General election advertising will soon begin in earnest. Just last week, Hillary Clinton announced she was launching her general election ad campaign in many battleground markets, but there is no indication where and when Donald Trump’s campaign will respond.  There was a lot of political advertising on TV during the primaries. By one count, ads were shown nearly three-quarters of a million times. Media consultants will tell you the primary ads worked. But you’d expect that, because it’s often in their financial interest to persuade candidates to advertise. What is more surprising is that research from political scientists, some involving creative and carefully designed field experiments done in cooperation with real campaigns, corroborates this notion.  The evidence suggests that campaign ads have small effects that decay rapidly — very rapidly — but just enough of the impact accumulates to make running more advertising than your opponent seem a necessity.  It sets off an arms race of ads as candidates try to neutralize or displace their opponents. But will the 2016 general election be different? Mr. Trump has used unconventional campaign tactics and has relied on free media to get his messages out. All of this may render advertising less relevant.  A study estimated that most of the impact of an ad in a presidential election is gone within a day or two of its airing (I am one of the authors of this paper). In governor, congressional and Senate elections, the effects last a bit longer: three or four days. Fleeting effects on campaigns have been shown by various authors in the lab; in Canada; in the 2000 and 2004 general elections; in the 2006 midterm elections; in the 2012 general election; and in field experiments in a Texas governor’s primary in 2006 and a general election in 2014.  The takeaway from these studies is simple: Even though the effects from an ad imbalance are small and go away fast, candidates cannot allow them to pile up. Election Day may be far away, but candidates may still want to match their opponents’ daily advertising in the months before the vote because they care about publicly released news polls that convey information to voters — and donors — about their viability and the closeness of the race.  Candidates, consultants and media advisers seem to understand this cycle of seemingly daily one-upmanship. In the last presidential election, it began as early as the summer of 2012, with Barack Obama’s campaign trying to define Mitt Romney as rich and out of touch. Only a few days after the start of Mr. Obama’s onslaught, Mr. Romney’s campaign began a counterassault, matching the buys in markets in Florida and Ohio.  So will it be any different with Mr. Trump and Mrs. Clinton? I suspect not, and here’s why.  Using data from Kantar Media on all ads aired between September 2015 and the end of May 2016, it’s easy to see that Mr. Trump (or groups advertising on his behalf) ran a lot of ads during the primaries. In fact, they ran nearly as many ads as Jeb Bush and his supporters, and many more than those supporting John Kasich or Ben Carson. Most of the ads aired by or for Mr. Trump were positive in tone — promoting his own ideas and not attacking his opponents, while most of Ted Cruz’s and Mr. Bush’s ads were attacks.  With all that attack advertising on the Republican side — roughly 60 percent of the ads aired for or by Mr. Cruz and Mr. Bush and 40 percent for or by Marco Rubio — how did Mr. Trump emerge unscathed as the Republican nominee? A look at the content of these attacks provides an explanation.  Most of the early attack ads run by Mr. Cruz, Mr. Bush and Mr. Rubio were not aimed at Mr. Trump; they were aimed at one another. A small batch of attacks on Mr. Trump hit the airwaves in December, but after that, it wasn’t until after the South Carolina contest that anti-Trump ads began again in earnest.  On March 14 — the day before the Florida, Illinois and Ohio primaries — more than 4,000 airings of attack ads were directed at Mr. Trump. Most were run by Our Principles PAC and accused Mr. Trump of hiring undocumented workers at his construction sites, engaging in illegal and deceptive conduct with Trump University and scamming Floridians who bought in to a real estate investment in Tampa.  Some of these attack ads ran nationally, and many were featured in news reports. It was a coordinated effort by many Republicans who considered this a critical moment to blunt Mr. Trump’s momentum. Polls suggest that the ad barrage in the spring took a little bit of the shine off Mr. Trump’s lead (and indicate he may be vulnerable to sustained attacks on the airwaves). But then the volume of attacks stopped, and his poll numbers bounced back to where they were before the attacks.  If the attacks had continued or if they had happened earlier, there’s some evidence that Mr. Trump would have responded with more of his own ads to rebalance the equilibrium. Just as these attacks were rising sharply, Mr. Trump increased his promotional advertising. This response suggests a candidate fully engaged in the ad-war battle.  There are more than 140 days left in the 2016 presidential election, with the most intense campaigning still to come. Given Mr. Trump’s use of advertising in the primaries, there is little reason to believe that he will forgo television advertising in the general election.  Lynn Vavreck, a professor of political science at U.C.L.A., is a co-author of “The Gamble,” about the 2012 presidential campaign. Follow her on Twitter at @vavreck.  The Upshot provides news, analysis and graphics about politics, policy and everyday life. 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He has started off slowly with ads, but there’s a reason he won’t skip them, even though the effects are small and short-lived.