We've all heard this one. The myth here is that horizontal stripes draw the eye across the body, making you appear wider, and vertical stripes draw the eye up and down the body, making you appear longer and leaner.
Believe it or not, a study done by a perception expert at the University of New York found that people don't perceive others wearing horizontal stripes as heavier -- they actually perceive them as thinner!
Like any pattern, though, stripes will draw attention to the area where they're worn. You can minimize that effect by reducing the area with stripes -- like wearing a blazer or a cardigan over a striped top.