[gojomo]

I think that's a great theory.

You don't even have to assume that past expressions permanently effect facial contour (though that's probably true). Whenever a person is being evaluated -- and taking a mugshot for a study certainly counts -- they adopt a pose/expression. They are almost certainly sending a message about themselves with their choice of expression, and that choice is biased by what's worked in that person's past interactions with others. Unless they're deeply dishonest, they'll send a generally-true message about their personality.

You could start to test this by seeing if the accuracy of third-party estimations holds up when people evaluate photos of sleeping/anesthesized faces.

On the other hand, a bleaker interpretation is also possible. Maybe you are born with a certain kind of face, and will always be judged that way no matter how you behave. So over time, only by playing-to-type do you get positive feedback, and the facial stereotype becomes destiny.