Finally, a simple method to looking younger that doesn't cost you a cent, a new study suggests that by simply showing off your pearly whites and looking happy, you have a greater chance of people underestimating your age.

The study conducted by the Max Planck Institute for Human Development in Berlin, found that the age of a person with a neutral facial expression is most accurately identified, possibly why we are told not to smile for passport photos. But that it is much harder to read the age of faces that have an emotional expression, with people smiling the most likely to be underestimated.

The German study published in the journal Psychology and Aging included 154 men and women representing all adult age groups, were asked to estimate the age of over 1000 people whose faces were photographed displaying a particular emotion.

Each face showed emotions of happiness, anger, fear, sadness, disgust, or were alert but neutral. The most accurate estimate of was if the face had a neutral expression or if guessing the age within the same age group.

"Our study is the first to show that facial expression affects both accuracy and bias in age estimation," says lead author Manuel Voelkle, a research scientist at the Max Planck Institute for Human Development in Berlin.

"Pictures of happy faces can be misleading because smiling or laughing can flex muscles around the eyes and mouth, creating temporary wrinkles. Since it's hard to tell temporary wrinkles from real ones in photos, people give a less accurate age estimate," Voelkle says.

It was also suspected that people smiling were generally perceived to be more positive and therefore more attractive, younger looking people.

Interestingly the study found that in general young people underestimated the age of older people, although older people were less accurate picking the age of younger or older people, indicating that age estimation ability decreases with age.

Generally, the age of older faces was more difficult to estimate than the age of younger faces. Although good news for women, older female faces were estimated to be at least three years younger than older male faces, even though make-up was not used.

Voelkle suggests that ladies may pay more attention to their physical appearance than guys do. Or it could be that women have a longer life expectancy than men. Looking at it from this perspective, he says, "it's possible to conceive of age as distance to death, rather than distance from birth." Now there is something to smile about.