I think the more fundamental flaw in PG's argument, which is just as present here, is that it assumes the populations are otherwise identical. That's obviously

I think the more fundamental flaw in PG's argument, which is just as present here, is that it assumes the populations are otherwise identical. That's obviously not the case -- there's no random assignment for bias -- so this sort of test can't tell you anything direct about casuation. Any credible statistical test for bias should be framed in the language of causal inference, e.g., as described by Judea Pearl: http://ftp.cs.ucla.edu/pub/stat_ser/r350.pdf http://ftp.cs.ucla.edu/pub/stat_ser/r350.pdf