Reversion to the mean. It's the idea that asset prices tend to eventually return to their historic mean (or average) levels. The concept often becomes reality. Gravity sets in for high-flying stocks. Beaten-down stocks rebound.

Granted, all stocks don't revert to their historical means. But many do. This phenomenon might even present a great opportunity for investors right now. Should you buy this year's worst-performing S&P 500 stocks for 2024?

The S&P 500's low five of 2023

It's been a great year for the S&P 500 overall. The index is on track to finish 2023 with a gain of well over 20%. However, more than one-third of the S&P 500 stocks are in negative territory as the year comes to a close. Five of them have delivered especially dismal performances.

Chemical manufacturer FMC (FMC 1.14%) is on pace to be the worst S&P 500 stock this year. Its shares have plummeted more than 51% as of this writing. That horrible performance even reflects a rebound in recent weeks.

Moderna (MRNA 1.69%) is in a close second for coming in dead last. The biotech stock is down 50%. Like FMC, Moderna's share price was even lower just a few weeks ago.

Enphase Energy (ENPH 3.80%) stands out as another big S&P 500 loser in 2023. It's only slightly ahead of Moderna with a year-to-date decline of nearly 50%.

Then there's Dollar General (DG -0.41%). The discount retailer's shares are available at a much bigger discount after sinking 47% in 2023.

Moderna isn't the only COVID-19 vaccine stock that's been a dumpster fire this year. Pfizer (PFE 0.55%) has given up all of its pandemic gains and then some with its shares free-falling by nearly 46%.

Why these stocks plunged

FMC's steep sell-off this year was caused in part by what the company called "abrupt and unprecedented reductions in inventory by growers" in the second quarter. This channel destocking continued in the third quarter, with sales for FMC's agricultural chemicals especially hit hard in Latin America.

Enphase Energy blamed macroeconomic conditions -- especially higher interest rates -- for the declining sales of its microinverters used solar and battery systems. This caused the company to provide disappointing guidance for both Q3 and Q4, which sent investors headed for the hills.

The huge declines for Moderna and Pfizer reflect the deterioration in demand for the companies' COVID-19 vaccines. Pfizer's sales of its COVID-19 antiviral pill Paxlovid have also fallen significantly.

Like Enphase, Dollar General's management pointed to a challenging macroeconomic environment to explain its declining same-store sales in 2023. However, Dollar Tree and Walmart (two of its top rivals) saw their stocks perform much better. Perhaps unsurprisingly, Dollar General's board decided to replace the company's CEO with Jeff Owen with his predecessor, Todd Vasos, who oversaw a more prosperous period between 2015 and 2022.

Are they good picks for the new year?

Investors looking for a quick rebound in 2024 might be disappointed with most of these underperformers of 2023. I'm not confident that any of the companies will turn things around in the near term.

Of the five, though, I suspect that Moderna could be the most likely to change its narrative in the new year. The company's COVID-19 vaccine sales probably won't bounce back much, if any. However, Moderna hopes to win U.S. and European approvals for its respiratory syncytial virus (RSV) vaccine mRNA-1345.

I also think that Pfizer and Dollar General remain solid picks for long-term investors. Pfizer's valuation is attractive with its forward earnings multiple below 9.8. Its dividend yield of over 6% should appeal to income investors. Meanwhile, Dollar General appears to be making the right moves to fix its problems. These two stocks might not be huge winners in 2024, but I don't expect either of them to make a repeat appearance among the S&P 500's biggest losers.