Not all of the conspiracy theories addressed in the YouGov poll actually have a big impact on public opinion. I suspect, for example, that many of the respondents had never even heard of Pizzagate until they saw it in the survey. But it is still notable that so many are credulous enough to believe such claims when they do hear them.
Rational ignorance helps explain why people are much more likely to believe outlandish political conspiracy theories than similar ones about their personal lives. Most of us have very strong incentives to be well-informed about issues in our lives where our choices are likely to make a difference. The person who falsely believes that his family members or co-workers are conspiring against him will impose tremendous costs on himself if he acts on that assumption. By contrast, individual voters suffer no such obvious penalties when they embrace political conspiracy theories. That’s one reason why millions of people who embrace absurd conspiracy theories about political events are generally rational in their everyday lives.
The perverse incentives that cause political ignorance also help exacerbate partisan bias. Because there is so little chance that any one vote will make a difference, voters have little incentive to evaluate political information objectively, and carefully consider opposing views. Unlike in many other aspects of their lives, they can afford to indulge political biases with very little chance of of suffering any adverse consequences. Just as it is rational for most voters to be ignorant about politics, it is also rational for them to make little effort to control their biases in evaluating new political information.