Biocultural evolution suggests that humans place a stronger emphasis on socioemotional processes than biological
factors in regard to sexual selection. Substantial evidence from past studies reinforces the proposition that mating
preferences have a basis in one’s health and well-being. The indication that biological entities prefer to
pass on favorable genes to their offspring can be traced back to Darwin’s theory of natural selection. Consistent
findings have been observed regarding the influence of development on facial symmetry. Research suggests that facial
symmetry reflects developmental stability, indicating how successful one’s genes are in shaping a symmetric
organism despite environmental assaults. In line with previous research, the current research proposal examined
the effect of facial symmetry (i.e., a biological process) and socioeconomic status (i.e., a socioemotional process)
on judgment of perceived health. It was hypothesized that women would judge a man’s health according to his
facial features and annual income. Stimuli consisted of individual faces that were separated into two distinct
groups based on the degrees of fluctuating asymmetry and then were counterbalanced with two levels of socioeconomic
status (i.e., low and high income). Findings indicated the relative importance of both biological and socioeconomic
processes in perceived health. Although the hypothetical results are preliminary, these findings suggest practical
applications for understanding the biocultural evolutionary process and characteristics of natural selection in
As early as the nineteenth century, evolutionary
researchers (e.g., Charles Darwin) have been studying the direct association between physical attractiveness and
health. Such a connection between the mind and body is imperative to the study of sexual selection. Recent studies
have focused on the basis of physical attractiveness judgments, placing little emphasis on the relationship between
physical attractiveness and health (Weeden & Sabini, 2005). Findings thus far have concentrated on morphological
factors (e.g., body size and shape), overlooking the importance of health and non-physical factors, such as socioeconomic
status and attraction. Based on the “good genes” sexual selection theory, Weeden and colleagues explored
the importance of body and facial characteristics in relation to perceived health via the “good genes” theory.
The “good genes” theory of sexual selection suggests that health is necessary for reproduction and
thus genetic propagation. It can be inferred then that mating preferences have a basis in health, because individuals
want to pass only “good genes” to their offspring. One such indication of “good genes” is
the degree of symmetry of an organism. Symmetrical characteristics are thought to reflect the stability of an individual’s
growth development, thereby modulating their perceived physical health judgment.
Past studies revealed the relative importance of facial
asymmetry in regard to perceived health by introducing fluctuating asymmetry. Fluctuating asymmetry describes deviation
from bilateral symmetry, which may reveal developmental instability brought on by genetic and environmental stress. A positive
correlation was found between facial attractiveness based on symmetrical features and perceived health. One important criterion
of sexual selection is the mate’s perceived attractiveness. Hence, a person who is perceived as attractive is also
perceived as healthy (Weeden & Sabini, 2005). To control for selection bias in stimulus appearance and asymmetrical
fluctuations, photographs of monozygotic twins were used to explore the relationship between facial symmetry and perceived
attractiveness. The twins with less fluctuating asymmetry were rated more attractive than their respective counterparts
(Mealey, Bridgstock, & Townsend, 1999).
Evidence further implied a relationship between facial
symmetry and perceived attractiveness using facial abnormalities (Kowner, 1996). It was argued that facial features played
an important role in social interaction across cultures, which in turn facilitated sexual selection (Gallup, Frederick, & Pipitone,
2008). The linkage between facial attractiveness and sexual selection seems to be universal. Facial attractiveness is not
of social but biological construction. Individual differences in facial attractiveness are thought to underlie biological
properties, such as health and well-being. In a series of four experiments, Kowner (1996) explored perceived judgment manipulating
age and facial expression. Facial asymmetry modulated judgments of attractiveness in older people to a greater degree than
it did with children and young adults. Chimeric images, or left-left and right-right mirror-image computer simulations,
of the faces were created and compared to natural faces. The variation due to age most likely resulted from differences
in morphological development as a result of biological and environmental factors. Using photographs with similar age range,
participants were asked to choose the more attractive photograph, one being an individual with an asymmetric smile and
the other a symmetric smile. The results revealed a preference for asymmetric smiles. In the latter experiment, the same
photographs were used but participants were asked to choose the photograph with the more genuine smile. Findings overall
indicated that an asymmetric smile was considered more genuine or natural. Facial asymmetry might be preferred due to cultural
evolution because humans may have begun to place more emphasis on social cues instead of biological ones (Kowner, 1996).
