Professional profile photos, or smiley ones?

People unconsciously react to our emotional expressions. So, for my the question is if you would rather prefer a professional reaction or a smiley one.

Why not both? You can be professional while smiling. :) I'd say this comes down to whatever feels the most like "you" - What image/aspect of yourself are you trying to portray, the audience you're engaging with (super buttoned up or normal humans).