Members of Congress now make $174,000 a year — not a bad living. But usually they can at least quintuple that salary by switching over to lobbying once they retire. And many of them do just that.

A new study by three political scientists has some good data on the trends. Jeffrey Lazarus, Amy McKay, and Lindsey Herbel went all the way back to 1976 to see who went on to lobby after leaving Congress. Their results are reported in a new article, "Who walks through the revolving door? Examining the lobbying activity of former members of Congress," in the journal Interest Groups & Advocacy. The below image comes from that article.

As the graph makes clear, the rate of retiring members going to lobby has grown steadily over time, though it seems to have peaked around 2000. The Senate exhibits more up and downs because fewer senators retire each year, making the percentage trends more sensitive to small fluctuations.

Back in the 1970s and '80s, it was relatively rare for former Congress members to become lobbyists. This makes sense, since the lobbying industry was not nearly as big then. The real growth of lucrative Washington lobbying has been since the 1990s, trends that I document in my book The Business of America Is Lobbying: How Corporations Became Politicized and Politics Became More Corporate. Now reported lobbying is a $3.2 billion-a-year activity.

The obvious caveat in these measures is that they only represent registered lobbyists. Many former members prefer not to register, but still do work that looks very much like lobbying, with former Sen. Tom Daschle (D-SD) being a prominent example. Political scientist Tim LaPira estimates that there's probably twice as much lobbying in Washington as shows up in disclosures.

And prior to 1995, when Congress passed the Lobbying Disclosure Act, registration requirements were even looser, though registration rules did exist.

So while these percentages are almost certainly an undercount, what's notable is the trend. It grows over time, consistent with the growth of lobbying as a business generally.

There are a few other notable findings in the paper:

Retiring members are more likely to become lobbyists if they were party or committee leaders, or they were on the powerful Ways and Means Committee in the House or the Senate Finance Committee (which have authority over tax and trade policy — always popular lobbying issues). This is especially true for House members. Backbenchers are less likely to become lobbyists.

Democrats and Republicans were equally likely to go on to become lobbyists.

Very conservative Republicans were less likely to become lobbyists than moderate Republicans.

Most retiring members (78 percent in the House, and 87 percent in the Senate) who became lobbyists went to lobbying firms that represent multiple clients, rather than going in house at a company, association or other group. This makes sense, since former members generally specialize in access, which they can then sell to multiple clients.

How outraged should you be?

Generally, members of Congress are prized as lobbyists because they are well-connected. They also know how the process works, and are generally skilled in political strategy. They may even have some policy expertise.

In a comprehensive study of lobbying, Lobbying and Policy Change, five political scientists looked at 98 issues to try to find what explained why some lobbying efforts succeed while others fail. In general, they had a hard time finding any reliable predictors. But one that did contribute was hiring more "revolving door" lobbyists. They found that 63 percent of the time, the side with more former government official lobbyists won.

Lazarus and McKay also find in previous research that clients that hired revolving door lobbyists were more likely to get earmarks than those that didn't.

In general, former members charge considerable sums for their lobbying time, because the market will bear it, which means that mostly they wind up representing corporate clients who can pay for their high rates. This contributes further to the already wild imbalance in who gets represented in Washington. In my book, I found that for every $1 spent by public interest groups and unions combined, corporations spent $34. Some of this disparity, no doubt, is because those corporations are paying for the pricey former members of Congress.

While other research I've done suggests that tightening restrictions of the revolving door may have contributed to fewer former members and staffers registering as lobbyists, I'm not so sure this matters all that much in the end. Former members find plenty of ways to lobby without having to register.

Besides, the bigger problem is not that members of Congress become lobbyists. It's that they primarily become corporate lobbyists. If they all went to work as lobbyists for public interest groups, there would probably be little outrage.

Perhaps, then, we ought to figure out how to make it more likely that former members use their connections and experience to represent general interest lobbying causes. Ideas are welcome.

This post is part of Polyarchy, an independent blog produced by the political reform program at New America, a Washington think tank devoted to developing new ideas and new voices. See more Polyarchy posts here.