If you believe that banning sex education in schools will decrease teen pregnancy, and it turns out that teen pregnancy rates actually increase when teens aren't educated, then your belief about sex education isn't rational.

Many - possibly most - public policy decisions are open to this kind of testing.

The fact that formal testing rarely happens, and that it's consistently ignored even when it does, suggests that politics itself is inherently pre-rational.

Arguably it's then irrational to expect it to be rational - but that's a deeper problem that doesn't just apply to politics.

Before people knew the earth was a globe, it was entirely rational to think everything that goes up goes down again.

And for all testing you can do, ie, throwing an apple up, it'll come down again.

It is entirely rational to conclude from the available evidence that everything falls down when thrown.

But recently we've discovered that things you throw up hard enough may not fall down at all.

Thusly, the rational thought was still rational but wrong.

Predictability is important when you want to evaluate scientific models (ie, assume earth is flat, assume earth is round, assume earth is a mostly round potato all have different predictive quality, though the first ones are easier and sometimes sufficient)