Left-handers have been persecuted by right-handers for millennia. This right bias is evident cross-culturally, linguistically (right is literally and figuratively ‘right’, with lefties being described as ‘gauche’, ‘sinister’ and ‘cack-handed’), and environmentally (e.g., equipment design, including power tools, ticket machines, and lecture-room desks). Despite this, the proportion of left-handers has remained constant at approximately 10% of the hominid population, implying that though there are costs associated with left-handedness (if there were not, the proportions of left- and right-handers would be 50:50), left handers must also enjoy fitness advantages that maintain the genes for left-handedness in the population. This paper reviews the costs and benefits of being left-handed, exploring research examining the effects of handedness on brain structure, cognitive function, and human behaviour. The research confirms a variety of left-hander advantages, including some cognitive superiorities, higher wages, and greater sporting and fighting prowess. On the other hand, left-handedness is also associated with significant fitness costs, including an increased risk of accidents, higher substance abuse susceptibility, and earlier death, in comparison with right-handers. In sum, left-handedness confers both costs and benefits, with the latter outweighing the former, maintaining the genes for left-handedness in the population.