(Editor's note: The following is an edited version of a commentary that first appeared in the Summer 2010 issue of Impact, the quarterly newsletter of the Mackinac Center.)

My late friend and colleague Joe Overton understood the power of ideas to change public policy. Change ideas, he understood, and you change the range of options available to policymakers. That's why he devoted his career to the think tank business rather than politics. It's also why a book titled with his name recently rose to No. 1 on The New York Times bestseller list.

Think tanks' influence on policy is real, but it's indirect. Joe wanted a way to explain how think tanks like the Mackinac Center affect policy even though think tanks don't vote in the legislature and they typically don't lobby or command legions of activists. So he devised a model of policy change he called the Window of Political Possibility.

As far as lawmakers are concerned, there are only two kinds of ideas: those that are inside the window of political possibility, and those that are outside.  Ideas inside the window are acceptable to the electorate. Support those ideas and gain (or keep) your constituents' support. Ideas outside the window are perceived as not acceptable to voters. Advocate those ideas and risk being replaced at the next election.

This much is common sense, but Joe saw how the window actually shifts over time along a range of ideas to encompass new policies and exclude old ones. If public policy ideas are arranged on a scale from top to bottom, with little government interference at the top and a lot at the bottom, the window will slide up and down depending on whether society accepts limited government or expansive government.

The ideas that influence society determine what laws that society will accept. When think tanks (and others) develop and promote ideas, they can shift the window of acceptable new laws. Politicians who want to remain in office take heed.

After we tragically lost Joe seven years ago, we renamed his model the Overton Window. We trained hundreds of think tank professionals in how it works. Political blogs began to buzz with the idea. This inspired popular and controversial talk show host Glenn Beck to write a political thriller that he titled "The Overton Window." Without giving away too much, I can tell you the novel's bad guy stole his big idea from "a think tank in the Midwest."

When Glenn Beck invited me to be a guest on his radio and television programs, we saw it as an opportunity to educate large audiences on the role of think tanks and the power of ideas. My appearances drew more than 25,000 individuals to our special website. More than 1,400 of those people asked us to keep in touch with them, and many became financial supporters.

To learn more about Joe Overton's life and work, please click here.

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Joseph G. Lehman is president of the Mackinac Center for Public Policy, a research and educational institute headquartered in Midland, Mich. Permission to reprint in whole or in part is hereby granted, provided that the author and the Center are properly cited.