Twin Study 1

Fraternal (non-identical) twins are just as genetically different from each other as regular siblings. However, being twins often causes us to think of such individuals as being somehow more related or connected to one another than regular siblings. Indeed, plenty of fraternal twins share very close bonds, due to both sharing many childhood experiences together (e.g. same birthdays, same level in school, easier to share toys and clothes) as well as facing constant social pressure to live up to their identity as twins.

From an aesthetic point of view, twins are the same age, so all photos where both twins are present show them at precisely the same calendar age, though not necessarily the same physical level of aging. An individual's look can subtly or even dramatically change as they age (their skin will begin to wrinkle and sag, and they may gain weight as their metabolism becomes more inefficient), so it is unfair to compare two individuals at different ages. Since we know twins are the same age, this makes it easy to fairly compare their faces and bodies directly.

Identical twins are often used in scientific studies in order to examine the effects that differing lifestyles and environments have on the body (because these twins are genetically identical and presumably react similarly under a specific environmental stressor). Fraternal twins can be excellent aesthetic case studies, showing us just how different two closely-related individuals can look.

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For instance, let's look at Barbara Bush (left) and Jenna Bush (right), twin daughters of former US President George W. Bush and Laura Bush.

Had I not written the long introduction about twins, it would be easy to believe they came from entirely separate families!

Barbara has an Oval face and ectomorphic body:

While Jenna has a much wider, Pentagon-leaning, face and has put on more weight as she aged:

Looking at a picture of when both sisters were young, Jenna's look (left) appears to have changed more over the years than Barbara's (right):