President Donald Trump this morning on Twitter hit at his typical theme that his pet border wall is vital to fighting crime and protecting national security. Can you spot the flaw with this argument?

BUILD A WALL & CRIME WILL FALL! This is the new theme, for two years until the Wall is finished (under construction now), of the Republican Party. Use it and pray! — Donald J. Trump (@realDonaldTrump) January 23, 2019

We blog about crime stats enough here at Reason that this is an easy one: Crime has been going down steadily (with a few recent bumps) since the 1990s. Here's a nice handy selection of charts from Pew Research Center showing the actual truth:

If crime has been going down steadily throughout much of America without this wall, isn't that evidence we actually don't need it? Or evidence that whatever influences crime rates, it's most certainly not illegal immigration numbers? The illegal immigration population in America actually increased as the crime rate was dropping (now both crime rates and illegal immigration numbers are dropping slightly):

Trump is both a reflection of and a contributing factor to a common attitude among Americans: People think there's much more crime than there actually is. In polling, huge numbers of Americans—we're talking more than 50 percent of those who were polled—believe that crime is up over previous years' numbers even when the exact opposite is true:

So, in that sense, Trump's tweets are wildly out of step with the facts, yet also likely compelling to a certain number of Americans, and that's frustrating. He's far from the first politician who uses fear as a way of selling bad policy prescriptions, and he won't be the last. But he's also completely and utterly wrong about crime and what's causing it.