In a similar vein, Singh (1995) studied the influence
of sociocultural and biological cues on judgment of male attractiveness using body shape and financial status. Participants
viewed four individual pictures with a pictorial representation of a specific waist-to-hip ratio contained an occupation
and an annual income. Males with average waist-to-hip ratios and high financial status were judged as most attractive,
suggesting that women choose men who are healthy to ensure propagation of “good genes” and men who are financially
stable to ensure quality parental care. The question of whether social and/or biological cues can influence people’s
perception of health is still unclear. Based on the theoretical assumption that facial symmetry equates physical attractiveness,
the current study replicated Singh’s (1995) findings while incorporating various manipulations obtained from Weeden
and Sabini’s (2005) research on monozygotic twins. The goal was to investigate the degree to which the different
types of facial symmetry and socioeconomic status might modify judgment of perceived health using photographs of monozygotic
Fifty-four undergraduate students were randomly recruited
from Purdue University Calumet campus to participate in the current study. All of the participants ranging in ages from
18 to 35 years had normal or corrected-to-normal visual acuity and were naïve to the purpose of the experiment.
On the Purdue University Calumet campus, 39 percent of the students are identified as ethnic minorities [(16 percent)
African American (1 percent) American Indian, (6 percent) Asian American, (14 percent) Hispanic American, and (2 percent)
Materials and Stimuli
Black and white photographs of two male and
female monozygotic twin pairs were obtained from Weeden and Sabini (2005) and then used as stimuli in the present
study. Photographs were chosen on the basis of clarity and age as well as how clear the person appeared in the
photograph. It was important that the age of the photographed individuals were similar to those of the participants
in order to control for the effect of age differences on their judgment. The process of using the photographs of
monozygotic twins controlled for extraneous components other than facial symmetry related to attractiveness (e.g.,
eye and hair color). Each individual photograph was cropped using Photoshop. Hence, the individual’s head
was in a neutral position and his/her eyes looked straight into the camera lens. Each frame consisted only of the
individual’s head, neck, and partial shoulders centered on the frame (Figure 1).
Figure 1. The photographs used as stimuli in the present study of Twin 1 on the left (A1) and Twin 2 on the right
(B1). Participants were asked to make perceived health judgment. Courtesy of Mealey et al. (1999).
Each photograph was then measured for fluctuating asymmetry
using the overall facial asymmetry (OFA) index method proposed by Kowner (1994). Based on the OFA value calculated, the
twin pairs were then divided into two groups; group A with high facial symmetry and group B with low facial symmetry (Figure
2). Those groups were each divided again for a total of four groups, A1, A2, B1, and B2, so that the twin pairs remained
in the same group number (1 or 2). Finally each of the four groups was arbitrarily assigned a yearly income falling into
one of two categories: low socioeconomic status (yearly income $10,000-40,000) and high socioeconomic status (yearly income
+$40,000). The four categories of stimuli follow: A1 (high facial symmetry/low SES), A2 (high facial symmetry/high SES),
B1 (low facial symmetry/low SES), and B2 (low facial symmetry/high SES).
Figure 2. The landmark points used for measurements
of facial symmetry. Courtesy of Grammer and
The stimulus materials were presented in a booklet,
one for each stimulus category. Each page of the booklet contained a 3”x5” photograph of a twin and the twin’s
assigned yearly income typed below. At the bottom of each photograph, a Likert scale ranging from 1 (least healthy) to
7 (most healthy) was displayed. Below the scale listing, health was specifically defined for the participants as cardiorespiratory
endurance, or how well the entity in the photograph can perform on an exercise stress test.
