How come is not incorrect but it’s considered too casual for formal writing these days. It’s actually a shorter way to say a more formal-sounding phrase such as “How does it come to be that …” In fact, a Google search for that phrase turns up 1,360,000 hits, so people are apparently still using the full phrase as well. It could be used to good effect in the example sentence above, I think, to add more weight to the question, because it puts an emphasis on the mechanisms behind the pay disparity.
“How come” first appears in print in Bartlett’s Dictionary of Amercianisms, 1848. Bartlett’s claims that it likely came over from England. I’m with the OED on that point — they believe how come came into being on these shores. Doesn’t it sound American to you, shortening “how comes it” to “how come”?
- How comes it then that this her cold so great is not dissolved through my so hot desire . . . (British poet Edmund Spenser in “Sonnet 30,” 1611)
- How comes it that the Church has attained such greatness in temporal power . . . (Machiavelli, in The Prince, 1513)
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