The basis of this “infrequency advantage” is that the vast majority of opponents faced by left-handers are right-handers (the majority of the population), so that they are well practised at dealing with the kind of asymmetry offered by other right-handers. It is, however, relatively rare for a right-hander to face a left-hander (which make up a small minority of the population), and are therefore much less practised at dealing with the different angles, stances, etc, employed by left-handers.
The Fighting Hypothesis also claims to explain the greater frequency of left-handed males than left-handed females, which is repeatedly found in studies, on the grounds that male-male fighting is a more common occurrence than other combinations and so it is the males who stand to profit the most from the left-handed fighting advantage.
Although far from conclusive, a 2004 study by Charlotte Faurie and Michel Raymond of nine undeveloped societies in five continents found some support for their hypothesis that left-handed combatants in such societies hold an advantage. They found that, in those primitive societies with higher levels of violence and higher murder rates, the incidence of left-handedness was higher, (ranging from 3.4% among the largely pacifist Dioula community in Burkina Faso, to 22.6% in the notoriously violent Yanomamo culture in South America), perhaps indicating that such violent societies favoured left-handed criminals.
Spiral staircases and towers in castles typically spiral clockwise going up, and this was purportedly a deliberate design to afford an advantage to the (largely right-handed) defending soldiers at the top of the stairs, who had plenty of room and good angles to swing their weapons at the (largely right-handed) attackers below, who had little room and were impeded by the central column of the stairs. Left-handed attackers, it is argued, therefore held an advantage (or at least less of a disadvantage) in such circumstances.
The famously left-handed Kerr clan supposedly built anti-clockwise spiral staircases in Ferniehirst Castle in the Scottish Borders during the Middle Ages in order to give themselves a similar advantage (although one would have thought that this would also yield an advantage to the right-handed attackers below!)