When governments and companies borrow by issuing bonds, investors need to assess the risk of not being repaid. Credit rating agencies do this professionally — analysing financial statements, business models, economic environments, and political risks to produce a standardised opinion of creditworthiness. Their ratings shape the global cost of borrowing.
The rating scale
Why the investment-grade/junk line matters
The BBB/BB dividing line between investment grade and high yield (junk) is enormously consequential. Many institutional investors — pension funds, insurance companies — are legally required to hold only investment-grade bonds. A downgrade from BBB to BB forces these institutions to sell, flooding the market with bonds and pushing prices down sharply. Being a "fallen angel" (recently downgraded to junk) is often more damaging than already being junk.
The conflict of interest problem
Rating agencies are paid by the issuers whose debt they rate — a fundamental conflict of interest. This contributed directly to the 2008 financial crisis: agencies gave AAA ratings to mortgage-backed securities that were filled with subprime loans, giving investors false confidence. Post-crisis reforms improved the system but didn't eliminate the conflict.
What this means for you
If you own bond funds, the average credit rating of the holdings determines how much risk you're taking on. An "investment grade" bond fund holds predominantly BBB or above; a "high yield" fund holds BB and below — with higher expected returns but significantly higher risk of default. When buying individual bonds, always check the rating — and be sceptical of unusually high yields, which often signal hidden credit risk.