The next time you get mad when you see the local park or median strip filled with campaign signs, remember this:

Not only are they ugly and potentially dangerous, they are also useless.

If the point of election campaigns is to persuade voters, then lawn signs (unless they are on well-known people’s lawns) are a waste of time and money. There is no evidence that good signs or bad signs have swayed a single race, but campaigns keep putting them out every year, mostly to keep volunteers happy.

But the sign printers don’t need to worry about their businesses being disrupted when this information gets out. At least in general elections, there is no evidence that anything really works. Political scientists David Brookman from Stanford and Joshua Kalla of the University of California looked at 40 experiments that measured the impact of different campaign strategies, and created nine more experiments of their own.

ZERO EFFECT

They found that in a general election in which a Democrat is running against a Republican, there is no perceivable effect of any campaign technique. The primaries are more fluid, but once the nominees have been selected nothing changes enough minds to be a factor.

Television advertising, which sucks up billions of dollars in every election and is the main reason candidates have to spend all their time raising money, cannot be shown to sway any votes. The same is true for direct mail, phone canvassing and even door knocking. Essentially, everything you will see candidates do over the next 2½ weeks has been proven to be ineffective at persuading voters.

“Our best estimate of the direct effects of campaign contact on Americans’ candidate choices in general elections is essentially zero,” the authors write. “Our findings throw cold water on the notion that it is easy, overall, for campaigns to persuade voters.”

So what are the campaigns doing out there? It’s more about making sure that your unpersuadable voters show up, and hope the other guy’s unpersuadable voters stay home. It doesn’t make for a lively debate, unless you like math.

In Maine, the math favors Republicans, at least in the midterm elections when we pick governors. Going back to the election of 2002, we average 150,000 fewer voters in the midterms than there were in the last presidential election, held two years earlier. And the shrinkage is not bipartisan. The Democrats have the bigger share of the “melt.”

The number of votes cast for Republican candidates for governor in those years averaged 74 percent of the Maine votes cast for the Republican presidential candidate in the last election. For instance, Paul LePage’s 218,065 votes in 2010 were 73 percent of the 295,273 Maine votes that John McCain got in 2008.

But Democratic gubernatorial candidates average between 52 percent and 64 percent of their last presidential candidate’s total, depending on whether you count independent Eliot Cutler’s share of the 2010 vote as “Democratic votes.”

The pattern is constant. When Democrats show up, there are more of them, but Republicans are more reliable about showing up. If the trend continues, Republican Shawn Moody will be our next governor.

For Democrats to win, voters will have to break the pattern and behave differently than they have in the past,

Which explains what the campaigns are up to. Brookman and Kalla find that even though you can’t persuade many voters, you can make sure the right ones turn out.

TALKING PAST EACH OTHER

TV commercials and mailers may not change your mind, but they do remind you that an election is coming.

Knocking on doors won’t win anyone over, but it might bug them enough to fill out an absentee ballot.

This kind of thinking would explain the lackluster campaigns being run in Maine this year. Even in debates, the candidates have gone out of their way to avoid challenging their opponents, speaking past each other to their own voters.

The candidates have been praised for civility, but I wonder if there isn’t something else going on. In these races, it’s all about who votes, not what they are voting for. It’s more like a census than an election.

You have to ask, which came first, political tribalism or campaigns that don’t try to persuade anybody? It’s hard to say, but one definitely feeds the other.

When you think about it like that, the lawn signs at the intersections don’t seem so bad.

At least state law requires them to be removed within a week of the election. This other stuff is going to be with us for a long time.