Markets tend to hate US government shutdowns and the uncertainty they cast on the economy and big companies.

And they really hate the long, drawn-out variety.

New number crunching out of RBC Capital Markets strategist Lori Calvasina on Monday found that in the lead-up to the last seven government shutdowns (of 10 days or more) dating back to 1976, the median S&P 500 decline was 10.2%.

The largest decline of 19.8% came ahead of the government shutdown that lasted from Dec. 21, 2018, to Jan. 23, 2019. The smallest decline in stocks tallied 3.7% ahead of the 21-day shutdown that ended on Jan. 6, 1996.

During the actual government shutdowns, Calvasina's team found the S&P 500's median drop was 2%.

There are at least two positives when it comes to markets and shutdowns, however.

First, markets often roar back after bickering politicians find agreement on thorny issues and reopen government.

In the 12 months following a government shutdown of 10 days or more, the S&P 500 has gained a median of 18.9%, according to RBC's research.

And two, data from Truist analyzing the last 20 shutdowns going back to 1976 reveals that stocks have been up 50% of the time during the actual shutdown period.

"Fraught politics tend to be a symptom of challenging times generally," Calvasina pointed out. "The good news for US equity investors today is that the S&P 500 has already fallen 5.9% from its late July 2023 high. A 10% drop, which would be in line with the average and median pullback heading into extended shutdowns in the past, would take the index a little north of 4,100."

Data analysis along these lines is critical for investors at the moment as the government knocks on the door of another shutdown.

The US government is set to shut down this coming weekend unless Congress strikes a last-minute agreement to pass a dozen spending bills. Sept. 30 is the funding deadline.

During a shutdown, the government is only able to spend money on essential services such as law enforcement and public safety. Hundreds of thousands of government workers will see their paychecks delayed, injecting a fresh dose of uncertainty into an economy already dealing with the lagged effects of rising interest rates and rolling UAW strikes at GM, Ford, and Stellantis.

EY economist Greg Daco estimated that $6 billion in real GDP, or 0.1%, could be cut from the fourth quarter if the government is closed for one week.

Pros such as Calvasina contend that markets could stay under pressure in the near term as the shutdown risk grows. But any eventual deal could be met with renewed buying stock prices that have pulled back a good amount in September.

"Government shutdowns tend to be high profile though low-impact market events," Truist chief markets strategist Keith Lerner said. "While uncertainty around these events tends to heighten investor angst and add to short-term market volatility, the historical evidence suggests a minimal lasting market impact.

"That said," Lerner added, "we continue to anticipate that equities will remain in choppy waters near term as we remain in a seasonally weak period, stocks continue to consolidate large year-to-date gains, and we are lacking an obvious near-term catalyst."

Brian Sozzi is Yahoo Finance's Executive Editor. Follow Sozzi on Twitter @BrianSozzi and on LinkedIn. Tips on deals, mergers, activist situations, or anything else? Email brian.sozzi@yahoofinance.com.

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