My apologies. I wrote that while awake by insomnia, and I didn't want to go rummaging around in unpacked boxes of books for Professional Software Development
or The Psychology of Computer Programming (Silver Anniversary Edition)
both of which cite the results of testing programmers with the Myers-Briggs Type Indicator test, and contrasts that with the general public. (IIRC introverts outnumber extroverts by about 3 to 1 among programmers, which is exactly the reverse of the general public. But don't quote me on that because I don't have those books beside me now either.) But you are right that I should be more specific so that people can double-check claims like that. I also tend to not be sympathetic to claims that sound like unsubstantiated broad generalizations.
Incidentally I should note that I am in the minority of programmers who are extroverts.