People don't only tell us things because they are true. They also tell us things because they want us to believe it. Believing things that others want us to believe is often a very, very bad plan. Especially if we advertise that we believe things because people tell us that we ought to.

Even if you can tell when people are lying, it's still a bad plan. If Alice tells you the IQ scores of people only if it will adjust your perception of that person downward and Alice dislikes that person, you're going to reliably treat people that Alice hates worse.