U.S. stock-market investors should know better than to base their investments on superstitions or trivia, but there’s a curious trend that has persisted for decades that could serve as yet another reason to be cautious about markets.

Since the presidency of Theodore Roosevelt, who left office in 1909, every single Republican president has seen a recession take hold in their first term.

While analysts consider this a fluke of timing more than an explicit economic reaction to Republican policies, the odds of this streak continuing during the presidency of Donald Trump are seen as plausible, particularly as he presides over what is already the second-longest bull market in history.

“Republican presidents seeing recessions has more to do with cycles — both political and economic — than policy,” said Sam Stovall, chief investment strategist of U.S. equity strategy at CFRA. “Most economic cycles last five to six years, presidential terms are four years, and you usually don’t see more than two straight terms of the same party. Right now we’re already late in an economic cycle that’s already much longer than average ones.”

In an April interview with MarketWatch, Stovall noted that bull markets don’t die of old age, but because of worsening economic conditions. However, he sees signs the economy may have already peaked, prompting him to forecast a recession at some point in Trump’s term. He added, “By the way, that’s not a brave forecast.”

The first year of the Trump administration was a positive one for the U.S. stock market by almost any metric. There was historic low volatility, a record number of records, and an unprecedented length of time that the S&P 500 SPX, +1.59% went without falling 5% or even 3% from record levels. The gains in the Dow Jones Industrial Average DJIA, +1.33% over Trump’s first year were the best for any president since Franklin Roosevelt.

Such an environment came to a swift end in the first quarter of 2018, a sign that stocks may be starting to wake up to the prospect of the bull market’s eventual end.

Major indexes have been rangebound for weeks, with both the Dow and the S&P in their longest stretch in correction territory in a decade. Volatility has been elevated throughout the year, and markets just entered the month of May — historically a treacherous one, particularly in midterm years. If current “sideways trading” trends persist, the S&P threatens the ominous technical pattern of a “death cross” by Memorial Day.

At the same time, leadership in the market is at multiyear lows, and Morgan Stanley recently calculated that expectations for future returns were at their lowest level since before the financial crisis. Furthermore, the yield on the U.S. 10-year Treasury note TMUBMUSD10Y, 0.657% recently hit a four-year high, a cautious signal for stocks for a variety of reasons, and changes to Federal Reserve policy — including both rising interest rates and a shrinking balance sheet — could further diminish the attractiveness of stocks.

“I view central banks as being like dads at Disneyworld, carrying their kids on their shoulders,” Stovall said. “They look pretty vibrant at 10 a.m., but now it’s 4 p.m. and they look pretty exhausted from supporting the economy.”

Read more:Why the Fed is ‘Public Enemy No.1’ for the stock market

There’s no shortage of potential that could threaten equity performance, ranging from escalating trade tensions to issues with North Korea and other geopolitical concerns. Domestically, both the upcoming midterm elections and the investigation currently being conducted by special counsel Robert Mueller are seen as potential risks.

More fundamentally, while earnings growth has been at a multiyear high in the first quarter, some analysts have suggested growth may have peaked in the quarter. GDP growth came in at 2.3% in the first quarter, its slowest pace in a year. While slowing growth isn’t the same as a contraction, it adds to the concerns that the period of synchronized global growth is coming to an end.

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Earlier this week, the Wells Fargo Investment Institute wrote that the U.S. economy was “entering the late stage of expansion,” a phase marked by moderating growth, earnings pressure, rising interest rates and confidence peaking. While it doesn’t see an imminent recession — defined as a significant fall in activity across the economy, with the National Bureau of Economic Research considered the ultimate arbiter — it indicated that one could only be a matter of time, pointing to how the yield curve was moving in the direction of inverting.

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“If a trade war breaks out and spreads, that could clearly have implications for the global economy, especially since there are already a few signs that global growth may be peaking,” said David Joy, chief market strategist at Ameriprise Financial. He added that Ameriprise hadn’t forecast a recession, and that “maybe the earliest we could see one would be 2020.”

Despite that, “we could certainly reach a point where trade headwinds overwhelm the tailwinds of tax reform. A deteriorating trade situation could overwhelm what we still see as strong fundamentals.”