Hi, this is an extract from a chapter from an evolutionary psychology book on dating titled ‘youth’ in section “Standards of Physical Beauty” of “The Evolution of Desire: Strategies of Human Mating” by David M. Buss, can you summarise this section?

Standards of Physical Beauty
A preference for youth is merely the most obvious of men’s preferences
linked to a woman’s reproductive capacity. Evolutionary logic leads to an
even more powerful set of expectations for universal standards of beauty.
Just as our standards for attractive landscapes embody cues such as water,
game, and refuge, mimicking environments beneficial to our ancestors, so
our standards for female beauty embody cues to women’s reproductive
capacity.14 Beauty may be in the eyes of the beholder, but those eyes, and
the minds behind the eyes, have been shaped by millions of years of
human evolution.
Our ancestors had access to two types of observable evidence of a
woman’s health and youth: features of physical appearance, such as full
lips, clear skin, smooth skin, clear eyes, lustrous hair, and good muscle
tone, and features of behavior, such as a bouncy, youthful gait, an
animated facial expression, and a high energy level. These physical cues to
youth and health, and hence to reproductive capacity, constitute key
elements of male standards of female beauty.
Because physical and behavioral cues provide the most powerful
observable evidence of a woman’s reproductive value, ancestral men
evolved a preference for women who displayed these cues. Men who failed
to prefer qualities that signaled high reproductive value—men who
preferred to marry gray-haired grandmothers lacking in smooth skin and
firm muscle tone—would have left fewer offspring.
Clelland Ford and Frank Beach discovered several universal cues that
correspond precisely with this evolutionary theory of beauty.15 Signs of
youth, such as clear skin and smooth skin, and signs of health, such as the
absence of sores and lesions, are universally regarded as attractive. Any
cues to ill health or older age are seen as less attractive. Imagining
passionately kissing a lover’s acne is more likely to provoke feelings of
disgust than feelings of sexual desire. Poor complexion is always
considered a sexual turnoff. Pimples, ringworm, facial disfigurement, and
dirtiness are universally seen as unattractive. Clarity and smoothness of
skin and freedom from disease are universally attractive.
For example, the anthropologist Bronislaw Malinowski reported that,
among the Trobriand Islanders in northwestern Melanesia, “sores, ulcers,
and skin eruptions are naturally held to be specially repulsive from the
viewpoint of erotic contact.” The “essential conditions” for beauty, in
contrast, were “health, strong growth of hair, sound teeth, and smooth
skin.”16 Specific features, such as bright eyes and full, well-shaped lips
rather than thin or pinched lips, were especially important to the
Islanders.
Cues to youth are also paramount in the aesthetics of women’s
attractiveness. When men and women rate a series of photographs of
women differing in age, judgments of facial attractiveness decline with the
increasing age of the woman.17 The ratings of women’s beauty decline
regardless of the age or sex of the judge.
Most traditional psychological theories of attraction have assumed that
standards of attractiveness are learned gradually through cultural
transmission and therefore do not emerge clearly until a child is at least
three or four years old. The psychologist Judith Langlois and her
colleagues have overturned this conventional wisdom by studying infants’
social responses to faces.18 Adults evaluated color slides of white and
black female faces for their attractiveness. Then infants of two to three
months of age and six to eight months of age were shown pairs of these
faces that differed in their degree of attractiveness to adults. Both younger
and older infants looked longer at the more attractive faces, suggesting
that standards of beauty apparently emerge quite early in life. In a second
study, Langlois found that twelve-month-old infants showed more
observable pleasure, more play involvement, less distress, and less
withdrawal when interacting with strangers who wore attractive masks
than when interacting with strangers who wore unattractive masks.19 In a
third study, Langlois found that twelve-month-old infants played
significantly longer with attractive dolls than with unattractive dolls. No
training seems necessary for these standards to emerge. This evidence
challenges the common view that the idea of attractiveness is learned
through gradual exposure to current cultural standards.
Many, but not all, constituents of beauty are neither arbitrary nor
culturally capricious. When the psychologist Michael Cunningham asked
people of different races to judge the facial attractiveness of photographs
of women of various races, he found great consensus about who was and
was not good-looking.20 Asian and American men, for example, agree
with each other on which Asian and American women are most and least
attractive. Consensus has also been found among Chinese, Indian, and
English samples, between South Africans and Americans, and between
black and white Americans.21
Photographic manipulation provides evidence supporting evolutionary
theories of female beauty. To find out what makes for an attractive face,
Langlois and her team generated composites of the human face by means
of computer graphics technology. These faces were then superimposed
upon each other to create new faces. The new composite faces were made
up of a differing number of individual faces—four, eight, sixteen, or thirty-
two. People then rated the attractiveness of each composite face, as well
as the attractiveness of each individual face that made up the composite.
