Kospi decouples from S&P 500, now tracks Philadelphia Semiconductor Index

The Kospi's decoupling from the S&P 500 has deepened sharply, even as the South Korean benchmark moves in near-lockstep with the Philadelphia Semiconductor Index. With Samsung Electronics and SK Hynix together accounting for more than half of the Kospi's total market cap, analysts say the domestic market has effectively become a "semiconductor index" that mirrors global chip-sector conditions and AI investment sentiment.

An analysis published Thursday found that the monthly correlation coefficient between the S&P 500 and the Kospi has fallen to 0.025 this month — down from around 0.5 in February, a level at which the two indexes were considered meaningfully synchronized.

A correlation coefficient ranges from -1 to 1, with values closer to 1 indicating that two indexes move in the same direction. A reading above 0.5 is generally considered significant, while a figure near zero suggests little discernible directional relationship.

Recent market moves illustrate the divergence clearly. The S&P 500 closed up 0.42% in New York on Friday, yet the Kospi tumbled 8.95% the following Monday when domestic trading resumed.

The contrast in volatility is equally stark. From last month through Wednesday, the Kospi recorded 12 trading sessions in which it moved more than 5% in a single day. The S&P 500 did not post a single daily swing of that magnitude over the same period.

Analysts point to the Kospi's overwhelming concentration in semiconductors as the primary driver of the decoupling. The S&P 500 spans financials, health care, energy, consumer goods and other sectors alongside technology, which limits how far a shock in any one industry can ripple through the broader index.

Concentration risk is rising in the US market as well, with the "Magnificent Seven" mega-cap technology stocks commanding an ever-larger share. According to Goldman Sachs, the top 10 S&P 500 constituents now account for roughly 40% of the index's total market cap — the highest level since 1990. Even so, the Kospi's concentration is far more extreme: Samsung Electronics and SK Hynix alone represented 52.30% of the entire index's market cap as of Wednesday's closing prices. It takes the top 10 S&P 500 names combined to reach 40%, whereas just two stocks make up more than half of the Korean benchmark.

That structure means the Kospi tends to outperform US equities when optimism about AI investment grows, but falls far harder when concerns arise about a peak in memory chip demand or doubts surface over the sustainability of AI spending. As global investors increasingly scrutinize the durability of the AI investment cycle and capital expenditure plans at US big-tech companies, volatility in the domestic market has risen in tandem.

Some analysts now argue the Kospi should be benchmarked against the Philadelphia Semiconductor Index rather than the S&P 500. "The Kospi is a market where the movements of Samsung Electronics and SK Hynix determine the direction of the index," said Seo Sang-young, a researcher at Mirae Asset Securities. "The decoupling from the S&P 500 stems from differences in index composition. When you compare the Kospi with the Philadelphia Semiconductor Index, the two move in virtually the same direction."

The numbers bear that out: from the start of this month through Wednesday, the Philadelphia Semiconductor Index and the Kospi fell 13.36% and 14.06%, respectively — nearly identical declines.

Securities analysts expect the concentration to deepen further as chipmakers' earnings contribution continues to outpace their already-dominant market-cap weighting. "Samsung Electronics and SK Hynix's combined 12-month forward net profit contribution within the Kospi surged to 70.5% at the end of June this year, and is forecast to reach 71.8% by the end of the third quarter and 80.2% by the end of the fourth quarter," said Kim Yong-gu, a researcher at Yuanta Securities Korea. "Meanwhile, the two stocks' combined market-cap weighting stood at 56.4% as of end-June — well below their earnings contribution — which suggests that, despite short-term share price and liquidity volatility, further concentration in line with their profit contribution is highly likely."