Slim, barrel-chested, tummy or 6 pack abs, shirts for men look better tucked in. Why? A shirt hanging out makes you look bigger - ESPECIALLY if it covers the crotch. We can't see where your legs begin.
You never want to be more torso than legs --- it's always more flattering (for men AND women), to look more legs than torso. A longer leg line helps make you look taller.
However, it now seems that tucking in is also tied to something else for men - happiness.
There was a wonderful article in the Chicago Tribune the other week about a study done by Fruit of the Loom that asked 1,000 men, ages 25-60, about the quality of their lives in terms of happiness, optimism, social status, income and relationships.
The study found that 60% of men who tuck in their shirts at work, are happier. It also found that these men are 22% more likely to be optimistic about their future and also earn more money than the (sloppy) untuckers and have more dates! Woo hoo! These are a good things :)
Here's a video I did for women about leggings, but I also address the issue of looking like more torso than legs so you can see the difference.