Results proved startling. The composite faces were uniformly judged to be
more physically attractive than any of the individual ones. The sixteen-
face composite was deemed more attractive than the four-face or eight-
face composites, and the thirty-two-face composite was deemed the most
attractive of all. Because superimposing individual faces tends to eliminate
their irregularities and make them more symmetrical, the average or
symmetrical composite faces were more attractive than actual individual
faces.22
One explanation for why symmetrical faces are considered more
attractive comes from research conducted by the psychologist Steve
Gangestad and the biologist Randy Thornhill, who examined the
relationship between facial and bodily asymmetries and judgments of
attractiveness.23 Repeated environmentally induced injuries and diseases
produce asymmetries during development. These include not just injuries
and other physical insults, which may provide a cue to health, but also the
parasites that inhabit the human body and the mutations that inhabit the
human genome. Because parasites and genetic mutations cause physical
asymmetries, the degree of asymmetry can be used as a cue to the health
status of the individual and as an index of the degree to which the
individual’s development has been perturbed by various stressors. In
scorpionflies and swallows, for example, males prefer to mate with
symmetrical females and tend to avoid those that show asymmetries. In
humans as well, when Gangestad and Thornhill measured people’s
features, such as foot breadth, hand breadth, ear length, and ear breadth,
and independently had these people evaluated on attractiveness, they
found that less symmetrical people were seen as less attractive. Human
asymmetries also increase with age. Older people’s faces are far more
asymmetrical than younger people’s faces, so that symmetry provides
another cue to youth as well. This evidence provides yet another
confirmation of the theory that cues to health and cues to youth are
embodied 
in 
standards 
of 
attractiveness—standards 
that 
emerge
remarkably early in life.

Body Shape
Facial beauty is only part of the picture. Features of the rest of the body
provide abundant cues to a woman’s reproductive capacity. Standards for
bodily beauty vary somewhat from culture to culture, along such
dimensions as a plump versus slim body build or light versus dark skin.
Emphasis on particular physical features, such as eyes, ears, buttocks, or
genitals, also varies by culture. Some cultures, such as the Nama, a branch
of Hottentots residing in southwest Africa, consider an elongated labia
majora to be sexually attractive, and they work at pulling and
manipulating the vulvar lips to enhance attractiveness. Men in many
cultures prefer large, firm breasts, but in a few, such as the Azande of
eastern Sudan and the Ganda of Uganda, men are reported to view long,
pendulous breasts as the more attractive.24
The most culturally variable standard of beauty seems to be in the
preference for a slim versus plump body build. This variation is linked
partly with the social status that body build conveys. In cultures where
food is scarce, such as among the Bushmen of Australia, plumpness signals
wealth, health, and adequate nutrition during development.25 In cultures
where food is relatively abundant, such as the United States and many
western European countries, the relationship between plumpness and
status is reversed and the rich signal their status through relative
thinness.26 Men apparently do not have an evolved preference for a
particular amount of body fat per se. Rather, they have an evolved
preference for whatever features are linked with status, which vary in
predictable ways from culture to culture. Clearly such a preference does
not require conscious calculation or awareness.
Studies by the psychologist Paul Rozin and his colleagues have
revealed a disturbing aspect of women’s and men’s perceptions of the
desirability of plump versus thin body types.27 American men and women
viewed nine female figures that varied from very thin to very plump. The
women were asked to indicate their ideal for themselves, as well as their
perception of men’s ideal for the female figure. In both cases, women
selected a figure slimmer than average. When men were asked to select
which female figure they preferred, however, they selected the figure of
average body size. American women erroneously believe that men desire
thinner women than they actually do prefer. These findings refute the
belief that men desire women who are runway-model thin; most men do
not.
While men’s preferences for a particular body size vary, one preference
for body shape that is fairly invariant, the psychologist Devendra Singh
discovered, is the preference for a small waist size relative to hip size.28
Before puberty, boys and girls show a similar fat distribution. At puberty,
however, a dramatic change occurs. Boys lose fat from their buttocks and
thighs, while the release of estrogens in pubertal girls causes them to
deposit fat in their lower trunk, primarily on their hips and upper thighs.
Indeed, the volume of body fat in this region is 40 percent greater for
women than for men.
In other words, the waist-to-hip ratio is similar for the sexes before
puberty, but after puberty women’s hip fat deposits cause their waist-to-
hip 
ratio 
to 
become 
significantly 
lower 
than 
men’s. 
Healthy,
reproductively capable women have a waist-to-hip ratio between 0.67 and
0.80, while healthy men have a ratio in the range of 0.85 to 0.95.
