It is generally a psychological tactic to make you publicly seem innocent. The typical resting-face mugshot make people not seem as friendly, and therefore guilty.
The issue is when it comes to trial it does the exact opposite. I've heard time and time again to be nothing but stoic if you ever get booked because the prosecution will claim you smiling in your mugshot shows you have no remorse.
It is generally a psychological tactic to make you publicly seem innocent. The typical resting-face mugshot make people not seem as friendly, and therefore guilty.
The issue is when it comes to trial it does the exact opposite. I've heard time and time again to be nothing but stoic if you ever get booked because the prosecution will claim you smiling in your mugshot shows you have no remorse.