Investors have increasingly expressed concerns about rising index concentration in the U.S. stock market, primarily driven by strong performances from technology stocks. Currently, these stocks represent approximately 35 percent of the S&P 500 index and 28 percent of the MSCI World. Consequently, any decline in U.S. tech stocks could lead to a significant downturn for both the U.S. and global markets.
While the U.K. stock market is not as heavily reliant on technology, it faces its own concentration risks. A notable disparity in total returns has been observed between the FTSE 100 and the FTSE 250, with a striking gap of 13 percentage points recorded this year. This difference is largely attributed to just two sectors and six key stocks.
Examining the sector compositions reveals significant contrasts between the FTSE 100 and the more diversified FTSE 250. Typically, this diversification benefits the FTSE 250 by reducing risks, but when market performance is heavily weighted towards a handful of sectors and stocks, it undermines the FTSE 250’s potential.
In 2025, performance surges have been prominently driven by bank and aerospace/defense stocks, which hold a larger weight in the FTSE 100 compared to the FTSE 250. Because of this concentration, it has been calculated that 8.5 percentage points of the total performance gap between the two indices originated from bank stocks, while 4.2 percentage points arose from aerospace and defense. Without these sector contributions, the FTSE 100 would have nearly equaled the FTSE 250’s performance.
On an individual stock basis, the disparity becomes even clearer. Notably, Rolls-Royce contributed 3.3 points to this performance gap despite being only the fifth-largest stock by market capitalization. Similarly, Lloyds Banking Group, ranked thirteenth, accounted for an additional 2.1 points. Further contributions from additional banks and defense stocks affirm that a small number of entities are propelling the FTSE 100’s performance.
While many investors in the FTSE 100 may embrace the current upward trend following years of lackluster returns, this analysis raises concerns about the sustainability of such narrow performance leadership. Should the rally in bank and defense stocks falter, the FTSE 100 could quickly fall back into underperformance.
The rise in defense stocks appears poised to continue as European nations bolster military spending. However, the outlook for banks is more uncertain. Currently benefiting from record profits—which are primarily a result of higher interest rates—banks may face challenges as the Bank of England signals a potential reduction in interest rates. The recent flattening of the yield curve could pose additional headwinds for bank performance, particularly heading into 2026.
Interestingly, it seems that not all investors have fully capitalized on the FTSE 100’s recent surge. The limited breadth of performance among leading stocks has made it especially difficult for active fund managers to outperform the index. Since the inflation spike in 2022, performance has been constrained primarily to banks, oil and gas, and defense sectors.
According to data from Trustnet, only about 10 out of 210 UK All Companies equity funds surpassed the FTSE 100 after costs this year. Over the previous five years, this figure dropped even lower, with just 14 managing to beat the index. In the last decade, the situation improved slightly, resulting in 23 funds outperforming the FTSE 100 after accounting for costs.
In this challenging investment landscape, the most successful funds are likely to be those that can identify the few stocks within the index that will drive overall performance. The skill of fund managers will play a crucial role, but luck will also undoubtedly influence the outcomes.
