But this is also not how isolating variables works. If you give someone no information, and require them to make a decision based only on speech patterns, that

But this is also not how isolating variables works. If you give someone no information, and require them to make a decision based only on speech patterns, that does not actually tell you very much about how they would make a decision if they had lots of information including speech patterns. Compare: a) only speech b) lots of complex skills-based information that is (verified in a separate experiment) equivalent on balance, plus also speech It's quite possible that you could see a large difference in decisions in (a) without any detectable difference in (b). For example, imagine in this study hiring managers are (perhaps subconsciously) estimating education level from speech. If they already have lots of details about candidate's education, that "screens out" the rough proxy of their speech patterns.