In their 2012 study published in the journal Psychology and Aging, the scientists write that despite the fact we use age as one of the primary ways we describe someone we've just met -  along with gender, race, and height - very little research has been conducted on how accurately we guess another person's age.  So, whether you're on the witness stand testifying in court or meeting someone for the first time at a party, how does a person's facial expression affect how old you think they are?

To answer this question, the team used a set of more than 2,000 photographs of models showing a variety of different emotions.  The models ranged in age from 19 years to 80 years, and each adult was photographed conveying 6 different expressions: angry, afraid, sad, disgusted, happy and neutral.  Same people showing different moods; without jewelry, makeup or eyewear, all in similar gray shirts.

Then the researchers asked a total of 154 young, middle-aged and older study subjects to rate the age of each of the models from their photographs.  In all, more than 135,000 ratings were collected and analyzed.

The result?  The most accurate age guesses happened with models in a neutral expression, suggesting it's easier to estimate a person's correct age when they are not conveying an emotional facial expression.  This likely explains why most official documents like passport photos require a neutral facial expression.

The least accurate guesses were of smiling models; these adults were judged on average to be about two years younger than their actual age.