"AI Has Tied Korean and U.S. Stock Markets Together"...Concerns in Japan Over Samsung and SK hynix Influence Too
Rayliant Research Analysis
KOSPI-Nasdaq 100 60-Day Correlation Coefficient
Highest Level in Five Years Since 2021
There is growing analysis that the increase in artificial intelligence (AI) investment is driving a high degree of synchronization between U.S. technology stocks and the Korean stock market. The diversification effect of investing in both markets, which investors once enjoyed by holding stocks from both countries, has essentially been rendered meaningless. In Japan as well, concerns have been raised about synchronization effects, as the Japanese stock market is sensitive to movements by Samsung Electronics and SK hynix.
"Investment risk rises as U.S. and Korean tech stocks become more synchronized"
On July 27 (local time), CNBC reported, citing research firm Rayliant, that the 60-day correlation coefficient between Korea’s KOSPI Index and the U.S. Nasdaq 100 Index has recently reached about 0.50, the highest level since 2021.
This is attributed to Samsung Electronics and SK hynix—companies that account for more than half of KOSPI’s market capitalization—positioning themselves as key suppliers of DRAM for AI servers, causing large-scale AI investment by American big tech companies to impact both stock markets simultaneously. According to Futurum Group, the share of data centers in global DRAM demand expanded from about 40% in 2025 to over 50% in 2026.
There is also an evaluation in the market that the Korean stock market is acting as a barometer for global AI investment sentiment ahead of the U.S. opening. The stock prices of Samsung Electronics and SK hynix react first to overnight AI-related news, providing a sense of how U.S. technology stocks might move. Analysts also note that Samsung Electronics announces its earnings approximately two weeks earlier than U.S. semiconductor companies, making it a leading indicator of AI demand.
Experts warned that as tech stocks in both countries become more synchronized, investment risk could increase. In the past, investing in both the Korean and U.S. stock markets offered regional diversification benefits, but now that both markets move according to the same variable—AI—the effect has diminished significantly. Concerns were also raised that if U.S. big tech companies slow capital expenditure (CapEx) on data centers and related areas, not only Samsung Electronics and SK hynix but also the overall KOSPI may be hit harder than other markets.
However, CNBC projected that in the long term, distinctions in stock performance could re-emerge between Samsung Electronics, SK hynix, and U.S.-based Micron, depending on each company’s capital investment, product mix, and U.S. government semiconductor support policies. The rise of the Chinese memory semiconductor producer Changxin Memory Technologies (CXMT) was also cited as a factor to watch.
"Japanese volatility also blamed on Samsung and SK hynix leveraged ETFs"
Similar concerns have been raised in Japan as well. Nikkei, Japan’s leading financial daily, reported on the 28th that volatility in the benchmark Nikkei 225 Index has recently been driven in part by Samsung Electronics and SK hynix. The explanation is that active trading of single-stock leveraged exchange-traded funds (ETFs) tracking Samsung Electronics and SK hynix has amplified price swings not only in these companies but also in Japanese semiconductor stocks such as Kioxia.
Driven by this year’s semiconductor rally, Kioxia overtook Toyota Motor to become Japan's largest company by market capitalization in June, only to fall to fourth place within the last month. Notably, the recent change in Kioxia’s shareholder structure, with SK hynix effectively becoming its second-largest shareholder, has been flagged as a key point for investors to watch.
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Nikkei analyzed on the same day that “Korea and Japan are in the same time zone and their market trading hours overlap,” adding, “with increasing trading in leveraged single-stock ETFs that move at twice the rate of Samsung Electronics and SK hynix shares, price movements of Korean semiconductor stocks have become even more volatile.”
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