The Vanguard S&P 500 ETF (VOO) is wrapping up 2025 with something investors don't see very often: eight consecutive months in the green.

As of December 26, the benchmark index tracking America's 500 largest companies sits up nearly 1% for the month. With three trading sessions left in the year, it looks set to officially join one of the market's most exclusive clubs—a streak of eight straight monthly gains. The last time this happened? 2017. And across the entire postwar period, it's occurred just 10 times total.

So naturally, the question on everyone's mind is whether this party is about to end. Should investors be taking chips off the table after such a powerful run?

History, it turns out, has a pretty interesting answer.

The Rare Company of Eight-Month Streaks

According to research from Carson Investment Research analyst Ryan Detrick, the S&P 500 has managed eight or more consecutive monthly gains only 10 times since World War II. That rarity alone makes the current moment worth paying attention to. But what really matters is what happened next.

The immediate aftermath has been, well, unremarkable. The month right after these epic runs ended with average returns basically flat and gains showing up only about half the time. Nothing to write home about.

But zoom out just a bit, and the picture changes dramatically.

Three Months Out: Momentum Kicks In

Three months after hitting an eight-month winning streak, the S&P 500 averaged gains of 4.3%. That's nearly double what the index typically delivers over any random three-month stretch. Even better, nine out of 10 times the market kept climbing. The single exception came after November 1980, when stocks stumbled 6.6% over the following three months.

Six months out? The trend gets even stronger. Average gains reached 6.6%, comfortably ahead of the market's typical six-month return of 4.5%. Median returns hit 7.8%, and once again, nine of the 10 historical cases produced positive results.

The One-Year View: Still Bullish

Looking out a full year, the pattern held. The S&P 500 posted average gains of 11.5% in the 12 months following these extended winning streaks, beating its long-term annual average of 9.2%. And positive returns showed up eight times out of 10, better than the market's typical postwar success rate of around 73%.

Now, no historical pattern is a crystal ball. Past performance doesn't guarantee future results, and all that standard disclaimer stuff. But the takeaway here is pretty clear: when the market builds this kind of momentum, it doesn't usually just stop on a dime. More often than not, strength begets more strength.

What the Numbers Show

The historical track record breaks down like this. After previous eight-month (or longer) winning streaks that ended in February 1950, April 1954, October 1958, July 1964, November 1980, March 1983, July 1995, June 1996, January 2007, and November 2017, returns varied widely in the short term but skewed positive over time.

The best one-year performance came after April 1954, when the market surged 34.3% over the next 12 months. The worst was after November 1980, with a 10.1% decline. But those are the extremes. The median one-year return was 8.2%, and most periods delivered solid gains.

Breaking it down by timeframe: the next month averaged just 0.3% with gains only half the time. Three months out averaged 4.3% with a 90% success rate. Six months out averaged 6.6%, also with a 90% hit rate. And one year out averaged 11.5% with an 80% success rate.

Compare that to all periods in market history, where the average month gains 0.7%, three months average 2.2%, six months average 4.5%, and one year averages 9.2%. The consistency of outperformance after these winning streaks is striking.

Momentum Feeds on Itself

What's going on here? One explanation is that extended winning streaks don't emerge from nothing. They typically reflect genuine underlying strength—whether that's economic growth, corporate earnings expansion, accommodative policy, or some combination thereof. And those conditions don't usually reverse overnight.

Another factor: investor psychology. When markets build momentum, it tends to pull in more buyers rather than scare them away. Fear of missing out is a powerful force, and it can sustain rallies longer than skeptics expect.

Of course, there are exceptions. The November 1980 streak ended badly, and the January 2007 streak preceded the financial crisis. But even in 2007, the market didn't collapse immediately—it took months for the real damage to materialize.

So What Should Investors Do?

The data doesn't tell you to blindly buy more stocks or to ignore risk entirely. What it does suggest is that knee-jerk profit-taking solely because the market has gone up for eight months straight might not be the smartest move.

If you're sitting on gains and want to rebalance or take some risk off the table for personal reasons, that's perfectly reasonable. But if the only reason you're thinking about selling is "we've had eight green months, surely this can't continue," history says you might want to reconsider.

Strong trends have a way of staying strong. The market's current eight-month run is rare, yes. But rather than signaling an imminent top, it might just be another reminder that momentum matters—and that fighting a powerful uptrend is often a losing battle.