A study on the appearance of twins was conducted back in 2009, examining the impacts of stress and factors like divorce on one compared to the other.
These identical twins were photographed side-by-side and examined by experts, and posted by NBC's health team online.
Factors like smoking, which can damage proteins which keep your skin firm and lead to wrinkles and sagging according to Healthline, have also been considered in the examination of the twins' appearances.
But each situation is different, as doctors behind the study found out about their backgrounds to better inform their explanations when analysing their snaps.
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It's not the first time twins have been the subject of a health experiment, with a pair of twins also making headlines after one underwent Botox treatment for 20 years while the other didn't, highlighting a number of long-term effects.
Unlike the previous study, this has six sets of twins as the subjects, and the reasons why they looked different in age were incredibly varied.
Charlotte and Rebecca, 48
These twins went through two different type of gastric weight-loss surgery, with Rebecca (right) losing 80lbs, while Charlotte (left) lost a jaw-dropping 220lbs.
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The latter then, was four pounds lighter at the time of this snap. Dr Bahman Guyuron, a plastic surgeon at Cleveland's Case Western Reserve University, said this huge loss of fat caused the skin to loosen, like the surface of a balloon when deflated, though her face is thinner while her wrinkles are 'deeper'.
Laurie and Lisa, 46
Lisa (right) weighs 38lbs more than her sister Laurie (left), which the surgeon says explains why the former has deeper smile lines and visible eye wrinkles.
Apparently, Lisa's extra weight gives her some filling for smile lines and a smoother look.
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"In twins over 40, more weight gives a younger appearance," Dr Guyuron admitted.
Janet and Jean, 54
Dr Guyuron explained that Jean (right) looks older than Janet (left), with darker areas under her eye and deeper smile lines.
He added that the American also has 'more numerous and deeper vertical frown lines', but suggested that her aged appearance is due to a divorce after 27 years of marriage, while Janet has been married for over three decades.
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Marital issues, worries about money and personal life problems can affect appearance, the surgeon said.
Brenda and Barb, 52
'The festoons of loose skin' under Brenda’s (left) eyes are down to her smoking habit, Dr Guyuron said, adding: “When I see that [on patients], I don’t have to ask if they smoke. I know they do.”
This, together with sunning, and lower weight means that Brenda looks older than sister Barb (right), as she even quipped that she 'loves being called the younger one'.
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Brenda has smoked half a pack a day for 14 years while Barb has never touched the stuff, while the former has also experienced seven times more sun exposure.
Jeanne and Susan, 61
Jeanne (left) told researchers that her sister looks 'ten years older' that her, though the experts said it was nearer to 11+ years older.
This was due to the fact that Susan (right) smoked for 16 years, sunbathes, and weighs less.
Susan spent a lot of time in the sun, having lived on Florida's east coast, while Jeanne avoids the sun.
Dr Guyuron said that Susan's lifestyle has resulted in dark, patching discolouration and age spots, as UV light reduces skin elasticity and increases wrinkles.
Dina and Gina, 27
The youngest of the bunch, Dina (left) weighs 50lbs less than her sister Gina (right), appearing younger.
“The added weight on Gina takes away definition in her jawline and projection in her cheekbones,” Dr Guyuron said.
These are key factors in a youthful face prior to the age of 40, though Dina has also had more sun exposure than her sister.
But for women in their 20s and 30s, sun damage isn't fully visible, and doesn't age you as much as it would in your later years.
Joshua Nair