Dear Professor Mankiw: please learn the difference between "reign" and "rein". Here's a hint: don't use "reign" unless you're making fun of somebody's monarchical pretensions, or discussing the Who's discography. You'll almost always be wanting to say "rein".
I'd have emailed you, but you don't seem to have an email address on your blog anymore.
[Update: somebody pointed it out to him since I first squawked. No point in removing the post, but it's no longer incorrect.]