Left-handers in competitive oppositional sports (like tennis, or boxing, or fencing, or baseball) have advantages in that right-handers often have less experience with them than they have with the more-common righties.

I can’t think of any mechanistic physiological advantage they would have, other than an easier time being ambidextrous – left handers seem to genetically have no clear hand preference.

I agree with Darby here, most if not all of the advantages left-handed people have in sports is due to their opponents being more used to deal with right-handed people.

Of course, in certain sports like football, at least the left back and left winger are usually expected to be left-handed (and thus left-legged) players, so they have more chances to play and succeed in their game, since right-handed players have more competition for their respective wide positions (e.g. there are about ten times more right wingers fighting for that position) 🙂

About the disadvantages, well, some are obvious. Most of the apparatuses and tools we use are designed for right-handed people – using many types of scissors is very difficult for lfet-handed people, for example. In some cases this can even increase the risk of injury.

Interestingly, some studies (and my high school biology teacher) suggest that left-handed people have shorter average life span than right-handed persons. Quick googling shows that this finding seems to be highly debated nowadays, though. But for example, in 2007 there was an article in the Journal of Epidemiology that was summarized by Reuters as:

"(Reuters Health) – A study suggests that women who are left-handed have a higher risk of dying, particularly from cancer and cerebrovascular disease – damage to an artery in the brain or an artery that supplies blood to the brain." … "When left-handed women were compared with the other women, and the data were adjusted for a number of potentially confounding factors, lefties had a 40 percent higher risk of dying from any cause, a 70 percent higher risk of dying from cancer, and a 30 percent higher risk of dying from diseases of the circulatory system."

Maybe left-handedness is a slight genetic disorder that may be linked to some other problems, or maybe it has nothing to do with higher mortality. But whatever the case, any effect there might be must be very small, otherwise there would be no such debate! If someone knows a reliable study about the topic, let me know 🙂