"While hundreds of studies have looked into how environmental factors, such as sun damage and smoking, are associated with facial ageing, conclusive data has been elusive. The reason is that despite the size and thoroughness of the studies, they weren't able to control one of the most important contributors of ageing – genetics. Because the twins' genetic make-up was identical, the differences in how old they looked could be attributed solely to external factors, and not to "good or bad" genes."
The body mass index (BMI) of each twin was was calculated by dividing their weight in kilograms by their height in metres squared. The ideal BMI for an adult is between 18.5 and 25. Anyone with a BMI lower than 18.5 is classed as underweight. "The twins were divided into groups based on a four-point BMI difference," explains Guyuron. "A BMI higher by four points was found to result in a younger appearance of between two to four years in women over 40 years old."
Rajiv Grover, Consultant Plastic Surgeon and Secretary of the British Association of Aesthetic Plastic Surgery, has welcomed the findings. "It gives scientific credence to what we always thought but couldn't prove, which is that it is not what your mother looks like, but volume-loss that makes you look older," he says. ''The more fat that is preserved in the face, particularly the cheeks, the more you will preserve the facial proportions of youth.
"Excessive dieting will give a woman a gaunt appearance by leading to volume loss in the midface [cheeks]. This volume loss can be compounded by yo-yo dieting, where not only do you create volume loss, but also stretching of the facial supporting ligaments due to repeated facial volume gain and volume loss, which causes deeper nose to mouth lines and jowls.''
Some of the other external factors that influence ageing come as no surprise: for example those who enjoyed an outdoor life, or who were careless with their sun protection aged faster. Twins who smoked and drank also aged faster, and more visibly.
But other findings were more surprising. Marital status was found to be a key factor in the ageing process, with women who have divorced looking older than their married or single counterparts. "The twin who is divorced appears about 1.7 years older than the twin who is not divorced," says Dr Guyuron.
In addition, a twin who suffered from depression and who took anti-depressant medication for the condition looked "significantly older" than a twin who had no history of anti-depressant use. Research suggests that the reason for this could be the ageing effects of stress, and the persistent relaxation of facial muscles which can be an effect of long term anti-depressant use.
While loss of volume in the face can be combated by maintaining a healthy weight, what about the things that are harder to control, such as a predisposition to depression or a partner asking for a divorce?
"Of course you can't control these things, but you can control how you handle them", says Dr Linda Papadopoulos, a psychologist and author of several books on psychology and skin disorders. "The idea that being in a happy relationship is going to keep you looking younger, for me is a no-brainer. Being in a happy marriage, or indeed any social support system, has a protective effect on our health and our looks.
"We accept that stress can cause skin conditions such as psoriasis and eczema, but research has also shown that one of the biggest contributing factors to skin ageing is stress," she says. "It's not so much the fact that you are divorced that will give you wrinkles, it's how you deal with it. If you are divorced and happy, then you'll look good.
"Emotional stress influences the immune system and this will manifest in changes in the skin. When we are tense, the body produces cortisol which can speed up the metabolism, resulting in stress break-outs on the skin surface and premature ageing. So yes, divorce, or being depressed, could age you for this reason."
Dr Nick Lowe, consultant dermatologist at the Cranley Clinic in London, routinely refers patients with skin conditions to psychotherapists, because the underlying cause of skin problems and deterioration is often stress-related. "Stress interferes with the systems that repair and regulate the skin, and hormones associated with stress, such as cortisol, weaken the skin.
Rajiv Grover agrees that stress and depression are key factors in ageing. "I believe ageing occurs in spurts rather than in a linear fashion. One trigger for an ageing spurt is a period of stress – recent divorce, illness, and bereavement or work problems. All these are related to depression which could accelerate volume loss from the midface, which therefore loses its youthful heart shape." However, he adds that a youthful outlook and joie de vivre are also significant characteristics in those who retain a young appearance.
Dr Papadopoulos says the new findings are positive ''but only if we read them in context. You shouldn't look at it and think, 'I'm divorced and depressed etc, so that's it…' If you turn it on its head, it is really encouraging because it suggests that if you learn to cope with stress, and learn to accept your body, then you cannot only change the way you think, but the way you look, too."
Dr Guyuron agrees: "What we've essentially discovered that is, when it comes to your face, it is possible to cheat your biological clock.