You could be facing a surprisingly dangerous amount of concentration risk without realizing it. Fixing that problem starts with knowing exactly what you own.
Most Investors Aren’t as Diversified as They Think: Are You? | Kiplinger
Previously, Uncle Warren said - “Consistently buy an S&P 500 low-cost index fund,” Buffett said in 2017. “Keep buying it through thick and thin and especially through thin.”
BUT FOLLOW WHAT THEY DO, NOT WHAT THEY SAY
“SEC filings data from March (2025) revealed that (Warren) Buffett’s company Berkshire Hathaway unloaded its entire positions in the Vanguard S&P 500 ETF and SPDR S&P 500 ETF Trust — two low-cost exchange-traded funds the company had previously held for years.”
I would be cautious about using Buffet’s decisions about his multinational holding company to inform someone’s decision about their individual retirement account. Buffet famously indicated in 2013 that his will stated that most of his wife personal holdings should be primarily in a S&P 500 index fund. “One bequest provides that cash will be delivered to a trustee for my wife’s benefit,” he wrote. “My advice to the trustee could not be more simple: Put 10% of the cash in short-term government bonds and 90% in a very low-cost S&P 500 index fund.” I have not seen any information that he has updated his will.
Another caution. At that time, Buffets wife was in her late 60’s. The asset allocation he decided on of 90% equities/0% bonds/10% cash is not appropriate for almost everyone in that age range. It reflects the massive amount that she will presumably inherit.
Given his wealth, he could invest in something very lossy, and his heirs would still love him.
My point is… in the past he used to tout the “one and done” S&P500 ETFs, but his own company reversed course on that in March.
Berkshire Hathaway managers are smart people - they are holding 50% in Cash now. They’ve had a lot of cash for a long time. They know there are dangers.
Over the years, most “professional” financial advisors I spoke with emphasize “re-balance” (not exactly market-timing 
) of your portfolio at least once a year. It is the time I re-direct my investment with current events and historical reference, aka the crystal-ball time. 
Unfortunately, I just did not see one crazy man can mess up my investment so badly at the end of last year. 
Index funds appear to be a good choice from historical data, if you are not close to retirement. But I am not firm on adding new money in NOW. It may not be a right decision - time will tell. 
I starting standing to one side in early 2008 during the initial Great Financial Crisis shocks. In fact, I lowered my target allocation to about 40%, and I was only 46 at the time. After “666” was hit in March 2009, AKA “The Obama Low” (he called it), I started ploughing all of my dry powder back in. I dollar cost averaged back in during all of 2009 and into 2010.
If you accumulate cash, have a plan for putting it to work eventually.
These things can take YEARS to complete.
While stock diversification is important…the reality is that gains come disproportionately from relatively few stocks in the market. As for asset class diversification, stocks haven’t just outperformed gold, bonds, and real estate over time, they have totally crushed them.
https://pages.stern.nyu.edu/~adamodar/New_Home_Page/datafile/histretSP.html
No one is disputing that stocks are the growth engine for a portfolio, but if you take that fact and run it to a ridiculous conclusion, then everyone should all the time be 100% stocks. That usually don’t work out in real life.
Most investors “say” they are long term investors, that they are risk-tolerant, etc, but when they get hit in the mouth they run.
I know it’s true for me… I want to maximize measures of risk-adjusted return. Sharpe Ratio, Sortino Ratio, minimized Ulcer Index, lower drawdowns. That makes me able to live with my portfolio, and I haven’t done anything stupid… like cash out at the bottom in 2009 and never get back in.