From what I understand, left-handedness is a recessive trait, meaning that there should be no more than 25% of the population with it at any time. Say the gene for being right-handed is H and the gene for left-handed is h, the only combinations you can have are HH, Hh (twice), and hh. Only one quarter of the time is left-handedness expected.Now, add into factors that in many places, it's taboo to favour the left hand (considered dirty, immoral, etc.) and in many places lefties are forced to switch their preference to the right hand. So if that happens to half of the lefties, the percentage of lefties in the world is actually 12.5% - exactly as someone else pointed out.