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The Federal Reserve held interest rates at a multi-decade high for more than a year. Investors took notice, piling into money market accounts to grab yields that haven't been available in more than a decade.

But since the Fed slashed rates by half a percentage point on Sept. 18, the flows into money market accounts haven't stopped. In fact, through Oct. 10, research provided to Yahoo Finance from Crane Data shows that money market fund assets have increased by about $180 billion since the Fed began cutting rates.

This reveals several truths about the surge of "cash on the sidelines" some have argued could be a reason for the stock market rally to continue.

For starters, it could be a nod to the uncertainty some feel about where things will head over the next year.

On Friday, Goldman Sachs chief equity strategist David Kostin wrote in a note to clients that "history does not lend much support to expectations of a cash-to-equity rotation." Kostin's research shows that since 1984, over the first three-, six-, and 12-month periods after the Fed begins cutting, flows into money market funds are greater than into equity or bond funds.

Kostin expressed a view we've written about in the past, noting that whether or not equities see inflows following rate cuts has more to do with why the Fed is cutting than the cuts themselves.

"Money market funds have historically experienced inflows following rate cuts regardless of the economic backdrop," Kostin said. "On the other hand, equity funds typically recorded inflows if the US economy avoided a recession and outflows if the US economy entered a recession shortly after the start of the cutting cycle."

This would tell us that some folks in cash may just be in wait-and-see mode. Just because the Fed is cutting doesn't mean the money needs to leave the sidelines and play in the game.

The continued surge into money market funds is also a reminder that while rates are lower than they were a month ago and are expected to continue falling, they're still higher than they've been in years. For instance, a Fidelity Government Money Market Fund is currently offering an average annual return of more than 4.5%, compared to the 10-year average of about 1.4%.

"At the moment, rates are just good enough and Americans are just nervous enough for cash to look like an attractive asset," Ritholtz Wealth Management chief markets strategist Callie Cox told Yahoo Finance.

Cox noted this phenomenon doesn't always last for long. Dating back to 1980, on average, flows into cash start declining 14 months after the Fed starts cutting.

Cash on the sidelines is often referred to as a reason to be bullish about the backdrop for future buying of equities. The logic is that all this money moving into money market funds will eventually be put to work. And while Cox believes this argument could go on a list of reasons to be bullish about the stock market, "it's not at the top."

"There's a lot of opportunistic cash in money markets that could make its way out over time but I think that's when people have overestimated the effects of that," Cox said. "It's not this chunk of money that's flowing in after every rate cut. It's a little more complicated than that."

So eventually, the money usually leaves cash. It could either move into bonds as the economic backdrop weakens and investors want to grab an attractive yield before they fall lower. Or, it could move into equities if the fundamental story surrounding the Fed's rate-cutting cycle continues to scream soft landing.

If the prospect of not knowing which scenario will win out scares you, there is still a reasonable option to earn nearly 5% a year. And you don't have to look beyond the growing pile of cash on the sidelines to see that plenty of investors are continuing to choose that path.

Josh Schafer is a reporter for Yahoo Finance. Follow him on X @_joshschafer.

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