State of the Union speeches. What are they good for?

Turns out, not much.

Political scientists who have studied the effects of presidential speeches have found little systematic evidence that they shift public opinion on an issue or cause Congress to act on legislation.

Even popular presidents like Ronald Reagan and Bill Clinton struggled to pass much of their legislative agendas.

The challenge in studying this issue is discerning what would have happened in the absence of a presidential appeal. Consider President Barack Obama's 2010 State of the Union.

According to data from the Policy Agendas Project, Obama dedicated nearly 14 percent of his policy appeals to “banking and finance.”

This constitutes the largest policy allocation during a State of the Union speech in his four years in office, aside from economic issues.

Months later, Congress passed Dodd-Frank, also known as the “Wall Street Reform and Consumer Protection Act.”

The question is: Did Congress act because of Obama's appeal?

Not necessarily.

According to Gallup poll data months before that State of the Union, the banking and financial services industry was widely unpopular and growing less popular by the day. Many Americans favored greater regulation of the banking industry in response to the financial crisis.

So the crux of the issue is: Does the president typically preempt public opinion or simply react to it?

For the most part, political scientists endorse the latter view.

In “Who Leads Whom,” Brandice Canes-Wrone shows that presidents typically advocate policies that are already popular. Presidents, she concludes, are rarely successful in changing mass opinions.

And in the aptly titled “On Deaf Ears,” George C. Edwards II finds that for the most part Congress, the president and the media all respond to exogenous events outside their control. Indeed, Edwards finds that presidents are generally unsuccessful in leading public opinion and pushing Congress to act on legislation that would have failed otherwise.

How might this apply to Tuesday night's State of the Union? Consider the issue of gun control. In his past four States of the Union the president never mentioned gun control.

Tuesday night, by comparison, Obama made four specific appeals:

1) Passing universal background checks.

2) Regulating the resale of guns.

3) Limiting military-style assault weapons.

4) Limiting high-capacity magazines.

Congress will no doubt draft and pass legislation to address these issues.

But the important point is that these proposals are already popular

According to a Gallup survey conducted in December:

■ 92 percent favor background checks.

■ 58 percent feel that the sale of firearms should be made “more strict.”

■ 44 percent support an assault weapons ban.

■ 62 percent support a ban on high-capacity magazines.

Thus, the passage of items 1, 2 and 4 is not evidence of presidential leadership per se. The passage of these items would be fully consistent with the conventional wisdom detailed above.

An assault weapons ban, by comparison, would, in my view, constitute evidence of presidential leadership.

However, I'm skeptical that the president's coattails are long enough to get an assault weapons ban through Congress.

In the coming months we'll see just how strong the “bully pulpit” is on the issue of gun control.

But when considering the effect of presidential rhetoric on policy outcomes, it's important to put first things first.

Jordan Ragusa is an assistant professor of political science at the College of Charleston.