Index concentration risk is the reason Monday’s Kospi crash felt like the whole Korean market was falling — because, mechanically, it kind of was.
On July 13, the Kospi plunged 8.95% to close at 6,806.93, triggering the year’s seventh circuit breaker* (*circuit breaker: an automatic 20-minute trading halt triggered when an index falls 8% or more in a day). SK Hynix fell 15.37% — its steepest single-day loss on record. Samsung Electronics dropped 10.70%. Almost nothing else in the index moved anywhere close to that.
Four separate things happened to land on the same afternoon.
First, SK Hynix’s own Nasdaq debut days earlier had already opened a pricing gap between its Korean shares and its new U.S. listing — and once that listing-week excitement faded, some of the buying pressure propping up the Korean stock faded with it.
Second — the real concept here — Samsung and SK Hynix together now make up 55% of the Kospi’s total market value and 63% of its daily trading volume, up from 36% and 28% a year ago. In a cap-weighted index* (*cap-weighted index: an index where each company’s influence is based on its total market value, not an equal split), “buying the market” quietly becomes “betting on two companies.”
Third, leveraged ETFs tied to these two stocks reset their exposure every single day. When the underlying stock falls, the fund has to sell more just to hold its target ratio — turning a drop into a bigger drop. South Korea’s central bank flagged exactly this risk on July 5. Eight days later, it played out almost exactly as described.
Fourth, a fresh spike in oil prices tied to Middle East tensions added a risk premium on top — a separate pressure that simply happened to land on the same day.
None of these four is unique to Korea. Any index dominated by a handful of giants carries the same structural risk; the mechanism just stays invisible until the day it isn’t.
My take: I can explain index concentration risk in my sleep — and I still called two stocks “diversification.”
Not advice. Just how I see it.