Female body has always been a source of aesthetic pleasure or sign for fertility and reproductive capabilities. In early ages, it was more of a sign of fertility. One of the reason were during those days when contraception was unheard of, number of children a women can bear was directly correlated with number of times a man can make love with her. Things have changed over centuries and decades. From two body types of Apple and Pear, now we have more than 12 types of body types. Even though some of the body types got fame due to marketing campaigns and the love of classification, to make the readers and viewers engaged, writers will make a detailed list of pros and cons of each body type giving some kind of temporary satisfaction and relief to readers who doesn't come in the "perfect" category of body shape.
There is nothing called "perfect" body shape. Unless you are too fat or too skinny. Body shapes are never going to make a difference to your lifestyle or your career even if you are into modelling industry. All you need to know is what feature is best in you and how to flaunt it. Ignore the rest because literally no one is going to dig deep into your body to find the flaws once they have seen the best. Remember "first impression is best impression"
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So, what is the reason why different humans have different body shapes? The culprits are Estrogen and Testosterone. These are hormones produced in the body of male and female. Estrogens are produced considerably higher in females causing to develop breasts and widen hips during puberty. This is the reason why breasts get enlarged during pregnancy due to the increased production of estrogen in body. Estrogen is also the reason for fat storage in hips, thighs and buttocks. Testosterone, on other hand develops muscle which prevents breast development and causes to add mass to the arms (Biceps and Triceps) and thighs. Female body shape is the play of all these factors in addition to various other localised reasons.
How it is determined
Generally, it is determined and expressed in terms of three infliction points (36-24-36) where first unit stands for Bust, middle for waist and last unit for Hip. By the way infliction point is a point where a curve changes from concave to convex or vice versa. But, this numbers cannot be used a standard way of measurement because above measurement (36-24-36) is considered athletic for a female with a height of 5.8" and the same measurement for a women of 5ft height can look very different. Hence, these are measurements to gauge a female's relative body shape. No single measurement is considered perfect. Various factors like height, localised fat, muscle etc.. decides the overall healthiness of a person.