What causes some people to be left-handed, and why are fewer people left-handed than right-handed?

These theories of hand preference causation are intriguing because they can account for the fact that the side of hand preference of individuals with the C gene (most left-handers and some right-handers) can be influenced by external cultural and societal pressures, a phenomenon that researchers have documented. These theories can also explain the presence of right-handed children in families with left-handed parents and the presence of left-handed children in families with right-handed parents. If the familial genetic pool contains C genes, then hand preference becomes amenable to chance influences, including the pressures of familial training and other environmental interventions that favor the use of one hand over the other. The proposed genetic locus that determines hand preference contains an allele from each parent, and the various possible genetic combinations are DD individuals who are strongly right-handed, DC individuals who are also mostly right-handed, and CC individuals who are either right-handed or left-handed. These genetic combinations leave us with an overwhelming majority of human right-handers and a small, but persistently occurring, minority of left-handers.

Schools once went through a phase of forcing lefthanded kids to write with the wrong (right) hand. The history is ancient:

Most animals are fifty/fifty right handers/left handers. Two million years ago all humans became right handed - that's one hundred percent of them and that had its advantages. And then about a hundred thousand years or so ago another mutation occurred which meant that left handedness could come into being and it must have had some advantage to it and left handers have stayed around since then. So they�re there not because they're an evolutionary throw back to a primitive state, but because they're a recent mutation with additional advantages. Now what they are is anybody guess.

I don't remember anything at school, but family pressure can work just as well. there is also a considerable religious pressure:

Moreover, the Scriptures themselves amply attest to the preeminence of the right hand and the depravity of the left. Thus the right hand confers blessing and signifies strength, while the left hand is treacherous and deadly (Gen. 48:13-20; Exod. 15:6; Eze. 21:22; Rev. 1:16-17; Judg. 3:15, 20:16; 2 Sam. 20:9-10). A place at one's right hand is the seat of honor and dignity (1 Kings 2:19; Ps. 45:9, 110:1). Sagely does Qoheleth teach that "a wise man's heart inclines him toward the right, but a fool's heart toward the left" (Eccles. 10:2). In like manner, both the passivity and the inferiority of the left hand are apparent in the solemn injunction forbidding us to let our left hands know what our right hands are doing (Luke 22:50). And it is by no accident that the elect are to stand like innocent sheep at the right hand of the Eternal Judge, while the reprobates cower and whimper like noisome and tick-infested goats on His left, awaiting their dizzying descent into sulfurous fumes and unfathomable miseries in the mind-bending agonies of eternal damnation (Matt�25:31-46).

I think I'll stick to the recent mutation with evolutionary advantages. It sounds about right left to me.