By: Katie Gomez

Why is the Russell 2000 lagging the S&P 500? The short answer is that small-cap stocks and large-cap stocks respond very differently to today’s market environment. Companies in the Russell 2000, tracked by the IWM ETF, tend to be more sensitive to interest rates, borrowing costs, domestic economic growth, and changes in investor risk appetite. Meanwhile, the market-cap-weighted S&P 500 has benefited heavily from strength in mega-cap technology and AI stocks. Understanding those differences helps explain why IWM can struggle even when SPY and the broader S&P 500 appear strong.

IWM vs. SPY: What’s the Difference?

The Russell 2000 tracks about 2,000 small-cap U.S. companies, while the S&P 500 represents many of the country’s largest publicly traded companies. IWM is one of the primary ETFs tracking the Russell 2000, while SPY tracks the S&P 500.

That distinction matters because these indexes represent very different parts of the U.S. stock market. Russell 2000 companies tend to be smaller, more domestically focused, and more sensitive to economic conditions. By contrast, the S&P 500 is heavily influenced by massive global companies like Nvidia, Microsoft, Apple, Amazon, and Meta. Comparing IWM vs. SPY is therefore not simply comparing two ETFs—it is comparing small-cap stocks with some of the largest and financially strongest companies in the world.

Higher Interest Rates Hurt Small-Cap Stocks More

One of the biggest reasons for Russell 2000 underperformance is the relationship between interest rates and small-cap stocks.

Smaller companies generally have less cash, weaker balance sheets, and greater dependence on outside financing than the mega-cap corporations that dominate the S&P 500. When interest rates stay high, borrowing and refinancing cost more. That pressure can reduce earnings, slow expansion, and make investors less willing to take risks on smaller businesses.

Large-cap companies often have an advantage in this environment. Many have significant cash reserves, stronger credit ratings, and easier access to capital. They may be better positioned to absorb higher borrowing costs than smaller companies.

That creates a structural reason why higher-for-longer Federal Reserve interest rates can hurt IWM more than SPY. When investors expect borrowing costs to remain elevated, small-cap stocks often feel that pressure first.

The S&P 500 Has a Mega-Cap AI Advantage

Interest rates are only part of the story. Another major reason the S&P 500 has outperformed the Russell 2000 is simply what each index owns.

The S&P 500 has significant exposure to the world’s largest technology and AI-related companies. When semiconductor stocks rally, enthusiasm around artificial intelligence grows, or mega-cap technology stocks surge, SPY captures much more of that upside.

IWM does not have that same advantage. Small-cap stocks exclude the mega-cap technology companies that have driven a significant portion of recent S&P 500 performance.

This also highlights an important point: the S&P 500 is not necessarily the same thing as the entire stock market. Because the index is market-cap weighted, its largest companies have an outsized influence on performance. A handful of enormous technology stocks can push SPY higher even as hundreds or thousands of smaller stocks struggle.

That is why stock market breadth matters. If gains remain concentrated in a few mega-cap names, the S&P 500 can keep outperforming the Russell 2000. If participation broadens into financials, industrials, healthcare stocks, regional banks, and other smaller companies, IWM may have a better chance to catch up.

Small Caps Are More Sensitive to the U.S. Economy

Russell 2000 companies also tend to have greater exposure to the domestic U.S. economy than many S&P 500 corporations, which often generate substantial revenue internationally.

That makes small-cap stocks more sensitive to consumer spending, bank lending, labor costs, inflation, credit conditions, and overall U.S. economic growth.

Profitability matters too. The Russell 2000 contains more younger, speculative, and unprofitable companies than the S&P 500. When investors are optimistic and capital is inexpensive, that risk may be rewarded. During periods of economic uncertainty, tighter credit, or higher rates, investors often favor larger companies with stronger balance sheets and more predictable cash flow.

This is also why Federal Reserve rate cuts do not automatically mean IWM will outperform. Lower rates caused by cooling inflation and healthy economic growth can create a favorable environment for small caps. But rate cuts caused by recession fears or rapidly weakening demand may signal deeper economic problems that still hurt smaller businesses.

Why rates are falling can matter as much as the fact that they are falling.

What Would Make the Russell 2000 Outperform the S&P 500?

For the Russell 2000 to regain leadership, several pieces of the market environment would likely need to improve.

Falling Treasury yields and lower borrowing costs could ease pressure on small-company balance sheets. Improving small-cap earnings, stronger domestic growth, healthier credit conditions, and easier Federal Reserve policy could also make investors more comfortable taking risk outside of mega-cap stocks.

Market breadth may be the most important signal to watch. The Russell 2000 does not necessarily need Nvidia, Microsoft, or the rest of the mega-cap technology trade to collapse. It simply needs more stocks to start participating.

A rotation away from concentrated mega-cap leadership and toward financials, industrials, regional banks, biotechnology, and other economically sensitive sectors could quickly change the IWM vs. SPY relationship.

Valuation may also play a role. After years of large-cap dominance, some small-cap stocks may look relatively inexpensive compared with mega-cap companies. However, cheap does not automatically mean ready to outperform. Small caps still need a catalyst, whether that comes from improving earnings, lower financing costs, stronger economic growth, or a broader shift in investor sentiment.

For active traders, monitoring relative strength between IWM and SPY can provide valuable insight into whether market leadership is beginning to change. Unusual volume, breakouts, sector rotation, and increasing small-cap momentum can all signal that participation is broadening beyond the largest technology stocks.

Bottom Line: Why Is IWM Lagging the S&P 500?

The Russell 2000 is lagging the S&P 500 because the two indexes are exposed to fundamentally different parts of the market. Small-cap companies face greater pressure from higher borrowing costs, economic uncertainty, and weaker balance sheets. At the same time, the S&P 500 continues to benefit from concentrated strength in mega-cap technology and AI stocks.

For IWM to regain leadership, investors will likely need to see lower financing costs, stronger small-cap earnings, healthier U.S. economic growth, and broader stock market participation. The key may not be waiting for large-cap technology to fall—it may simply be waiting for the rest of the market to catch up finally. 

Frequently Asked Questions

Why is the Russell 2000 underperforming the S&P 500?

Small-cap companies are generally more sensitive to interest rates, borrowing costs, domestic economic growth, and investor risk appetite. Meanwhile, the S&P 500 has benefited heavily from mega-cap technology and AI stocks.

Do lower interest rates help the Russell 2000?

They can. Lower borrowing costs generally benefit smaller companies, but rate cuts help most when they come alongside healthy economic growth rather than a recession.

What would make IWM outperform SPY?

Falling yields, improving small-cap earnings, stronger U.S. growth, easier credit conditions, and broader stock market participation could all help IWM outperform.

Is the Russell 2000 undervalued?

Small caps can look inexpensive relative to large-cap stocks after extended S&P 500 outperformance, but low valuations alone don’t guarantee an immediate rally.​

For active traders, spotting that rotation early can make all the difference. Trade Ideas helps you monitor real-time relative strength, unusual volume, breakouts, and small-cap momentum so you can see when market leadership starts shifting beyond mega-cap stocks.