Suppose X is some statement, and you currently think that there is a 50% chance that X is true, and a 50% chance that it is false.
You ask two people about X. One says that it is true, and the other says that it is false. The one saying that it is true is somewhat more intelligent than the one who says that it is false. The evidence contained in these claims will surely not balance out exactly. At this point, then, is it more likely that X is true, or that it is false?
It is reasonable to say that at this point it is more likely that X is true. In fact, if we discovered that on average it would be more likely that X is false in such a situation, we should rename the thing we were calling “intelligence” and call it “unintelligence”, since it would not promote understanding but the lack of it.
But this is a question about what is true on average. It will not always be true once other factors are taken into account. And since belief is voluntary, and people believe things for other motives besides truth, under many circumstances intelligence can actually hinder the search for truth. For we can expect that a more intelligent person will on average be somewhat better at finding a way to attain his goals than a less intelligent person. Thus to the degree that those goals happen to hinder the search for truth, the more intelligent person will actually be less likely to come to the truth.
For example, several of the goals that people often have in believing things, or at least continuing to believe what they have believed in the past, is maintaining the appearance of stability, since instability is often seen as bad, and avoiding the shame of admitting that one was wrong. To the degree that a person has these goals, a more intelligent person will be more capable of finding ways to avoid changing his mind, even when his current position happens to be false. He will be more capable of finding subtle ways to defend his position, more capable of finding reasons to dismiss opposing arguments, and more capable of finding ways to avoid stumbling upon evidence against his current position.
Even apart from these two particular motives, there are of course any number of other motives which are potentially opposed to the search for truth in concrete cases. And to the degree that such motives are involved, intelligence will not be a help to the truth, but a hindrance.