President Obama’s birth certificate is once again in the news, thanks to Donald Trump. Trump, whose enterprises frequently seem to find themselves in bankruptcy court, is now intimating that he will run for president and appears to be making a play for the birther vote. Charges that President Obama was born in Kenya and has a fake birth certificate are nonsense, of course, but why won’t they just go away?

According to research by Brendan Nyhan and others, it is difficult to change people’s minds when they believe something false about Obama. Nyhan’s team found that showing people video of Obama denying he was a Muslim or affirming that he was a Christian had no statistically significant impact on their beliefs. The only exception was when the person administering the experiment was not white, perhaps indicating that participants felt pressure to give the “socially acceptable” answer. And when the experimenter was white and Republicans were shown the video of Obama declaring he was a Christian, they actually believed more strongly that he was a Muslim. So perhaps this explains why Obama has taken to joking about or wholly ignoring the hubbub around his birth certificate and let his effort to spread the truth wither since 2008—no matter what he says, he can’t convince people of the truth.