Two Stocks That Rise and Fall Together Aren't Actually Diversifying You
6 min read
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6 min read
You own BBCA, BBRI, BBNI and BMRI. Four different tickers, four different companies, four different stock codes on your holdings screen. That feels like diversification: if one bank has a bad quarter, the other three are still there. But diversification isn't measured by how many tickers you hold. It's measured by correlation, how much those tickers actually move together, and on the days that matter most, Indonesia's four biggest banks tend to move as one.
Correlation is a number between -1 and +1 that describes how two assets' returns move relative to each other. A correlation near +1 means they tend to rise and fall together. A correlation near -1 means one tends to rise when the other falls. A correlation near 0 means their movements have no consistent relationship at all.
Owning more tickers spreads out company-specific risk, a scandal, a bad earnings report, a lawsuit at one firm. It does very little against a risk all four companies share, and Indonesia's big banks share quite a lot: the same regulator, the same national credit cycle, and the same interest rate set by Bank Indonesia.
On 17 and 18 June 2026, Bank Indonesia's board raised the BI Rate by 25 basis points to 5.75%, the second hike in a month after a 9 June move to 5.50%. Higher rates are often framed as good news for banks, since they can typically reprice loans faster than deposits, widening net interest margin. That is not what happened to the share prices that day.
All four of Indonesia's biggest bank stocks fell together:
| Stock | Move | Closing price |
|---|---|---|
| BBRI (Bank Rakyat Indonesia) | -3.89% | Rp2,960 |
| BBCA (Bank Central Asia) | -3.18% | Rp6,075 |
| BBNI (Bank Negara Indonesia) | -1.84% | Rp3,730 |
| BMRI (Bank Mandiri) | -0.44% | Rp4,470 |
Source: Akses.co.id, "BI Rate Naik Menjadi 5,75 Persen Picu Pelemahan Empat Saham Bank Raksasa", reporting on the 17-18 June 2026 Rapat Dewan Gubernur decision.
Notice the magnitudes are not identical. BBRI fell almost nine times as much as BMRI did. That is exactly the point: correlation is not about two assets moving by the same amount, it is about moving in the same direction because of the same trigger. All four names answered to the same BI Rate decision that day, in the same direction, even though the theoretical case for bank stocks (better margins from a rate hike) argued the opposite way. When the trigger is shared, the direction gets shared too, regardless of what the fundamentals case says should happen.
Here is why BBRI falling 3.89% and BMRI falling only 0.44% on the same day doesn't mean they were weakly linked. Take two illustrative stocks, Stock X and Stock Y, not real securities, where Stock Y's return each period is always exactly 0.6 times Stock X's return, same direction, smaller size:
| Period | Stock X | Stock Y |
|---|---|---|
| 1 | +5% | +3.0% |
| 2 | -4% | -2.4% |
| 3 | +2% | +1.2% |
| 4 | -6% | -3.6% |
| 5 | +3% | +1.8% |
| 6 | -1% | -0.6% |
Because Stock Y's return is an exact multiple of Stock X's return every single period, the correlation between them works out to a perfect +1.0, even though Stock X moves nearly twice as much as Stock Y in every period. Perfect linear proportionality between two return series always produces a correlation of exactly +1 or -1, regardless of how different the magnitudes are. Real stocks are never this perfectly proportional, but the BI Rate day above shows the same shape: different-sized moves, same direction, same underlying cause.
If BBCA, BBRI, BBNI and BMRI all answer to the same BI Rate decision in the same direction, holding all four gives you less protection than the ticker count suggests against the one risk that matters most on days like 18 June 2026: a rate move nobody can control from an account screen. What it does still protect against is something happening to one specific bank and not the others, a governance problem, a bad loan book, a management scandal. That is a real benefit. It is just a narrower one than "I own four different stocks" implies.
Actual diversification means holding assets exposed to different dominant drivers, not just different company names. A bank stock like BBCA, a commodity exporter, physical precious metals and something denominated outside the rupiah each answer to a different primary trigger, so a single BI Rate decision cannot move all of them the same way at once.
The lesson is not "avoid bank stocks." It is: know what a holding actually reacts to before assuming a second, third and fourth ticker is giving you real protection. NetWort's Portfolio Health view shows your real holdings together in one place, so you can see how much of your portfolio sits in positions that answer to the same trigger. Check the Macro Context page the next time BI meets to see the BI Rate decision and the market's reaction on the same page, then look at your own holdings and ask which of them would move first.