By comparing photos of identical twins, researchers from Case Western Reserve University confirmed that smoking leads to premature aging of the face, contributing to more wrinkles, droopy skin, jowls, and under-eye bags.

For this study, 79 pairs of twins were selected between 2007 and 2010 during the Twins Days Festival in Twinsburg, Ohio. In some sets of twins, only one smoked. If both twins smoked, one twin had to smoke at least five years longer than his or her counterpart to take part in the study. Participants ranged in age from 18 to 78, with the average age being 48.

To rule out other factors that contribute to premature aging, like sun exposure, drinking, and weight gain, researchers asked about their sunscreen use, alcohol intake, and perceived work stress for each twin and determined that the differences were not statistically significant.

To determine facial changes caused by smoking, a panel of three plastic surgeons, who were not aware of the twins' smoking history, looked at professionally taken photographs of each twin's face (see some examples below).

They found that smokers, when compared to their non-smoking counterparts, had more sagging and bags under the upper eyelids, more smile lines, sagging jowls, and upper and lower lip wrinkles.

Among twins who both smoked, those who had a longer smoking history scored worse for lower lid bags, lower lip wrinkles, and baggy cheeks.

Most smoking-related differences were observed in the lower-thirds of the face, according to the study. In the lab, cells lose elasticity and collagen after exposure to tobacco extract, which could be to blame for the changes in this part of the face.

Additionally the "overall thickness of the skin is greater in nonsmokers than in smokers, perhaps making the upper lip lines and nasolabial creases less discernible," the authors write. Nasolabial creases refer to the lines that run from each side of the nose to the mouth, also known as smile lines.

The findings were reported in the journal Plastic and Reconstructive Surgery.

The twin on the left has smoked 17 years longer than the twin on the right. Note the differences in lower lid bags and upper and lower lip wrinkles.

Plastic and Reconstructive Surgery

Both twins are smokers. The twin on the right smoked 14 years longer than his brother.

Plastic and Reconstructive Surgery

The twin on the right is a smoker; the twin on the left is a non-smoker. Notice the difference in the smile lines.

Plastic and Reconstructive Surgery

The twin on the left is a non-smoker. The twin on the right smoked for 29 years. Note the differences in aging around the eyes.