The Best Big Lies?

Sometimes I slip into hyperbole and say things like:

Many folks figure that if evolution planned for them to believe a lie, they might as well believe a lie; that probably helps them achieve their goals.  But I want, first and foremost, to believe the truth.

Now I won’t accept any possible harm no matter how large to believe any truth no matter how small.  But I do have a very strong presumption toward telling and especially believing truth.

Many folks, however, say there are important cases where we are better off believing lies.  So I now ask you all: what are some big truths where we are overall better off to have most folks be mistaken?

I’m not looking for extreme rare examples like lying to keep Nazis from finding Jews in WWII; nor do secret passwords or launch codes count.  I’m looking for real and important examples today, where it is the lie, not just the uncertainty, that gives the advantage.

Yes, by mentioning such truths here you will risk more people believing them, but you might also convince many of us to join your cause to suppress important truths.  So you don’t need to tell us about our most important lies, but you do need to tell us about lies big enough to convince us that there are many big lies worth keeping.