comment what you think about this topic. Ok, so back to stats!
With all these measurements in mind, the data was collated and compared to check
whether there was a statistically significant difference in the shape and size of the eye for
men compared to women. Looking at the descriptive statistics, we can see that all
the data was normally distributed, shown here by non-significant p-values from a Shapiro wilk test,
that is, p-values that are not special, defined as an p-value greater than 0.05, (and are hence
normal). Also, just by eyeballing the descriptive statistics, we can see that men tend to have a
greater average Sclera Size Index, Sclera Surface Ratio and a greater Width to Height ratio of eye
outline when compared to women. The same data also shows us that women will have a greater average
Relative Iris Luminance when compared to men. However, to see whether these differences in
eye shape and size are scientifically significant and not simply due to chance brought about by the
small sample, a comparative statistical ‘t-test’ needed to be done. Looking at this table, any
p-value under 0.05, will be seen as statistically significant. This is the opposite to what we were
looking at before with the normally distributed data, as now we want a special, abnormal result,
as that would signal that we have a special effect in the data. As you can see,
all of the eye measurements have significant differences between men and women, except the
Sclera Surface Ratio. When we compare the male and female averages for these significant results,
we can infer that men have a greater Sclera Size index compared to women, and a greater Width to
height ratio than women. On the other hand, women have a greater Relative Iris Luminance than men.
The final thing to look at with this analysis of ocular sex differences is the effect size,
whereby a number closer to 1 indicates a stronger effect. As can be seen from this part of the
table, the Width to Height Ratio of eye outline shows the strongest effect, meaning the most
prolific difference for men compared to women. All of this information can be re-understood through
another interpretation of the data, whereby the green slope refers to women’s measurements,
and the red slope refers to men. Slopes further to the right indicate a larger measurement.
Now, this paper, and our video, has proven that there is sexual dimorphism in the shape and size
of eyes for the two sexes. Further analysis by the paper attempted to see if these differences
between men and women have been due to generations of sexual selection for dimorphic eyes. In other
words, over time, have our eyes adapted to be distinct for our gender, because that was seen as
more attractive to potential partners. The paper did this by attempting to correlate the scleral
characteristics found for men and women to widely acknowledged markers of facial beauty, that is,
ways of measuring facial beauty that are accepted all throughout the field of facial aesthetics.
These facial beauty markers were individual expressions of facial sexual dimorphism and
averageness. As expressed by the author of the paper, “neither averageness nor facial
sexual dimorphism was correlated with the examined characteristics of eye morphology.” In easy terms,
sexual dimorphism for eye shape and size are not due to a sexual preference for dimorphic eyes,
but are perhaps due to non-adaptive processes which have developed over time.
As is always important at QOVES studio, it is important not to attain a eurocentric bias by only
focusing on a caucasian sample. If we look towards the discussion of this paper, references can be
made to earlier studies by Hajnis and colleagues who developed their own eye measurement scale to
quantify ocular morphology. This eye measurement scale, known as the ‘eye fissure index’ measures
the height and width of the eye, in a measurement scale very comparable to the WHR scale that is
used all throughout this paper. Most importantly, Hajnis studied differences between genders in a
range of populations, including African-American’s and Chinese demographics. Results show that
for all 3 demographics, men have significantly wider eye-fissures compared to women, which is
a statistic that is further evidence towards the finding found from this paper. While this
result shows elements of generalisability from the Daniels study, it was noted that in Hajnis’ study,
the effect was greatest for Caucasians. I believe the field of cross-cultural sexual dimorphism for
eye shape would be further improved by even more studies like Hajnis’ one, really attempting to
scientifically quantify gender differences in eye shape and size for different cultures.
Overall, The research has shown that there are gender differences in the shape of eyes,
yet have analysed that these differences that have emerged over time are not due to sexual selection.
An important taking from this finding is that, as mentioned in a previous video, sexual dimorphism
is a symbol of attractiveness, and recognising sexual dimorphism in eye shape allows us to
infer how the eye can be shaped to maximise dimorphism and hence facial attractiveness.