Abundant evidence now shows that the waist-to-hip ratio is an accurate
indicator of women’s reproductive status. Women with a lower ratio show
earlier pubertal endocrine activity. Women with a higher ratio have more
difficulty becoming pregnant, and those who do become pregnant do so at
a later age than women with a lower ratio. The waist-to-hip ratio is also
an accurate indication of long-term health status. Diseases such as
diabetes, hypertension, heart problems, previous stroke, and gallbladder
disorders are linked with the distribution of fat, as reflected by the ratio.
The link between the waist-to-hip ratio and both health and reproductive
status made it a reliable cue for ancestral men’s preferences in a mate.
Singh discovered that waist-to-hip ratio is indeed a powerful cue to
women’s attractiveness. In a dozen studies conducted by Singh, men rated
the attractiveness of female figures, which varied in both their waist-to-
hip ratio and their total amount of fat. Men find the average figure to be
more attractive than a thin or fat figure. Regardless of the total amount of
fat, however, men find women with a low waist-to-hip ratio to be the most
attractive. Women with a ratio 0.70 are more attractive than women with
a ratio of 0.80, who in turn are more attractive than women with a ratio
of 0.90. Finally, Singh’s analysis of Playboy centerfolds and winners of
beauty contests in the United States over thirty consecutive years
confirmed the invariance of this cue. Although both centerfolds and
beauty contest winners got thinner, their waist-to-hip ratio remained
roughly the same at around 0.70.
There is one more possible reason for the importance of waist-to-hip
ratio. Pregnancy alters this ratio dramatically. A higher ratio mimics
pregnancy and therefore may render women less attractive as mates or
sexual partners. A lower ratio, in turn, signals health, reproductive
capacity, and lack of current pregnancy. Men’s standards of female
attractiveness have evolved over thousands of generations to detect and
find attractive this reliable cue.

The Importance of Physical Appearance
Because of the bounty of fertility cues conveyed by a woman’s physical
appearance, and because male standards of beauty have evolved to
correspond to these cues, men have evolved to prioritize appearance and
attractiveness in their mate preferences. In the United States mate
preferences for physical attractiveness, physical appearance, good looks,
or beauty have been lavishly documented. When 5,000 college students
were asked in the 1950s to identify the characteristics they wanted in a
future husband or wife, men listed physical attractiveness far more often
than women did.29 The sheer number of terms that men listed betrayed
their values. They wanted a wife who was pretty, attractive, beautiful,
gorgeous, comely, lovely, ravishing, and glamorous. American college
women, at that time at least, rarely listed physical appearance as
paramount in their ideal husband.
Multi-decade mating studies in the United States from 1939 to 1996
found that men rate physical attractiveness and good looks as more
important and desirable in a potential mate than do women.30 Men tend
to see attractiveness as important, whereas women tend to see it as
desirable but not very important. The gender difference in the importance
of attractiveness remains constant from one generation to the next. Its size
does not vary throughout the decades. Men’s greater preference for
physically attractive mates is among the most consistently documented
psychological sex differences.31
This does not mean that the importance people place on attractiveness
is forever fixed. On the contrary, the importance of attractiveness
increased dramatically in the United States in the twentieth century.32 For
nearly every decade since 1930, physical appearance grew in importance
for men and women about equally, corresponding with the rise in
television, fashion magazines, advertising, and other media depictions of
attractive models. For example, the importance attached to good looks in
a marriage partner on a scale of 0.00 to 3.00 increased between 1939 and
1989 from 1.50 to 2.11 for men and from 0.94 to 1.67 for women. Similar
shifts have been observed in China, India, and Brazil well into the twenty-
first century.33 These shifts show that mate preferences can change. But
the sex difference so far remains invariant. The gap between men and
women has been constant since the late 1930s in all countries studied thus
far.
These sex differences are not limited to the United States or to Western
cultures. Regardless of the location, habitat, marriage system, or cultural
living arrangement, men in all thirty-seven cultures included in the
international study on choosing a mate valued physical appearance in a
potential mate more than women did. China typified the average
difference in importance attached to beauty: Chinese men gave it a 2.06
and women gave it a 1.59. This internationally consistent sex difference
persists despite variations in ranking, in wording, and in race, ethnicity,
religion, hemisphere, political system, and mating system. Among the
Hadza, more than five times as many men as women placed great
importance on fertility in a spouse.34 When asked how they could tell that
a woman could have many children, most Hadza men responded by
saying, “You can tell just by looking,” suggesting their awareness that
appearance conveys vital information about fertility. Men’s preference for
physically attractive mates is a species-wide psychological mechanism that
transcends culture. Women too value appearance in a mate, as they should
because appearance conveys health for both sexes, but they do not
prioritize it as much as men.