Design and Procedure
The four categories of stimuli [A1 (high facial
symmetry/low SES), A2 (high facial symmetry/high SES), B1 (low facial symmetry/low SES), and B2 (low facial symmetry/high
SES)] accounted for the 2x2 factorial between-subject design. The independent variable of facial symmetry had two
levels: group A with high facial symmetry and group B with low facial symmetry. The independent variable of socioeconomic
status also has two levels, high socioeconomic status (yearly income +$40,000) and low socioeconomic status (yearly
income $10,000-$40,000). The dependent variable, perceived health, was defined as cardiorespiratory endurance,
or how well the photographed entity can perform on an exercise stress test. The twin pairs were selected and eventually
divided into the four conditions as mentioned previously.
Prior to the testing phase, standard experimental instructions
were read to inform participants of any potential risks and/or benefits associated with the study. They were then given
a booklet containing the four photographs of monozygotic twins, and were asked to rate perceived health of the individual
in each photograph on a Likert scale of 1 (least healthy) to 7 (most healthy), with 4 being neutral. Participants were
required to rate each photograph in the order it appeared in their assigned booklet, and were told to make an educated
guess if they were unsure of their judgment.
A significant main effect was obtained for
facial symmetry, F (1, 208) = 5.95, p <. 02, h2 = .03. High facial symmetry had significantly
higher rating scores (M = 4.49, SD = 1.56) than did low facial symmetry (M = 4.02, SD = 1.63) (Figure
3). A significant main effect of socioeconomic status (SES) was also found, F (1, 208) = 52.14, p <.
001, h2 = .20, indicating that subjects of high SES (M = 4.95, SD = 1.41) had higher rating scores than
subjects of low SES (M = 3.56, SD = 1.49) (Figure 4). These significant main effects were coupled by an
interaction between facial symmetry and socioeconomic status, F (1, 208) = 8.57, p < .01,
.04. Regardless of their facial symmetry, twin individuals with high socioeconomic status received higher rating
scores in perceived health than did their counterparts of low socioeconomic status. The lowest rating was found
for the low facial symmetry and low salary condition, which suggested the relative importance of physical appearance
and SES on perceived health judgment (Figure 5).
Figure 3: The influence of facial symmetry on perceived health judgment based on a Likert scale ranging
from 1 (least healthy) to 7 (most healthy).
Figure 4: The influence of socioeconomic status on perceived health judgment based on a Likert scale
ranging from 1 (least healthy) to 7 (most healthy).
Figure 5: The influence of facial symmetry and socioeconomic status on perceived health judgment based on
a Likert scale ranging from 1 (least healthy) to 7 (most healthy).
The purpose of the present study was to examine
the effect of facial symmetry and socioeconomic status on perceived physical health. Results revealed the relative
importance of both biological and socioeconomic factors on perceived health judgment, supporting Kowner’s
(1996) theory and previous findings (Weeden & Sabini, 2005; Singh, 1995). Through cultural evolution, humans
may have begun to place more emphasis on social cues instead of biological ones during the sexual selection process.
Our findings overall indicated that both morphological
factors and social factors played a key role in the perception of one’s health. When applying these findings to theories
of sexual selection, it could be inferred that health has biological characteristics as well as social ones. Humans consider
both biological and social factors as a primary indication of health, which is necessary for reproduction and thus genetic
propagation. As Singh (1995) suggested, mates choose mates who are healthy to ensure propagation of “good genes.” According
to this theory, it then can be inferred that these “good genes” are exemplified both in facial symmetry and
in socioeconomic status. These “good genes” are thought to produce the fittest offspring.
Future research should manipulate the specific differences
in facial asymmetry to adequately control for the many distinct features of facial symmetry. If variation in asymmetry
was minimal, subjects might turn to other random factors (e.g., mouth shape) to judge a person’s health (Grammer,
Fink, Moller, & Manning, 2005). Hence, it would be interesting to examine whether or not people consider facial features
to have a socioeconomic rather than genetic basis. What other morphological factors and social factors play a role in sexual
selection? Are our judgments of perceived health constant through all ages and cultural backgrounds? Although our findings
are preliminary, the present research appears to shed light on the importance of human biological-cultural evolutionary
process with respect to sexual selection.
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