First it was left-handers were more prone to experiencing random accidents like burning one's self on the stove because the utensils were designed for a right handed person. Then it was left-handers were probably going to die sooner because of these accidents. Now, according to the British Medical Journal, for some reason, left-handed women who have had a child are more prone to breast cancer. Get the article here.
There is truth in the fact that many things are designed for a right-handed preference, but we really don't notice them (but I do). My things, however, are designed for lefties. :)